August 25, 2022
Researchers have identified 72 genes very strongly linked to autism spectrum disorders and more than 250 other genes with a strong link to ASD, according to a study published in Nature Genetics. The findings, based on analysis of more than 150,000 people's genetics, arose from a collaboration of five research groups whose work included comparisons of ASD cohorts with separate cohorts of individuals with developmental delay or schizophrenia.
"We know that many genes, when mutated, contribute to autism," and this study brought together "multiple types of mutations in a wide array of samples to get a much richer sense of the genes and genetic architecture involved in autism and other neurodevelopmental conditions," co–senior author Joseph D. Buxbaum, PhD, director of the Seaver Autism Center for Research and Treatment at Mount Sinai and a professor at the Icahn School of Medicine at Mount Sinai, both in New York, said in a prepared statement. "This is significant in that we now have more insights as to the biology of the brain changes that underlie autism and more potential targets for treatment."
Glen Elliott, PhD, MD, a clinical professor of psychiatry at Stanford (Calif.) University who was not involved in the study, said the paper is important paper for informing clinicians of where the basic research is headed. "We're still in for a long road" before it bears fruit in terms of therapeutics. The value of studies like these, that investigate which genes are most associated with ASD, is that they may lead toward understanding the pathways in the brain that give rise to certain symptoms of ASD, which can then become therapeutic targets, Elliott said.
Investigating Large Cohorts
The researchers analyzed genetic exome sequencing data from 33 ASD cohorts with a total of 63,237 people and then compared these data with another cohort of people with developmental delay and a cohort of people with schizophrenia. The combined ASD cohorts included 15,036 individuals with ASD, 28,522 parents, and 5,492 unaffected siblings. The remaining participants were 5,591 people with ASD and 8,597 matched controls from case control studies.
In the ASD cohorts, the researchers identified 72 genes that were associated with ASD. De novo variants were eight times more likely in cases (4%) than in controls (0.5%). Ten genes occurred at least twice in ASD cases but never occurred in unaffected siblings.
Then the researchers integrated these ASD genetic data with a cohort of 91,605 people that included 31,058 people with developmental delay and their parents. Substantial overlap with gene mutations existed between these two cohorts: 70.1% of the genes related to developmental delay appeared linked to risk for ASD, and 86.6% of genes associated with ASD risk also had associations with developmental delay. Overall, the researchers identified 373 genes strongly associated with ASD and/or developmental delay and 664 genes with a likely association.
"Isolating genes that exert a greater effect on ASD than they do on other developmental delays has remained challenging due to the frequent comorbidity of these phenotypes," wrote lead author Jack M. Fu, of Massachusetts General Hospital and Harvard Medical School, both in Boston, and colleagues. "Still, an estimated 13.4% of the transmission and de novo association–ASD genes show little evidence for association in the developmental delay cohort."
ASD, Developmental Delay, and Schizophrenia
When the researchers compared the cells where the genetic mutations occurred in fetal brains, they found that genes associated with developmental delay more often occurred in less differentiated cell types – less mature cells in the developmental process. Gene mutations associated with ASD, on the other hand, occurred in more mature cell types, particularly in maturing excitatory neurons and related cells.
"Our results are consistent with developmental delay-predominant genes being expressed earlier in development and in less differentiated cells than ASD-predominant genes," they wrote.
The researchers also compared the specific gene mutations found in these two cohorts with a previously published set of 244 genes associated with schizophrenia. Of these, 234 genes are among those with a transmission and de novo association to ASD and/or developmental delay. Of the 72 genes linked to ASD, eight appear in the set of genes linked to schizophrenia, and 61 were associated with developmental delay, though these two subsets do not overlap each other much.
"The ASD-schizophrenia overlap was significantly enriched, while the developmental delay-schizophrenia overlap was not," they reported. "Together, these data suggest that one subset of ASD risk genes may overlap developmental delay while a different subset overlaps schizophrenia."
Chasing Therapy Targets by Backtracking Through Genes
The findings are a substantial step forward in understanding the potential genetic contribution to ASD, but they also highlight the challenges of eventually trying to use this information in a clinically meaningful way.
"Given the substantial overlap between the genes implicated in neurodevelopmental disorders writ large and those implicated directly in ASD, disentangling the relative impact of individual genes on neurodevelopment and phenotypic spectra is a daunting yet important challenge," the researchers wrote. "To identify the key neurobiological features of ASD will likely require convergence of evidence from many ASD genes and studies."
Elliott said the biggest takeaway from this study is a better understanding of how the paradigm has shifted away from finding "one gene" for autism or a cure based on genetics and more toward understanding the pathophysiology of symptoms that can point to therapies for better management of the condition.
"Basic researchers have completely changed the strategy for trying to understand the biology of major disorders," including, in this case, autism, Elliott said. "The intent is to try to find the underlying systems [in the brain] by backtracking through genes. Meanwhile, given that scientists have made substantial progress in identifying genes that have specific effects on brain development, "the hope is that will mesh with this kind of research, to begin to identify systems that might ultimately be targets for treating."
The end goal is to be able to offer targeted approaches, based on the pathways causing a symptom, which can be linked backward to a gene.
"So this is not going to offer an immediate cure – it's probably not going to offer a cure at all – but it may actually lead to much more targeted medications than we currently have for specific types of symptoms within the autism spectrum," Elliott said. "What they're trying to do, ultimately, is to say, when this system is really badly affected because of a genetic abnormality, even though that genetic abnormality is very rare, it leads to these specific kinds of symptoms. If we can find out the neuroregulators underlying that change, then that would be the target, even if that gene were not present."
The research was funded by the Simons Foundation for Autism Research Initiative, the SPARK project, the National Human Genome Research Institute Home, the National Institute of Mental Health, the National Institute of Child Health and Development, AMED, and the Beatrice and Samuel Seaver Foundation. Five authors reported financial disclosures linked to Desitin, Roche, BioMarin, BrigeBio Pharma, Illumina, Levo Therapeutics, and Microsoft.
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