Mosaic trisomy 16 syndrome

Mosaic trisomy 16 syndrome (Orphanet code 1708) is a rare chromosomal disorder in which some cells in the body contain three copies of chromosome 16 instead of the usual two, while other cells have the normal two copies. Full (non-mosaic) trisomy 16 is the most common autosomal trisomy found in first-trimester miscarriages and is uniformly lethal; however, the mosaic form is compatible with survival, though it is associated with significant clinical consequences. The condition arises as a sporadic event during early embryonic cell division and is not typically inherited from parents. Mosaic trisomy 16 affects multiple body systems. Key clinical features include intrauterine growth restriction (IUGR), congenital heart defects (particularly ventricular septal defects and atrial septal defects), craniofacial anomalies, musculoskeletal abnormalities, and developmental delay. Affected individuals may also present with asymmetric growth, minor facial dysmorphisms (such as a flat nasal bridge, low-set ears, and micrognathia), and genitourinary anomalies. The degree of clinical severity varies widely depending on the proportion and distribution of trisomic cells across different tissues. Some individuals may be mildly affected, while others can have significant organ involvement. There is no cure or specific targeted therapy for mosaic trisomy 16 syndrome. Management is supportive and symptom-based, involving a multidisciplinary team that may include cardiologists, developmental pediatricians, orthopedic specialists, and other subspecialists depending on the organs affected. Early intervention programs for developmental delays, surgical correction of congenital heart defects when indicated, and regular monitoring of growth and development are cornerstones of care. Genetic counseling is recommended for affected families, though recurrence risk is generally considered to be very low.

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