Aiming for multiple targets for Huntington’s disease therapies: a hopeful report from the Yang lab at UCLA

 

This article is in commemoration of Huntington’s Disease Awareness Month (May).

 

One of the most impactful university labs focusing on Huntington’s disease, the X. William Yang Research Group at the University of California, Los Angeles (UCLA) employs a multi-pronged approach to investigating potential therapies for this deadly brain disorder.

 

Started in 2002 by X. William Yang, M.D., Ph.D., the lab has produced several key findings on HD, mainly through the study of genetically modified (i.e., transgenic) mice, engineered to carry the HD mutation and exhibit some of the disease-like phenotypes (characteristics).

 

Dr. Yang was inspired to focus on Huntington's disease because of his interaction with patients in Venezuela – the world’s largest clusters of HD families – and the HD scientists working there. In 2000 and 2002 he was invited to observe these families and assist with studies by Nancy Wexler, Ph.D., the president of the HD-centered Hereditary Disease Foundation (HDF) and leader of the landmark effort to identify the HD gene in 1993.

 

Dr. Yang's Venezuela experience cemented his resolve to study HD in his own lab. Indeed, the first research grant ever received by Dr. Yang was from HDF. Today he serves as its scientific advisory board’s vice chair.

 

Dr. Yang’s team has also collaborated with CHDI Foundation, Inc., the largest private funder of HD therapeutic research. Pharmaceutical firms such as Roche (the world’s largest) and Ionis Pharmaceuticals, Inc., the developer of the Roche drug now in its second HD clinical trial, have consulted Dr. Yang for his expertise.

 

Dr. Yang has emerged as a leading academic voice in HD science. Listed as the first author, in February he and two other important prominent HD researchers – Leslie Thompson, PhD., of UC Irvine and Myriam Heiman, Ph.D., of the Massachusetts Institute of Technology (MIT) – published a major co-edited book. Huntington’s Disease: Pathogenic Mechanisms and Implications for Therapeutics presents the latest work on the disease’s medical impact, genetics, the huntingtin protein, new tools and models for research, and an overview of therapeutic approaches and clinical trial programs.

 

 

The back and front covers of Huntington’s Disease: Pathogenic Mechanisms and Implications for Therapeutics (image courtesy of Dr. Yang) (Click on an image to enlarge it.)

 

‘The stars are aligned’ for developing HD treatments

 

Although the use of human data in HD research has increased dramatically, crucial research in mice has become more relevant to potential therapies because of new biotechnologies and the availability of so-called “big data” made possible by powerful computing systems.

 

“This is completely unprecedented in terms of the kind of study we can do,” Dr. Yang told me in a 40-minute interview on January 29, noting the advantages of a “21st century toolbox.” “Mouse models in this context are extremely useful.”

 

We met in Dr. Yang’s office in his lab, which is located in UCLA’s Gonda (Goldschmied) Neuroscience and Genetics Research Center. I was invited to Los Angeles to offer my perspective as an HD gene carrier on the first day of a two-day HDF scientific workshop, co-chaired by Dr. Yang.

 

“I know it's probably an oxymoron to say that it's time to be hopeful, because we’ve been to a hopeful stage many times before,” Dr. Yang said, acknowledging the negative results of some recent clinical trials. He added that “the stars seem to be aligned” for developing HD treatments.

 

 

Dr. Yang (left) with project scientist Chris Park, Ph.D. At the far left is a confocal microscope, which uses laser light to obtain high-resolution images of thick tissues. Behind the men is a light sheet microscope, also used for obtaining high-quality images of tissues (photo by Gene Veritas, aka Kenneth P. Serbin).

 

Focusing on the brain

 

Dr. Yang grew up in Tianjin, China, a port city located 80 miles from the capital, Beijing. In 1985, Dr. Yang was one of five students selected by the Chinese government to participate in the Rickover Science Institute, founded by Admiral Hyman G. Rickover to foster high-school science education for both domestic and international students. Rickover developed the first nuclear-powered engines and first atomic-powered submarine.

 

“I did a whole summer of research at the NIH [National Institutes of Health], working on signaling pathways in rat brains,” Dr. Yang wrote in a follow-up e-mail to our interview. “The research experience got me really interested in studying the mammalian brain.”

 

The Rickover program is now called the Research Science Institute (RSI). Among other prestigious alumni are Harvard University’s Steve McCarroll, Ph.D., a leading molecular geneticist who also works on HD; and MIT/Broad Institute's Feng Zhang, Ph.D., a CRISPR research pioneer.

 

After RSI, Dr. Yang briefly studied at Peking University, one of China's top universities, before transferring to Yale University, where in 1991 he completed the highly demanding joint B.S./M.S. program in molecular biophysics and biochemistry.

 

Over lunch Dr. Yang and I reminisced about our years at Yale. I was privileged to graduate from Yale in 1982. I told Dr. Yang that I had seen Admiral Rickover give a public lecture at the university – a poignant moment for me as a history major because of his military and scientific prominence. I told Dr. Yang of my interest in tracking the contributions of Yale and its graduates like him to HD science and medicine (click here, here, and here to read more.)

 

Dr. Yang completed the joint M.D./Ph.D. program at The Rockefeller University (Ph.D., 1998) and Weill Medical College of Cornell University (M.D., 2000) in New York City. In 2002, he finished postdoctoral research in the Rockefeller lab of Nathanael Heintz, Ph.D., which focuses on HD and other neurological and psychiatric disorders.

 

Gene Veritas (left) (aka Kenneth P. Serbin) with Dr. William Yang in his UCLA office. In the background: a mouse medium spiny neuron. In humans this neuron is one of the cells most affected by Huntington’s disease (photo by Nan Wang, Ph.D., of the Yang Research Group).

 

A ‘trustworthy and versatile’ invention

 

Dr. Yang and his lab have made key contributions to HD science, including understanding the causes and potential pathways to therapies. The team also studies Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative disorders.

