Metabolic Dysfunction-Associated Steatotic Liver Disease in a Patient with Phelan–McDermid Syndrome

Background: Phelan–McDermid syndrome (PMS), caused by SHANK3 variants or 22q13.3 deletions, often includes systemic features such as gastrointestinal and hepatic abnormalities. This study highlights the overlap between PMS and metabolic-associated steatotic liver disease (MASLD), focusing on PNPLA3 variants and underscoring the need for structured metabolic monitoring. Methods: We describe a 25-year-old male with PMS due to a 22q13.33 microdeletion involving SHANK3. He exhibited developmental delay, seizures, and hypotonia. Genetic testing revealed homozygosity for the PNPLA3 p.I148M variant. Clinical, anthropometric, biochemical, imaging, and metabolic investigations were performed, including liver ultrasound and metabolic profiling of lymphoblastoid cell lines. Results: Ultrasound revealed moderate hepatic steatosis consistent with MASLD. After ursodeoxycholic acid treatment and a Mediterranean-style diet, steatosis improved to mild. Metabolic profiling demonstrated increased nicotinamide adenine dinucleotide generation under metabolic stimuli, suggesting altered energy homeostasis. Conclusions: We highlight the contribution of PNPLA3 to MASLD in PMS and support systematic hepatic monitoring. Genotype–phenotype associations in PMS may provide insights relevant to MASLD research and clinical management.