Phelan-McDermid Syndrome Prevalence Study Reveals Significant Underdiagnosis and New Therapeutic Urgency

New research led by scientists at the Seaver Autism Center for Research and Treatment at Mount Sinai suggests that Phelan-McDermid syndrome (PMS) may be much more common than earlier estimates indicated. The findings, published in the journal Autism Research, estimate that the condition affects roughly 1 in 7,300 people, a significant upward revision that indicates the genetic disorder is far more prevalent in the general population than clinical records previously suggested.

Phelan-McDermid syndrome, a rare genetic disorder caused by a deletion or mutation involving the SHANK3 gene on chromosome 22, presents a multifaceted clinical profile. Individuals affected by the syndrome often navigate a broad spectrum of medical, intellectual, and behavioral challenges. Because SHANK3 is critical for synaptic function and neuronal communication, its disruption is a known contributor to neurodevelopmental variations. Clinical data confirms that the majority of individuals with PMS meet the diagnostic criteria for autism spectrum disorder (ASD), and genetic variations affecting SHANK3 are believed to account for as many as one percent of all ASD cases globally.

Methodology and the Scope of Genetic Data

To arrive at these refined prevalence estimates, the Mount Sinai research team undertook a massive data-aggregation project. Recognizing that traditional clinical counts were likely failing to capture the true scale of the syndrome, researchers collaborated with a consortium of genetic testing laboratories, academic medical centers, and prominent autism research programs.

The study analyzed genomic data from approximately 180,000 individuals with autism who had undergone various forms of genetic testing. By synthesizing information from ten distinct sources—including major diagnostic providers like GeneDx, Labcorp, and Ambry Genetics, alongside foundational research initiatives such as the SPARK research study and the Autism Sequencing Consortium—the researchers created a comprehensive dataset.

After performing rigorous statistical adjustments to account for undiagnosed populations, the inherent limitations of historical genetic testing technologies, and the cohort of individuals with PMS who do not exhibit the full suite of ASD symptoms, the team arrived at a prevalence estimate of 13.7 cases per 100,000 people. This translates to approximately 1 in 7,300 individuals. This statistical shift suggests that more than 45,000 people in the United States alone could be living with Phelan-McDermid syndrome, a figure that dwarfs earlier, more conservative estimates.

Addressing the Diagnostic Gap

The disparity between previous diagnostic counts and this new, higher estimate highlights a systemic failure in the current healthcare landscape. According to Tess Levy, MSc, Assistant Professor of Psychiatry at the Icahn School of Medicine at Mount Sinai and the study’s first author, the gap is not a result of a sudden spike in the condition, but rather a persistent lack of access to clinical genetic screening.

"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 explained. "Families may also face insurance barriers or may receive tests that do not adequately evaluate the SHANK3 gene, leaving the underlying cause of their loved one’s symptoms unidentified."

This barrier to entry—whether rooted in insurance reimbursement policies, a lack of clinical awareness among primary care physicians, or the high cost of comprehensive genomic sequencing—prevents thousands of families from receiving a definitive diagnosis. Without this "genetic key," patients and their families often struggle to access specialized support networks and informed medical guidance.

The Clinical Imperative: Why Genetic Testing Matters

The researchers emphasize that this is not merely a matter of academic interest; it is an urgent medical requirement. As the field of precision medicine advances, the ability to identify the specific genetic driver of a patient’s condition becomes the gateway to potential treatment.

Joseph D. Buxbaum, PhD, Director of the Seaver Autism Center and senior author of the paper, advocates for a paradigm shift in how neurodevelopmental disorders are diagnosed. "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 study, which received support from patient advocacy organizations CureSHANK and Neuren Pharmaceuticals, represents one of the most comprehensive epidemiological efforts to date. By establishing a clearer picture of the patient population, the research provides the necessary data to justify and structure larger-scale clinical trials.

The Path Toward Precision Medicine

For many years, the management of Phelan-McDermid syndrome was largely supportive, focusing on symptomatic relief through speech therapy, physical therapy, and behavioral interventions. However, the landscape is rapidly changing. Several clinical trials are now underway, investigating precision medicine approaches that target the specific biological pathways affected by the SHANK3 gene deletion.

For families, a diagnosis serves as more than a label; it is a clinical roadmap. It allows for participation in research studies, access to disease-specific patient support networks, and, perhaps most importantly, the potential to enroll in upcoming trials for disease-modifying therapies.

Rachel Groth, PhD, Head of External Innovation and Patient Advocacy at Neuren Pharmaceuticals, underscored the ethical dimensions of this research. "Neuren Pharmaceuticals initiated this landmark PMS prevalence study in collaboration with the Seaver Autism Center at Mount Sinai and CureSHANK because, with new treatments moving closer to reality, identifying these individuals has become an ethical imperative. Patients cannot benefit from these advances if they never receive a diagnosis."

Advocacy and the Future of Diagnosis

The findings of the study have been met with strong support from the advocacy community, which has long argued that the official numbers were failing to reflect the reality on the ground. Geraldine Bliss, Board Chair of CureSHANK, noted that the data validates the lived experiences of thousands of families who have navigated the healthcare system in search of answers.

"This study confirms what many families, clinicians, and advocates have suspected for years," Bliss said. "There are likely tens of thousands of individuals with Phelan-McDermid syndrome who have never received a genetic diagnosis. At a time when multiple therapeutics are advancing into clinical trials, finding these individuals has never been more important."

The results of the study are expected to bolster initiatives such as the "Start Genetic" campaign, a global effort encouraging patients, families, and healthcare providers to prioritize genetic screening early in the diagnostic process. The broader implication is clear: the promise of the genomic era in medicine—where treatments are tailored to the individual’s unique genetic makeup—is currently bottlenecked by a lack of identification.

Implications for the Healthcare System

As healthcare systems look toward the future, the integration of genetic sequencing as a standard of care for neurodevelopmental disorders faces both logistical and economic hurdles. Yet, the cost of under-diagnosis is also high. Families without a diagnosis may undergo years of "diagnostic odyssey," moving from specialist to specialist without finding a coherent explanation for their child’s condition.

By establishing a more accurate prevalence rate, the Mount Sinai study provides the necessary evidence for policymakers and insurance providers to reassess the necessity of genetic testing. If 1 in 7,300 people are affected, the economic case for early, comprehensive testing becomes increasingly robust. Early intervention based on a genetic diagnosis can reduce the burden on public healthcare systems by providing more effective, targeted therapies rather than broad-spectrum symptom management.

Furthermore, the collaboration between academic institutions, private pharmaceutical firms, and patient advocacy groups serves as a model for future research into rare diseases. By pooling data across institutional boundaries, these groups have overcome the limitations of small, isolated studies, proving that large-scale epidemiological data is reachable when data-sharing protocols are aligned.

As the scientific community moves forward, the focus will likely shift toward streamlining the diagnostic pathway. If researchers can turn the tide on under-diagnosis, the next decade may yield the first generation of patients who receive a genetic diagnosis at birth or early infancy, allowing them to access the precision therapies that are currently moving through the clinical trial pipeline. In the interim, the work led by the Seaver Autism Center stands as a critical reminder that in the realm of precision medicine, diagnosis is the essential first step toward progress.