 

As a Ph.D. student, Dr. Yang co-invented with Dr. Heintz and Peter Model, Ph.D., the first method to engineer Bacterial Artificial Chromosomes (BACs) to generate transgenic mice. BACs have the advantage of holding long strands of DNA with key regulatory elements that confer accurate gene expression in transgenic animals.

 

In an analysis of this research, which Drs. Yang, Model, and Heintz published in 1997, one leading biologist described their technique as “trustworthy and versatile” for cloning genes and the key task of learning the specific function of particular genes.

 

Indeed, scientists have used this method to generate a variety of transgenic animals, from zebrafish to mammals (click here to read more).

 

The key BACHD mouse

 

In 2008, Dr. Yang and other researchers published the results of a project creating the first BAC transgenic mouse model of HD, the BACHD mouse, their term for this mouse specifically engineered to study HD.

 

As Dr. Yang explained in our interview, the team inserted a long strand of a mutant (irregularly expanded) human huntingtin gene into the mice. Those genetic characteristics do not normally exist in mice. As they hoped, the mice developed dysfunction, displaying impaired movements, shrinkage to the same brain regions affected in HD, and damage to the synapses (the connections between brain cells).

 

“We developed different versions of these mouse models to allow us to ask, for example, which cell types in the brain with mutant huntingtin are important,” Dr. Yang said.

 

The team demonstrated the presence of mutant huntingtin in two key areas of the brain: medium spiny neurons in the striatum and pyramidal neurons (brain cells) in the cortex. (See the photo above with Dr. Yang, me, and an image of a medium spiny neuron. Also see the photo in the next section.)

 

In mice, humans, and other mammals, the cortex handles important processes such as cognition, memory, motor control, and sensory processing. The striatum – an area deep in the brain and greatly affected in HD – controls motor (movement), motor and reward learning, and executive function. In humans, this region is also known as the caudate and putamen. The Yang lab also examines communication between these regions.

 

Dr. Yang (left) and Nan Wang, Ph.D., a project scientist focusing on Huntington’s, in the lab (photo by Gene Veritas)

 

Using mice and genetics to understand HD

 

In detecting the impact of mutant huntingtin in those areas, that initial BACHD research revealed disease phenotypes in both striatum and cortex, Dr. Yang recalled. “That study turned out to be really important because, for the longest time, people thought the striatum, the medium spiny neuron, was really the primary site of action.”

 

Removing the mutant huntingtin from the cortex led to improvement in the mice’s behavior and even partially helped the striatum, Dr. Yang explained. Likewise, deleting mutant huntingtin from the striatum brought some improvement.

 

“But most importantly, if you reduce mutant huntingtin in both cortex and striatum, the BACHD model looks really, really good, almost as good as a normal mouse,” he added.

 

The BACHD work, he recalled, helped to convince the field that  the cortex is one of the key brain regions that should be targeted in HD clinical trials. The Roche/Ionis ASO lowers the level of huntingtin protein more in the cortex than in the caudate/putamen, according to preclinical studies in non-human primates.

 

In sum, Dr. Yang said, the studies of BACHD mice represent a “proof of concept that we can use this kind of a sophisticated – genetically as accurate as we could get – type of mouse model to inform about disease pathogenesis” – how HD develops, progresses, and, significantly, might be treated.

 

A mouse medium spiny neuron (image courtesy of Dr. Yang)

 

From disease switch to vulnerable neurons

 

The Yang Research Group has achieved other key findings, some in collaboration with other labs.

 

The Yang lab teamed with researchers at UC Irvine, UC San Francisco, the University of Pittsburgh, and the University of Tennessee to study the chemical modification of the huntingtin protein itself. This research focused on so-called “chemical tags” that naturally attach to the very beginning of the huntingtin protein, a small region acts like a disease switch.

 

In one experiment, this research used a BACHD-like mouse to mimic the tagging. That resulted in mice that had “very little disease despite having the HD mutation,” Dr. Yang explained. The results were published in 2009.

 

In 2015, the Yang Research Group published a separate study showing the genetic switch is necessary to prevent severe disease including neuronal loss and movement deficits, phenotypes reminiscent of those found in HD. These studies showed that the huntingtin protein itself and its chemical tags could be a source of new targets to develop therapies, Dr. Yang said.

 

From watching mice in ‘log rolling contests’ to unbiased genetic analysis

 

In 2013, the Federal Government announced the launch of the BRAIN initiative to enhance understanding of the human brain. The Yang lab was one of the first 59 in the country to receive support in the initial round of BRAIN funding. It now has its third grant. It receives support from other government agencies, as well as the HDF and CHDI. The lab’s achievements include developing a new, genetic way to label the complete, intricate shape of single brain cells, which allows the study of their function and dysfunction in diseases such as HD.

 

With big data and "the 21st century toolbox," the field of HD research has advanced from more traditional ways of observing diseased mice to more nuanced molecular, cellular and systems biology analyses, Dr. Yang explained.

 

In earlier research, by primarily relying on the behavior and pathology of individual mice, the work resulted in “relatively few readouts” of data, Dr. Yang observed. With that methodology, scientists had mice doing activities such as “spontaneously move” in an open area or on a rotarod, “like the ESPN log rolling contest,” he said. Scientists also routinely measured loss of brain matter.

 

Now, scientists can do a “big-scale, unbiased molecular studies” by examining tens thousands of datapoints, including analysis of DNA, RNA and proteins, Dr. Yang added.

 

Clues from gene expression about neuronal vulnerability

 

Collaborating with CHDI, the Yang lab’s work in this area has involved study of HD’s impact in different areas of the brain, moving beyond the standard understanding that most damage comes in the striatum. The lab has done this research using different types of engineered HD mouse models carrying different lengths of CAG repeats and measured the levels of tens of thousands of RNA transcripts ("RNA-seq," that is, RNA sequencing) in the mouse brains and peripheral tissues.

 

Published in 2016, the results noted that despite the presence of the mutant HD gene throughout the body, the disruption in gene expression in these HD mice is highly selective to the striatum, the most affected brain region in HD. The severity of the disruption is correlated with the length of CAG repeats in these mice. Moreover, the molecular defects in the striatum appear in young adulthood, worsening with age.

