A landmark study spearheaded by researchers at the Seaver Autism Center for Research and Treatment at Mount Sinai has fundamentally shifted the scientific understanding of Phelan-McDermid syndrome (PMS). Published in the journal Autism Research, the investigation suggests that the rare genetic condition, characterized by a deletion or mutation of the SHANK3 gene on chromosome 22, is far more common than historical medical literature had indicated. By synthesizing data from nearly 180,000 individuals who underwent genetic testing for autism spectrum disorder (ASD), researchers have calculated a prevalence rate of 13.7 cases per 100,000 people. This statistical recalibration suggests that more than 45,000 people in the United States alone may be living with the condition, many of whom remain undiagnosed.
Understanding the Genetic Basis of PMS
Phelan-McDermid syndrome, historically known as 22q13.3 deletion syndrome, is a neurodevelopmental disorder that manifests through a complex array of intellectual, behavioral, and medical challenges. At its biological core is the SHANK3 gene, which plays a critical role in the development and function of synapses—the junctions between nerve cells in the brain. When this gene is missing or mutated, the synaptic scaffolding necessary for healthy communication between neurons is compromised.
The clinical presentation of PMS is broad. It often includes moderate to severe intellectual disability, delayed or absent speech, hypotonia (low muscle tone), and a high likelihood of meeting the diagnostic criteria for autism spectrum disorder. Current data suggests that SHANK3-related changes account for approximately one percent of all ASD cases globally. However, because the phenotypic expression of the syndrome is highly variable, many individuals have historically been categorized under the broader, umbrella diagnosis of "autism" or "developmental delay" without ever receiving the specific genetic testing required to identify the underlying SHANK3 deficiency.
Methodology and Data Synthesis
The research team at Mount Sinai conducted what is arguably the most comprehensive prevalence study of PMS to date. To arrive at their findings, they collaborated with a vast network of genetic testing laboratories, academic medical centers, and prominent research consortiums. The data pool included contributions from GeneDx, Labcorp, Ambry Genetics, the SPARK research study, and the Autism Sequencing Consortium, alongside patient records from several major children’s hospitals.
The researchers employed a rigorous methodology to ensure the accuracy of their estimate. They accounted for systemic variables, including the prevalence of undiagnosed cases, the technical limitations inherent in certain genetic testing panels, and the subset of the PMS population that does not present with the traditional features of autism. By integrating information from ten separate sources, the team was able to bridge the gap between documented cases and the estimated total population, revealing a significant discrepancy that highlights a crisis of under-diagnosis in pediatric and adult neurology.
The Diagnostic Gap: A Barrier to Care
The study’s first author, Tess Levy, MSc, an Assistant Professor of Psychiatry at the Icahn School of Medicine at Mount Sinai and a certified genetic counselor at the Seaver Autism Center, emphasized that the prevalence of PMS is likely obscured by systemic barriers to healthcare.
"The large gap between known and estimated cases is likely due in large part to the fact that many individuals with developmental disabilities and autism are never offered genetic testing," Levy noted. "Families may also face insurance barriers or may receive tests that do not adequately evaluate the SHANK3 gene."
This diagnostic latency has profound implications. Without a specific genetic diagnosis, patients often fail to receive the specialized medical surveillance they require. Individuals with PMS are at increased risk for seizures, renal abnormalities, and gastrointestinal issues, which may go unmanaged if the clinician is unaware of the patient’s underlying genetic profile. Furthermore, the absence of a confirmed diagnosis limits a family’s ability to join patient advocacy networks or access clinical trials specifically designed for SHANK3-related disorders.
Clinical Trials and the Precision Medicine Shift
The timing of this research coincides with a pivotal era in precision medicine. For decades, the management of PMS was limited to symptomatic treatment—addressing individual symptoms like epilepsy or behavioral challenges as they arose. Today, the landscape is shifting toward disease-modifying therapies.
Joseph D. Buxbaum, PhD, Director of the Seaver Autism Center and senior author of the study, underscores the urgency of universal screening. "We recommend that every child with autism undergo genetic testing, because knowledge is power," Buxbaum stated. "These genetic findings allow researchers to design more targeted clinical trials for potential therapies. I truly believe that within the next five years, we’ll see successful examples of new treatments coming from these genetic discoveries."
This shift toward precision medicine is supported by organizations such as CureSHANK and Neuren Pharmaceuticals, both of which provided funding and logistical support for the study. The industry interest in PMS is growing, as the clear genetic mechanism (the SHANK3 mutation) provides a concrete target for drug development.
The Ethical Imperative of Identification
Beyond the scientific benefits of research, there is an increasing consensus regarding the ethical necessity of diagnosis. Rachel Groth, PhD, Head of External Innovation and Patient Advocacy at Neuren Pharmaceuticals, pointed out that the inability to identify patients is no longer just a medical oversight—it is a barrier to equity in healthcare.
"Neuren Pharmaceuticals initiated this landmark PMS prevalence study because, with new treatments moving closer to reality, identifying these individuals has become an ethical imperative," Groth said. "Patients cannot benefit from these advances if they never receive a diagnosis."
This sentiment is shared by advocacy groups who have long sought to raise awareness among primary care physicians. Geraldine Bliss, Board Chair of CureSHANK, remarked that the study validates the lived experiences of thousands of families who have spent years navigating the medical system without clear answers. "There are likely tens of thousands of individuals with Phelan-McDermid syndrome who have never received a genetic diagnosis," Bliss said. "At a time when multiple therapeutics are advancing into clinical trials, finding these individuals has never been more important."
Future Implications and Global Awareness
The findings from Mount Sinai are fueling a broader push for systemic change in how clinicians approach patients with neurodevelopmental differences. CureSHANK is currently leveraging this data to support the "Start Genetic" campaign, a global initiative that encourages families, healthcare providers, and diagnostic labs to prioritize genetic testing as a first-line clinical procedure rather than a secondary consideration.
The implications of this study extend beyond Phelan-McDermid syndrome. By demonstrating that a rare disorder is significantly more common than previously assumed, the researchers have provided a blueprint for analyzing other rare genetic conditions. The methodology used to identify these 45,000 "hidden" patients serves as a model for future studies aimed at quantifying the prevalence of other rare, under-diagnosed genetic variants.
As medical technology advances, the threshold for genetic sequencing continues to drop, making universal screening for children with autism and developmental delays more feasible than ever before. However, the study concludes that technological capability alone is insufficient. Real-world progress requires a fundamental change in clinical practice—one that views genetic testing as a standard of care rather than a niche, last-resort option. For the tens of thousands of individuals currently living in the "diagnostic shadow," the implications of this research are clear: identifying the cause is the first, and most critical, step toward securing a future of effective, targeted, and life-changing care.
