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.