 

“There's about 100 or so genes that have essential function selective to the striatal neurons that are most affected in Huntington’s disease,” Dr. Yang said. “And somehow the mutant huntingtin knows to go there and make them the sickest, which we thought was a remarkable find – a sense that there's some fundamental mechanism connecting this CAG expansion to selective neuronal vulnerability.”

 

‘Perturbing’ the mice to understand human modifier genes

 

Taking advantage of the gene signatures from RNA-seq studies, especially those selectively disrupted in the striatum, the Yang lab embarked on a study using such gene signatures to sensitively detect "modifiers" of the disease. To achieve this, they used these genes to genetically “perturb” the mice, Dr. Yang explained.

 

“We basically genetically perturb the huntingtin mouse and say, ‘which gene, if we perturb them just right, can make the disease worse – that's one thing that's interesting – but more importantly make them better. And if better, how much better.’”

 

Continuing this line of work, the lab has continued testing the impact of other genes. These experiments include study of some of the human HD modifier genes – about ten – previously identified by the Genome Wide Association Study (GWAS) from over 9,000 HD-affected individuals and their relatives. The modifiers found by the Genetic Modifiers of Huntington's Disease Consortium can delay or hasten HD onset.

 

In addition, the Yang lab tested over 100 other candidate modifier genes identified in the prior systems biology work.

 

The scientists have tested large number of genetic mutants in HD mice to determine whether this makes the disease better or worse, Dr. Yang said. Noting that the results are still unpublished, Dr. Yang said that the team is drilling down on discovering the best gene targets that could help advance therapies to alleviate the disease.

 

Three potential ways to treat HD

 

Dr. Yang also discussed his outlook for therapies to slow, prevent, or reverse the course of Huntington’s. As noted, he believes that “the stars seem to be aligned” for the development of treatments.

 

In exchanging ideas with other HD scientists, he proposed the model of a stool – which needs four legs to remain stable –  as a metaphor for the benefit of developing multiple therapies (polypharmacy) that could act synergistically for HD.  

 

“If one drug could work for HD, that will be great. However, for many diseases, like HIV or cardiovascular diseases, multiple drugs together can make the disease more manageable, and patients' lives much better.”

 

As of now, Dr. Yang said scientists are developing three potential legs of the therapeutic stool. Each leg represents a new angle in understanding HD and how it might be applied to slow or stop the disease.

 

The first leg: huntingtin lowering

 

As the first leg of the therapeutic stool, Dr. Yang pointed to so-called huntingin lowering – the reduction of the HD gene (DNA), RNA, or its toxic protein in the brain. Pioneered in patients by the above-mentioned Roche/Ionis clinical trial program, this approach has captured the attention of many academic and biopharma labs.

 

This Roche/Ionis drug is an antisense oligonucleotide (ASO), a synthetic strand of DNA that degrades the RNA from making the huntingtin protein. Other clinical trial programs aim to alleviate HD with ASOs, or other DNA or RNA targeting therapies. Some of them using small chemicals to reduce human huntingtin.

 

This approach has received ample coverage in this blog and elsewhere.

 

The second leg: GWAS/mismatch repair genes

 

Dr. Yang pointed to potential therapies based on the HD GWAS genes – which include DNA mismatch repair (MMR) genes – as the second leg of the stool.

 

“Lots of companies now are really excited about some of these genes,” Dr. Yang noted. “They are essential for aspects of repairing DNA. There's not much we know yet about the potential efficacy and safety liability of a drug targeting these genes. We and others are actively doing research in these areas.”

 

Dr. Yang said that some of these genes are known to “stabilize” the CAG repeats, which tend to expand in the brain areas affected by HD. Such "somatic" repeat expansion is thought to be a key mechanism in the disease.

 

A gene with great potential is MSH3, a MMR gene under investigation by academic labs and biopharma firms. Before it had to shut down for lack of funding, Triplet Therapeutics had planned to use an ASO to target MSH3 in a clinical trial.

 

“So far, I can tell you MSH3 looks pretty safe, at least in animal models,” Dr. Yang explained.

 

He cautioned that scientists still need to learn more about the basic biology of the HD GWAS DNA repair genes in the brain and select the best targets and therapeutics before advancing them in clinical trials in patients.

 

The third leg: huntingtin protein-protein interaction

 

The third leg of the therapeutic stool, he said, is how the huntingtin protein interacts with other proteins.

 

So far, researchers have discovered at least 100 proteins that could interact with huntingtin, including in different cell types and at different ages, Dr. Yang said. The interactions occur with both the normal and mutant versions of the protein.

 

At least one of these proteins, HAP40, binds very closely with huntingtin. Dr. Yang described HAP40 and huntingtin as “inseparable buddies.” The Yang lab is actively working on the normal function of HAP40 in the brain and whether it could have a modifier role in HD.

 

As with the GWAS genes, Dr. Yang stressed that research on protein-protein interaction and its potential benefit for patients is ongoing. He added that, in the search for potential drugs, the key is finding “a protein that binds to huntingtin and is required for disease, and ideally this protein is amenable to therapeutic intervention.”

 

Aiming to solve one of the ‘central mysteries of HD’

 

The recent HDF workshop’s focus on “cell-type specific biology” in HD took up the question of why certain brain cell types (i.e., neurons in the striatum and cortex) are vulnerable to degeneration.

 

Dr. Yang stated that it is unclear whether research on cell-type vulnerability could become the fourth leg of the therapeutic stool. “Cell-type vulnerability could be related to” the first three legs, “especially protein-protein interaction and GWAS mismatch repair genes.”

 

However, this does not diminish the importance of cell-type vulnerability.

 

“This question of  selective vulnerability is really a key feature for all neurodegenerative diseases,” Dr. Yang said. “So, for Huntington it's a striatal medium spiny neuron and some of the deep-layer cortical pyramidal neurons.” In Alzheimer’s and Parkinson’s, neuronal cell types in other brain areas are affected.

