Prader-Willi syndrome is caused by the loss of expression of paternally inherited genes in the 15q11-q13 region. This condition involves genomic imprinting, a phenomenon where gene expression depends on whether a gene is inherited from the mother or the father; in this region, certain genes are normally expressed only from the paternal chromosome, and when those paternal copies are missing or silenced, PWS results. Three main molecular mechanisms have been described. A deletion of the paternal 15q11-q13 region accounts for approximately 70 percent of cases. Maternal uniparental disomy, in which a child inherits both copies of chromosome 15 from the mother and none from the father, accounts for approximately 25 percent. An imprinting defect, in which the paternal chromosome is present but the imprinting pattern is incorrectly set, accounts for approximately 2 percent. MAGEL2, located on chromosome 15, is one of the paternally expressed genes in this region; loss of expression of genes in this region, rather than a single gene, drives the syndrome. Most cases arise de novo, but recurrence risk depends on the underlying molecular mechanism: deletions and uniparental disomy generally carry a low recurrence risk, while imprinting defects can carry a substantially higher recurrence risk depending on the specific cause. For this reason, identifying the precise molecular mechanism is essential to providing accurate recurrence risk counseling. Genetic counseling is recommended for affected individuals and their families to discuss inheritance, recurrence risk, and family planning.

Diagnosis of Prader-Willi syndrome is most often suspected from the clinical picture, particularly the combination of severe neonatal hypotonia and feeding difficulties, and is confirmed through molecular testing. Methylation analysis is the recommended first-line test because it detects all three of the major molecular mechanisms (deletion, uniparental disomy, and imprinting defect) and reliably establishes the diagnosis. When methylation testing is positive, additional testing is performed to identify the specific underlying mechanism, typically using fluorescence in situ hybridization (FISH) or chromosomal microarray to detect deletions and microsatellite analysis to confirm uniparental disomy. Identifying the specific mechanism is important for accurate recurrence-risk counseling. Many of the early features of PWS, including neonatal hypotonia and feeding difficulties, can be seen in other conditions, so molecular genetic testing is essential to confirm the diagnosis and to distinguish PWS from conditions with similar presentations.

Care for Prader-Willi syndrome is structured around the changing clinical priorities at each stage of life and is most effective when delivered through a multidisciplinary team. Foundational care includes growth hormone therapy, which improves height, body composition, and aspects of cognitive and motor development and is now considered standard of care for most individuals; strict dietary management and food security measures, which are essential for managing hyperphagia and preventing obesity; behavioral interventions for food-seeking and other behavioral challenges; sex hormone replacement at the appropriate developmental stage when indicated; and sleep studies with treatment for sleep apnea, which is common. Regular surveillance is an essential part of long-term care, helping to identify potential complications before they become serious; this includes endocrine, growth, scoliosis, sleep, and behavioral monitoring. VYKAT XR (diazoxide choline) is FDA-approved for use in eligible individuals with Prader-Willi syndrome to address hyperphagia. Eligibility for any medication depends on individual factors that are best discussed with the care team. Care is most effective when coordinated across an endocrinologist, geneticist, genetic counselor, nutritionist or dietitian, behavioral psychologist, and sleep specialist, with input from physical therapy, speech-language pathology, and other specialties as needed. Patient assistance programs may be available to help cover treatment costs. Patients should discuss treatment options with their healthcare team to determine which therapies may be appropriate for their specific situation, and genetic counseling is recommended.

The course of Prader-Willi syndrome unfolds across the lifespan. With current standards of care, including early growth hormone therapy, structured food security, behavioral and psychiatric support, and access to specialty multidisciplinary teams, outcomes have improved substantially. Many individuals attend specialized educational programs, develop meaningful skills, and live in supported settings as adults. Outcomes depend on a range of factors, including the specific molecular mechanism, the timing of diagnosis and initiation of growth hormone therapy, the consistency of food security and behavioral support, the management of obesity-related complications, and access to specialty care. Obesity-related complications, sleep apnea, and behavioral and psychiatric challenges are the most important long-term medical considerations and the most amenable to proactive management. Connection with a specialty Prader-Willi center, a genetic counselor, and a patient organization is among the most reliable ways for families to stay current with rapidly evolving options.

Prader-Willi syndrome is an active research area with several ongoing clinical trials investigating new treatment approaches. Major directions include oxytocin-related therapies aimed at behavioral and social symptoms, additional medications for hyperphagia and weight management, gene reactivation strategies aimed at restoring expression of the affected paternal genes, and improved tools for predicting and tracking behavioral and metabolic outcomes. Patient organizations maintain disease registries that help researchers understand the natural history of PWS and connect families to studies for which they may be eligible. Individuals interested in clinical trials can search ClinicalTrials.gov or consult their care team about eligibility, since eligibility often depends on age, the underlying molecular mechanism, and current clinical features.