Mosaic trisomy 17 syndrome

Mosaic trisomy 17 syndrome (Orphanet code 1711) is an extremely rare chromosomal disorder in which some cells of the body contain three copies of chromosome 17 instead of the usual two, while other cells have the normal two copies. This mosaic pattern arises from a postzygotic (after fertilization) nondisjunction event, meaning it is not typically inherited but occurs as a sporadic event during early embryonic development. Full trisomy 17 is generally considered lethal and incompatible with life, so virtually all liveborn cases represent the mosaic form. The clinical presentation of mosaic trisomy 17 is highly variable, depending on the proportion and distribution of trisomic cells across different tissues. Reported features include prenatal and postnatal growth restriction, intellectual disability of variable severity, craniofacial dysmorphism (such as a broad or prominent forehead, low-set ears, micrognathia, and hypertelorism), congenital heart defects, skeletal anomalies, and skin abnormalities. Some patients may also exhibit limb anomalies, seizures, and genitourinary malformations. The degree of clinical severity correlates broadly with the percentage of cells carrying the extra chromosome 17 and which organ systems are affected. There is no specific cure or targeted therapy for mosaic trisomy 17 syndrome. Management is supportive and symptomatic, addressing individual clinical manifestations such as cardiac defects (which may require surgical intervention), developmental delays (managed through early intervention programs, physical therapy, occupational therapy, and speech therapy), and seizures (treated with standard antiepileptic medications). Genetic counseling is recommended for affected families, though recurrence risk is generally considered very low given the sporadic nature of the condition. Long-term prognosis varies widely depending on the severity of organ involvement.

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