 

“So the big question is: why, for each disease, certain types of neurons die?” Dr. Yang asked. “If we can understand this fundamental question and elucidate its mechanism, we could use the knowledge to develop new disease-specific therapies to protect neurons from degeneration.   

 

With the workshop, Dr. Yang said, “we think the time is right to revisit what I consider one of the central mysteries for Huntington’s disease – why certain neurons are selectively vulnerable to degeneration despite that mutant huntingtin is expressed in all the cells in the body.”

 

As usual, this group of HD scientists used the workshop to explore new ways to solve this mystery and develop potential therapies.

 

At the HDF workshop: seated, from left to right, Mahmoud Pouladi, M.Sc., Ph.D., Osama Al Dalahmah, M.D., Ph.D., Ashley Robbins, Gene Veritas (aka Kenneth P. Serbin), Sarah Hernandez, Ph.D., William Yang, M.D., Ph.D. Standing, from left to right, Xinhong Chen, Andrew Yoo, Ph.D., Anton Reiner, Ph.D., Baljit Khakh, Ph.D., Nicole Calakos, M.D., Ph.D., Ed Lein, Ph.D., Beverly Davidson, Ph.D., Nathaniel Heintz, Ph.D., Harry Orr, Ph.D., Leslie Thompson, Ph.D., Myriam Heiman, Ph.D., Shawn Davidson, Ph.D., Steven Finkbeiner, M.D., Ph.D., Roy Maimon, Ph.D. (photo by Julie Porter, HDF)

 

Bonding with the scientists

 

Following our interview and tour of the lab, I made a PowerPoint presentation to Dr. Yang and other members of the lab: “Advocating for the care and cure of Huntington’s disease: a biosocial journey.”

 

I spoke about my family’s struggles with HD, my advocacy, and my deepening interest in the social and scientific history of the HD movement. Afterwards, I answered questions.

 

Once again, I bonded with a fellow Yale graduate immersed in the fight against Huntington’s disease and scientists dedicated to a cure.

 

 

The X. William Yang Research Group after hearing Gene Veritas speak on his Huntington’s disease story. Seated (from left to right) Chris Park, Ph.D., Xiaofeng Gu, M.D., Ph.D., Dr. Yang, Gene Veritas, Nan Wang, Ph.D. Standing (from left to right) Ming Yan, MPH, Masood Akram, Ph.D., Tien Phat Huynh, M.D., Ph.D., Daniel Lee, Ph.D., Nianxin Zhong, Henry Chen, Lalini Ramanathan, Ph.D., Alexandra Shambayate, Leonardo Dionisio, Amberlene De La Rocha, Linna Deng Ferguson

 

Thanks to Emily Farrell, Executive Assistant, Department of History, University of San Diego, for assistance with the interview transcript.

 

Disclosures: the Hereditary Disease Foundation covered my travel expenses to Los Angeles. In support of the HD cause, I hold a symbolic number of Ionis shares.

A veteran neurologist’s book offers tools for navigating the ‘turbulent world’ of Huntington’s disease

 

In the fight against any disease, affected individuals and health professionals can arm themselves with an invaluable tool: detailed, cutting-edge knowledge about a condition’s medical and social impact.

 

For the Huntington’s disease community and related disorders, Thomas Bird, M.D., has made a key contribution with his book Can You Help Me? Inside the Turbulent World of Huntington Disease, published last year (Oxford University Press).

 

A retired neurologist who has observed more than 1,000 individuals with HD, Dr. Bird has produced one of the most important – and most accessible – introductions to this devastating disorder. Can You Help Me? will remain relevant for years.

 

Dr. Bird is an Emeritus Professor of Neurology and Medical Genetics at the University of Washington in Seattle, WA. His career spanned more than 40 years, including pioneering work in the field of clinical neurogenetics (diagnosing and evaluating hereditary nervous system disorders). His patients included sufferers of Alzheimer’s disease (AD), Parkinson’s disease (PD), and other severe, chronic brain conditions.

 

“I have been […] constantly amazed, puzzled, distressed and impressed by the trials and tribulations of these families coping with it,” Dr. Bird writes of Huntington’s. (As with some in neighboring Canada and elsewhere, he calls it “Huntington disease.”) “Dealing with HD has been so moving, so unsettling and so challenging for me that I felt compelled to write about it.”

 

“Can you help me?” a desperate HD-affected man wrote Dr. Bird from the state penitentiary in 1980 seeking medical assistance. That question reverberated in Dr. Bird’s encounters with many other HD people and their families and caregivers.

 

With non-technical, limpid prose, Dr. Bird tells the full story of HD’s wide-ranging medical, socioeconomic, and legal implications through a series of poignant vignettes, based on hundreds of HD cases. He changed identifying information to preserve patient privacy, and in some cases created composites of two or three different individuals.

 

Can You Help Me? will ring familiar to veterans of the HD cause, but it also offers new revelations and insights about HD’s impact. Also, it provides an excellent primer for families new to HD.

 

 

Key lessons about bioethical challenges

 

Many of the stories in Can You Help Me? center on the bioethical challenges faced by HD families, such as the complex ramifications of genetic testing.

 

Dr. Bird retells the story of a deeply troubled young man from an HD family killed by the police after a life of drug use and many clashes with the law – anti-social behavior perhaps resulting from the disease. The man’s aunt pleaded with Dr. Bird to have the coroner confirm the man’s HD status, because he had a three-year-old daughter for whom the test result would someday hold great relevance. Divorced, the man had lost contact with the mother.

 

“Needless to say, this was uncharted territory for us,” Dr. Bird writes of his clinical team. “The appropriate legal or ethical aspects of this case were not clear to me, but I called the University Hospital genetics lab and explained the situation.” On Dr. Bird’s orders, the lab performed the test.

 

The test demonstrated that the man indeed did have HD – “another burden” for the extended family, as Dr. Bird writes.

