ARTICLE IN BRIEF
Investigators identified mutations in two genes that disrupt ubiquitination and result in a rare neurodegenerative syndrome characterized by reproductive failure, cerebellar ataxia, and dementia.
Using exome sequencing, researchers have identified a recessive genetic mutation in a previously unrecognized gene which, working alone or in concert with at least one other gene, disrupts ubiquitination and results in a rare neurodegenerative syndrome characterized by reproductive failure, cerebellar ataxia, and dementia.
Mutations in one or both of two genes were found in affected members of eight unrelated families, and genetic on-off modeling in zebrafish triggered or rescued similar neuropathologies, according to a study published May 23 in the New England Journal of Medicine.
It remains unclear how, why, or which of the two involved genes contributes — and to what degree — to the disorder, noted co-senior author Nicholas Katsanis, PhD, who directs Duke University's Center for Human Disease Modeling in Durham, NC.
Both of the mutated genes are involved in ubiquitination. Ubiquitin is a small protein found in almost all tissues and, like a traffic controller, it directs proteins within cells and flags unwanted proteins for destruction or recycling. One of them, ring finger protein 216, or RNF216, codes for an enzyme that attaches ubiquitin to proteins; the other, OTU domain containing 4, or OTUD4, codes for a protein that removes ubiquitin.
The researchers first perfomed whole-exome sequencing on a single patient with ataxia, hypogonadotropic hypogonadism, and dementia, from a family with both affected and unaffected members. After identifying the recessive mutations in that patient, they searched only for these defects in 12 members of the other families.
The study also involved investigators at Massachusetts General Hospital (MGH) in Boston, led by co-senior author Stephanie Seminara, MD, an assistant professor of medicine at Harvard Medical School and member of MGH's Reproductive Endocrine Unit.
According to Dr. Seminara, the study highlights for the first time the importance of the ubiquitin system in this syndrome — reproductive failure due to abnormal signaling from the brain or pituitary gland. It also demonstrates how combining genomics with detailed functional assays can unlock complex genetic architecture, she said.
Cerebellar ataxia is caused by lesions in the part of the brain responsible for coordination and balance. Genes linked to other ataxia syndromes have already been identified, but none have previously been associated with the rare combination of ataxia and reproductive failure.
“This syndrome was first described more than a century ago, but it has taken genetic sequencing technology and other analytical advances for us to get this first glimpse of what is occuring in affected individuals and families,” Dr. Katsanis told Neurology Today in a telephone interview.
“This highlights a very fundamental biological feature in a complicated disease process, and represents a very interesting paradigm in the deregulation of ubiquitin, which we are aware of in other diseases that also affect the brain, including Parkinson's disease,” he said.
Henry Houlden, PhD, professor of neurology and molecular neuroscience at the University College of London's Neurological Institute, also studies the genetics of cerebellar ataxias. “This is an interesting finding in a very rare syndrome, but the important question is whether [these mutations] also cause other types of ataxia or dementias,” he commented.
PROBING GENETIC CAUSALITY
Exome sequencing of the primary patient revealed rare variants in both copies of 13 genes, two of which were also found in samples from the patient's two affected siblings, but not in unaffected family members.
The researchers then sequenced both of these proteins in samples from nine additional affected individuals from seven other families. One patient had two different RNF216 mutations, while four other subjects — two in the same family — had mutations in a single copy of that gene, but none had mutated versions of OTUD4.
All of the individuals with RNF216 mutations had similar medical histories, characterized by a lack of normal hormonal secretion, progressive ataxia and dementia; and all of those with mutations in both genes died in their 30s or 40s.
Neuroimaging revealed similar brain abnormalities in individuals with RNF216 mutations — including atrophy of the cerebellum and cortex. The four patients without RNF216 mutations had very different histories, with less severe symptoms.
Nonetheless, all developed progressive ataxia and dementia, and brain autopsies showed neuronal loss in cerebellar pathways and in the hippocampus, while surviving hippocampal neurons contained ubiquitin-immunoreactive intranuclear inclusions. Defects were also found at the hypothalamic and pituitary levels of the reproductive endocrine axis.
“Despite a historical dichotomy between monogenic and complex traits, there exists a continuum of genetic causality, whereby mutations at a discrete number of loci cooperate to either cause the disease or modify its onset and severity,” said Dr. Katsanis.
“I think these findings show that we need to be as unbiased as possible with whole-exome data. In the past, when one genetic defect has been discovered it has been assumed to be responsible for a disorder, but what we have found is that there may be others involved that might be overlooked,” Dr. Katsanis told Neurology Today.
“My prediction is that we will find more and more conditions with similar segregation of genes, with an affecting driver and compounding influences by others,” he said.
Although exactly how these mutations lead to the symptoms seen in these individuals is unknown, the researchers noted that identifying these genes may someday lead to therapies — potentially including drugs currently being developed for other disorders involving ubiquitination, including Parkinson's disease — and enable genetic screening and counseling for affected families. They also hope to investigate whether less severe mutations in these genes may contribute to the presence of ataxia, dementia, or hypogonadism alone.
“This piggy-backing of biology and genomics is really just starting,” said Dr. Katsanis. “The next steps will involve investigating the collaboration of additional genes and trying to understand their influence on ubiquitination.”
The technology that made the study possible has only emerged within the past decade, said Brent L. Fogel, MD, PhD, assistant professor in the department of neurology program in neurogenetics at the David Geffen School of Medicine of the University of California, Los Angeles.
“The key step is the use of next-generation exome sequencing from the large affected family to identify potential disease genes. This is the future of clinical neurogenetics,” he told Neurology Today.
Whole-exome sequencing is becoming more mainstream, Dr. Fogel noted. The cost of exome sequencing is currently at around $5,000, with results usually available within a couple of months, similar to many individual gene tests already in use.
“Bioinformatics is still the main roadblock. Analysis can be very time-consuming, but with continuing improvements, the tests are going to start moving along more quickly.”
The researchers had “an ideal scenario” in the current study, he added. “They had a large consanguineous family to look for a recessive mutation, could also search in other families with similar phenotypes for confirmation, and then test their findings in an animal model. This is a good example of how to approach identifying a novel genetic disease.”
But then again, Dr. Fogel noted that it is still a challenge for such approaches to find novel genes in small families or individuals, dominantly inherited neurological conditions, or complex diseases involving multiple genes.
“What I find most encouraging is that physicians are realizing the value of exome testing and how this might be of great help to their patients, so more clinicians are paying close attention to studies like this.”