 

“To this day, I do not know if we followed the correct strategy in trying to help this family,” he concludes. It seemed like “quick thinking” to ask the coroner to save a sample of the dead man’s blood for HD testing, he observes. But many questions remained, including when and how to tell the girl of her at-risk status, he adds.

 

Dr. Bird believed that he might encounter the family again, when the little girl had grown up.

 

Dr. Thomas Bird (book jacket photo by Chang En Yu)

 

No ‘typical’ patient

 

In an appendix, Dr. Bird provides an overview of the genetics of HD and the genetic testing process. In general, as I noted in a previous article, Dr. Bird’s book has helped move the HD field from a traditional, more limited kind of genetic counseling to broader “genetic education.”

 

Can You Help Me? reflects on many other key issues for the HD community.

 

He offers valuable insight into the challenges faced by health professionals working with HD patients, as exemplified in the story of a patient clinic who shot himself. “Could we have done more?” Dr. Bird asks. “These situations are heavy burdens for everyone to bear.”

 

Dr. Bird’s vignettes underscore another crucial point in line with the latest medical and scientific research into the disease: there is no “typical” HD patient, as symptoms manifest uniquely in each case.

 

Notably, Dr. Bird does not describe HD as being like a combination of other diseases such as AD and PD, a shorthand some in the HD community use. Dr. Bird instead compares HD to these and other conditions, thus adding vital context.

 

Dr. Bird also emphasizes the need to end the false dichotomy between psychiatric “mental” diseases like schizophrenia and “brain” diseases like HD. Both originate in the brain, he points out, and both cause “mental illness.” Psychiatry and neurology should intersect more, he argues.

 

A contribution to the history of HD

 

In researching the book, Dr. Bird did important historical legwork. For instance, he painstakingly tracked down important episodes such as the use of lobotomies as an attempt to treat HD.

 

“It is estimated that 50,000 lobotomies were performed in this country between 1938 and 1955,” Dr. Bird writes. “Since persons with HD were often institutionalized and lobotomies were common, it is likely that many of these operations were performed on patients with HD.”

 

Although the procedures on HD people were “not easy to document,” Dr. Bird finally found evidence by examining medical texts. In all, Dr. Bird estimates that “perhaps more than 100” people with HD had the operation. It is not currently recommended for HD.

 

A ‘Princess in Pink’

 

Although many vignettes are gut-wrenching, Can You Help Me? also highlights the sometimes-brighter side of the HD story, such as individuals with late onset, mild symptoms, and productive lives.

 

That message holds two-fold meaning for me as an asymptomatic 61-year-old HD gene carrier who saw his mother develop the disease in her late 40s and die at 68.

 

First, I remember how fortunate I am to have reached this stage without symptoms.

 

Secondly, as a writer and advocate, it reminds me that, no matter how badly the disease has turned people like my mother into shadows of themselves, we should see them as humans struggling with disabling symptoms.

 

A caring community can seek to alleviate some of that burden.

 

One of my favorite stories from Can You Help Me? spotlights the “Princess in Pink,” Bobbi, a little girl who, although afflicted with juvenile HD, maintained her cheerfulness.

 

Bobbi’s fifth-grade teacher, Miss Perry, “decided to be proactive,” Dr. Bird writes. “She wanted to make Bobbi more comfortable in the classroom and educate her other students about Bobbi’s disease and how to relate to persons with disabilities.”

 

The class created the “Princess Project” to discuss HD and create a booklet ­– with a pink cover and a picture of Bobbi wearing a pink crown – about Bobbi and her condition. The classmates wrote perceptive and compassionate entries and also made drawings of Bobbi.

 

“It was a learning experience for everyone, including the adults,” Dr. Bird recalls.

 

Sadly, by age 15, Bobbi’s conditioned worsened, requiring a feeding tube, a frequent end-of-life measure for HD patients. She died in a hospice setting. Several of her old classmates attended the memorial service, where they fondly remembered Bobbi, Dr. Bird writes.

 

A resounding ‘yes’ in wanting to help

 

Can You Help Me? is one of the best and most important books about HD. It builds on the work of historian Alice Wexler, in particular her book The Woman Who Walked into the Sea, which uncovers many of the prejudices associated with HD (click here to read my review).

 

Dr. Bird has provided us with a deeply rich documentation of life in the HD trenches.

 

The title Can You Help Me? asks a question that I and so many other HD family members have posed when confronted with the frightening prospects of HD. (It also holds great value for people in the AD, PD, and other neurological disease communities.)

 

“Sometimes we can help a great deal, sometimes we can only help a little, and sometimes we just muddle through as best we can, navigating our way between suffering and harm,” Dr. Bird concludes, noting briefly that research towards treatments “heralds better days ahead for the world of HD.”

 

Fortunately, for the HD community, Dr. Bird’s book indicates a resounding “yes” regarding the desire by him and so many other professionals to alleviate the suffering caused by HD.

 

(For an interview of Dr. Bird about the book, click here).

I’m a Huntington’s disease gene carrier at age 60, so why haven’t I developed symptoms yet?

Huntington’s disease struck my mother in her late 40s, turned her into a debilitated, mere shadow of herself by her late 50s, and took her life at 68. I inherited from her the same degree of genetic mutation. Last December, I turned 60. So, doomed to suffer this inevitable and untreatable disease, why don’t I have any apparent symptoms yet?

Of course, I am thrilled to have avoided the dreadful scenario I imagined for myself after my mother’s diagnosis in 1995 and my positive test for the mutated, expanded gene in 1999. I did not believe that, by age 60, I would still be able to work, write, and not become a burden for my family. Indeed, in January, I marked fifteen years as a Huntington’s disease blogger.

I have written about my broad range of strategies for keeping healthy, including swimming, neurobics  (exercising the brain) and blogging, and taking supplements, some of which were ultimately proved ineffective. I stretch daily to keep limber, and I eat a healthy diet (no alcohol, sodas, or red meat; minimal processed foods; and lots of fish and fresh fruits, vegetables, and salads). I also consult a psychotherapist, meditate, and practice spirituality. 

I also have the benefit of a stable, solid-paying job and a close relationship with my wife and daughter. I cannot be sure whether any of these things help avoid HD, but they generally bolster health.

As Robert Pacifici, Ph.D., the chief scientific officer for the nonprofit, HD-focused CHDI Foundation, Inc., pointed out in a major interview last year, “lifestyle” is potentially very important. Evidence from at least one animal study suggests this, he said, although no scientific data yet prove this for HD in humans (click here to read more).

However, extensive, pathbreaking research based on humans has provided a new understanding of the genetics of Huntington’s and why people with the same size of gene mutation – the same CAG count, as explained below – can experience widely different ages of onset. A Huntington’s Disease Society of America (HDSA) webinar, presented by James Gusella, Ph.D., on November 19, 2019, explained the main points of this research and its relevance for HD families.

“You can relatively easily find people who’ve developed symptoms maybe 20 or more years later than you’d expect from the average, or 20 or more years earlier than you’d expect, and you can find people all along that range,” said Dr. Gusella, who titled his presentation “New Insights on Huntington’s Disease Age of Onset from Genetic Studies of HD Families.”

Dr. Gusella is the Bullard Professor of Neurogenetics at Harvard Medical School and the director of the Center for Human Genetic Research at Massachusetts General Hospital. He helped lead the efforts that narrowed the search for the huntingtin gene to chromosome 4 in 1983 and the discovery of the gene in 1993 (click here to read more). Since then, he and his collaborators have continued to make important discoveries about HD.

Above, Dr. James Gusella (left) during an interview with Gene Veritas (aka Kenneth P. Serbin) at the 7th Annual Huntington's Disease Therapeutics Conference, sponsored by CHDI,  in 2012. Below, a slide from Dr. Gusella's HDSA webinar presentation illustrating the average age of HD onset correlated with the CAG count.

The CAG count

Focusing on the discovery of so-called “modifier genes” for HD, Dr. Gusella delved into the reasons for the wide variations in onset – and the potential this research has for producing HD treatments.

As Dr. Gusella explained in the webinar, the human genome has 3 billion “letters,” or base pairs, which make up our DNA. The four letters that make up the bases of DNA are A (for adenine), C (cytosine), G (guanine), and T (thymine).

Like all genes, the huntingtin gene is made of a string of “three-letter words,” sequences from those four letters. Within the gene is a segment in which the word “CAG” is repeated a number of times. Normal genes have 10-25 CAG repeats. Repeat lengths of 26-34 do not ordinarily cause HD, but the repeat number can increase as the gene is passed to a child, leading to HD in the offspring. HD can occur in people with 35-39 repeats, and genes with 39 or more repeats “almost always” cause the disease, Dr. Gusella stated.

“CAG repeats” is the lingo of the HD community. Tested gene carriers like me usually know our repeats, and those of our affected parent and relatives. I have 40, as did my mother.

The “CAG count,” as it’s also known, became critical in my wife’s and my decision to conceive, especially because males (we were told) had a greater tendency to pass on a larger number of repeats. What if our child had a few more repeats or even more?

The CAG count has long factored heavily in genetic counseling and even in people’s decisions about moral dilemmas like abortion.

In general, the more repeats, the earlier the onset, leading even to juvenile HD – although, as Dr. Gusella emphasized, the age of onset varies widely.

New thinking about HD genetics

Since the discovery of the HD gene, scientists have published thousands of papers on HD, many of them based on studies in non-human organisms such as flies, mice, sheep, and primates – some of these organisms genetically modified (before birth) to later develop HD-like symptoms. However, because HD occurs only in humans, ultimately our species provides the best model for understanding and treating the disease, scientists say.

Scientific advances and the advent of clinical trials have made deeper research in humans more widespread and easier to carry out.

“We’re firm believers that, if you’re going to study a human disease, you’re best to study it first in people, rather than in trying to recreate it in other animals,” Dr. Gusella stated. “People really give you the information for what the disease is.”

Assessing genetic data collected over decades in more than 9,000 people affected by HD, Dr. Gusella and the Genetic Modifiers of HD (GeM-HD) Consortium have made discoveries that have changed standard thinking about Huntington’s genetics.

This type of broad-ranging study is known as GWAS, genome-wide association study. 

As Dr. Pacifici stated in 2015, human data are “precious” because they enable Huntington’s drug hunters to design and run better clinical trials, which are crucial for developing treatments.

Dr. Robert Pacifici (photo by Gene Veritas)

Explaining onset

In the webinar, Dr. Gusella detailed the research on CAG repeats and onset. A correlation definitely exists, he stated. However, other key factors come in into play.

“The inherited CAG length accounts for about 60 percent or so of the variation in age of onset, but there is a lot of variation” at each CAG count, he said.

“Just measuring the CAG repeat doesn’t give you an accurate prediction of when any given individual is going to have onset,” he emphasized. Research in thousands of people produces an average, “but it really doesn’t tell you much specifically about a given individual that would be useful diagnostically.”

However, the mass CAG data can help scientists explain why individuals diverge from the average, he stated.

Forty percent of the reason for onset must be due to factors other than the CAG count, Dr. Gusella continued. From their research, the GeM-HD Consortium concluded that 20 percent is due to other genes, that is, modifier genes “that are influencing when you have onset.”

Environmental factors ‘hard to study’

“The other 20 percent remains unexplained,” Dr. Gusella said. “It could be anything. It could be chance. It could be environmental factors.”

Environmental factors “are very hard to figure out and study,” he added. In answer to a webinar question about environment, diet, and exercise, Dr. Gusella could point to no study on the topic, although he noted that such research falls outside his expertise.

Indeed, in my more than two decades as an HD advocate and participant in numerous research studies, I’ve not been aware of any such study for presymptomatic gene carriers like me. The closest was PREDICT-HD, which collected samples of blood, urine, saliva, and cerebrospinal fluid from presymptomatic gene carriers. It also had them undergo a motor coordination exam and brain MRI scan and perform a battery of cognitive and mood tests. (Click here to read more).

Dr. Gusella added that the unexplained factors could also include “simply the diagnostic uncertainty, because you’re dealing with a motor onset.”

Motor onset marks the start of the involuntary movements typical in HD. Doctors have long used it as the standard way of diagnosing the disease, as opposed to other, initially often more subtle symptoms such as depression or cognitive difficulties.

However, as Dr. Gusella noted, diagnosing motor onset can be “a little bit subjective” on the part of the patient, the family, and the physicians. They all might also lack certainty about the exact time of onset.

Modifier genes influence age of onset

For the 20 percent of onset determined by modifier genes, the GeM-HD Consortium has hard evidence from the genetic studies of the 9,000-plus individuals.

It is “clear” that genetic variations “account for the differences” in age of onset for people with the same CAG count, Dr. Gusella said.

Everybody has genetic differences such as hair and eye color, and the overall number of differences among people is very large, he explained. By studying thousands of people, and using two methods of analysis, the scientists have detected 23 genes that influence the onset of HD.

Modifiers can come from both the affected and non-affected parent, Dr. Gusella pointed out.

As with many other genes, researchers have assigned these modifiers with very long, scientific names, which they have abbreviated to terms like FAN1. Delay in onset from the average varied from one to 20 years. FAN1 and most of the other modifiers are involved in the maintenance and repair of DNA, which, in general, helps cells remain healthy, he noted.

A slide from Dr. Gusella's presentation illustrating the location on the chromosomes of some of the currently identified Huntington's disease modifier genes

Dr. Gusella stressed that the GeM-HD research had not yet resulted in new types of genetic tests for individuals to discover whether they have favorable or unfavorable modifier genes. The research correlates to observations in thousands of people, but does not allow for prediction of age of onset in any given individual. 

The GeM-HD findings have shed light on other genetic aspects of the disease critical for families and family planning. When an affected parent passes on an abnormal CAG repeat, the count can increase or decrease, usually by one to three repeats, with a slight tendency to go up, and with a greater tendency for increases in CAG count when the gene is passed on by males, Dr. Gusella stated.

However, because of the action of modifier genes and the larger overall variation in onset, any attempt to “to predict onset from relatives” could “easily be wrong.”

So, Dr. Gusella asked, if such findings cannot directly inform individuals and their families, what are they good for?

Researchers can seek to investigate the “mechanism” by which the modifiers affect the “disease process” and then, based on that knowledge, design treatments to influence that process “in a much, much stronger fashion” than any of the modifiers does individually.

“Imagine if we had a drug that could delay onset of motor symptoms by 40 years!” Dr. Pacifici exclaimed, commenting on the discovery of the modifier genes. “My gosh, that would be fantastic. Nature’s kind of done that experiment for us. It’s told us that it is possible to modulate the disease.”

A slide from Dr. Gusella's presentation illustrating how age of disease onset is influenced by modifier genes, as shown in the different curves

The defective protein

Another key finding of the GeM-HD studies has also changed standard thinking in the HD field. This discovery involves the protein made by the huntingtin gene, also called huntingtin.

Each 3-letter “word” in the DNA encodes an amino acid to put into the protein the cell is making.  There are 20 different amino acids; proteins are made of long chains of hundreds or thousands of amino acids, which are then folded, linked, or otherwise modified to create the final product. Dr. Gusella described proteins as the “workers in the cell.” Cells are assisted in this process by RNA, which acts as a messenger to carry instructions from the DNA in the making of proteins.

In the case of huntingtin, there is a particular location in the gene where the word CAG appears many times in a row, as noted above. This leads to the creation of a protein that includes the amino acid glutamine many times in a row.

Since the discovery of the gene, scientists have assumed that HD onset occurred because of too many glutamines in the protein, supposedly resulting in cumulative damage to brain cells by the faulty protein, Dr. Gusella observed.

“This assumption is actually not correct,” he reported.

The gene drives onset

The GeM-HD researchers found that, after the string of CAG repeats in the gene, there is usually the “word” CAA and then another CAG, Dr. Gusella explained. The DNA “words” CAG and CAA both mean “glutamine” to the cell’s protein-making apparatus.  

“The vast, vast majority of Huntington’s disease individuals have that structure,” he continued.

However, in less than one percent of people with HD, there is no extra CAA-CAG – or there are two CAA-CAG combinations.

These genetic differences affect the measurement of the CAG count, making the actual section of the gene shorter or longer than the laboratory would measure using usual test methods, Dr. Gusella explained. Detecting these very small variations in DNA sequence in a small number of patients is difficult and costly. Also, as with modifier genes, getting tested for these differences would not benefit HD patients in any way, he added.

However, these uncommon variants in the DNA sequence permitted researchers to do something very important: to distinguish the effect of the CAG from the effect of the glutamine.

“It’s not glutamine that’s driving the time of onset,” Dr. Gusella explained. “It’s some property of the CAG repeat itself, some property of the DNA where the consecutive CAG that’s not interrupted by anything is determining roughly the time of onset.”

Here is an example: a typical person with HD might have a huntingtin gene with 42 CAGs followed by a CAA and another CAG. Because both CAA and CAG lead to glutamine, the gene test would say that he had 44 CAG repeats, and his huntingtin protein would have 44 glutamines in a row. But the testing in Dr. Gusella’s laboratory would show that there were only 42 CAG repeats before the CAA “interruption.” Another person might have 44 CAG repeats without a CAA interruption. Her gene test would show that she has 44 CAG repeats, the special test would show 44 repeats, and the protein would have 44 glutamines in a row. The first patient, however, who has a smaller actual number of CAG repeats before the interruption, would have a later onset age than the second patient.

This finding “makes a big difference for how you think about the disease and how you might go about trying to intervene in it,” Dr. Gusella concluded.

Dr. Gusella with long-time collaborator Marcy MacDonald, Ph.D., a member of the GeM-HD team (HSDA photo)

The CAG can expand over time

Another “special property” of the expanded CAG repeat is that the longer it starts out, the more likely it is to increase in size over time, Dr. Gusella said.

According to Dr. Pacifici, this so-called somatic expansion could be related to the appearance of symptoms. In this theory, brain cell damage and death occurs as CAG repeat lengths within the cell increase from 40-50 to 100 or more.

Several of the 23 modifier genes identified by the GeM-HD team appear to influence somatic expansion of the CAG; some modifiers seem to make it go faster, leading to early symptom onset, while others seem to slow somatic expansion, leading to a later onset of symptoms.

Onset (start of the disease) is different from progression (how the disease worsens over time).

Dr. Gusella cautiously answered a question from a webinar participant about whether a later onset could slow or hasten “progression” of the disease. He observed that the HD field has not yet established a clear definition of progression, with much debate on the matter. Clearly, as the GeM-HD data demonstrate, there’s a “lesser influence” of the CAG count on the changes in symptoms “than there was on getting there in the first place, of starting to have them.”

Implications for potential treatments

Taken together, the GeM-HD findings have helped to specify – over a large number of people – a number of genetic factors determining HD onset, and to show that it’s not a “cumulative damage as a result of the huntingtin protein,” Dr. Gusella summarized.

“The mechanism of toxicity is uncertain – it might involve huntingtin protein or might act by another mechanism involving the DNA or RNA of the HD gene,” he said.

The search for other modifier genes continues in the quest to clarify how the cells are being harmed, he said. Researchers are also examining how rapidly certain measures of health change before onset, how the disease changes after onset, and the differences in how the disease develops in people with very similar CAG length.

Dr. Gusella addressed the potential implications of the GeM-HD research for clinical trials in progress that seek actually to reduce the amount of the huntingtin protein in brain cells. Run by Roche, the first of these so-called huntingtin-lowering trials, GENERATION HD1, entered a critical and final Phase 3 in early 2019 (click here for the latest update on the trial).

“Those therapies are being applied at a point in time where you’re right around onset or after onset, which means that the expansion of the repeat that is leading to damage has gotten to the point where enough cells are damaged that you are close to or showing symptoms,” Dr. Gusella said. “If you now knock down the huntingtin [protein], if the huntingtin is the mechanism by which the expanded repeat ultimately kills the cell, then it should work. If it’s the RNA, it may work, depending on what the effect of the treatment is on the RNA level.”

However, Dr. Gusella emphasized that the GeM-HD findings do not address when a huntingtin lowering therapy should be given, or whether or how they work.

“I certainly hope that it does,” he added.

Other paths to drugs

Dr. Gusella addressed other ways in which the new understanding of HD genetics might help in the search for treatments. One possibility would be to interfere with the characteristic of the CAG repeat that is seen as driving onset, he said. Another approach could involve the modifiers engaged in DNA maintenance and repair – by manipulating them with drugs, suppressing them, or by activating them.

Yet another way would be to block the somatic expansion of the huntingtin gene, Dr. Gusella continued. Researchers could also use the new techniques developed for manipulating DNA and perhaps even change the number of repeats. Also, huntingtin-lowering drugs (if and when they are developed) could be used in combination with as yet undiscovered modifiers, he said.

Would more genetic information be helpful?

In addition to the Dr. Gusella’s 2019 webinar – his first such presentation for HDSA – I’ve also watched talks at scientific conferences by him, his long-time collaborator Marcy MacDonald, Ph.D., and Jong-Min Lee, Ph.D. According to Dr. Gusella, Dr. Lee “in particular has helped drive these studies.”

People in the HD community often speculate as to what “triggers” the disease. The GeM-HD research provides a partial but important answer with its discovery of modifier genes and other genetic factors that influence the age of onset.

Dr. Jong-Min Lee at the 2015 HD Therapeutics Conference (photo by Gene Veritas)

For many years, I have speculated about my age of onset, almost always referencing my mother’s situation. However, as the GeM-HD research now shows, that is not very helpful because of the great variation in age of onset.

Thus, as I’ve watched the research progress, I have wondered: could one or more modifier genes inherited from my parents have acted to delay my HD onset well beyond my mother’s?

I’ve also thought about somatic expansion: perhaps my mother’s 40 CAG repeats expanded to a much higher number more quickly than mine. Perhaps the other genetic factors outlined by Dr. Gusella have had an impact.

For now, at least, I can’t be tested for the modifier genes or these other factors. As Dr. Gusella indicated, even if I could, it’s not clear how predictive they would be, nor how helpful such knowledge would be.

From 1995 to 2000, my family went through three CAG tests: my mother’s, mine, and our daughter’s. Luckily, our daughter tested negative in the womb, but my wife and I waited for three agonizing months to learn her status.

After those difficult experiences, would I really want to go through more tests? If I could know my genetics to a more precise level, including moment of onset and how the disease would develop, would I really want such information?

Because of the lack of an effective treatment, most at-risk untested individuals decline testing for the CAG count. As Gene Veritas – the person who wanted to know the “truth in his genes” – I’m an outlier.

However, I cannot predict my feelings about further genetic testing until actually facing that possibility. I would only know at the moment they became available.

HD in the vanguard, but still highly complex

A decision to get tested again and my feelings about it would also depend on the availability of effective treatments. With the potential success of the Roche drug and others, doctors and HD clinics are preparing for the likely boom in testing for the CAG mutation, as people seek to learn their status before taking a drug.

As Dr. Gusella pointed out, HD stands in the vanguard of the attempt to apply protein-lowering and other cutting-edge techniques because, unlike the other major neurological disorders, it is monogenetic: it has a single genetic cause.

The critical GeM-HD discoveries could perhaps bolster the effectiveness of these other approaches or even result in unique medicines.

However, the new genetic research also underscores another reality of HD. Despite its monogenetic status, it is complex and features subtle genetic nuances. Huge challenges remain in developing treatments.

For HD-impacted individuals and their families, in the near term much will remain a mystery.

(For further background on the GeM-HD research, click here for the 2019 CHDI presentation “Genetic Modifiers” by Dr. MacDonald. Click here for the 2015 CHDI presentation by Dr. Lee.)