Mosaic trisomy 1 syndrome

Mosaic trisomy 1 syndrome (Orphanet code 1692) is an extremely rare chromosomal disorder in which some cells of the body contain three copies of chromosome 1 instead of the usual two, while other cells have the normal two copies. Full (non-mosaic) trisomy 1 is considered lethal and incompatible with life, typically resulting in very early miscarriage. However, in the mosaic form, the presence of a normal cell line alongside the trisomic cells allows survival, though affected individuals present with a range of congenital anomalies and developmental problems. The clinical features of mosaic trisomy 1 are highly variable and depend on the proportion and tissue distribution of trisomic cells. Reported findings include intrauterine growth restriction, psychomotor developmental delay, intellectual disability, craniofacial dysmorphism (such as asymmetric facial features, low-set ears, and micrognathia), skeletal anomalies, congenital heart defects, and various organ malformations. Body asymmetry and skin pigmentary anomalies following the lines of Blaschko may also be observed, reflecting the mosaic distribution of abnormal cells. Due to the extreme rarity of this condition, with only a handful of cases described in the medical literature, there is no established specific treatment. Management is supportive and symptomatic, tailored to the individual's particular clinical manifestations. This may include cardiac surgery for heart defects, physical and occupational therapy for developmental delays, and regular monitoring by a multidisciplinary team including geneticists, cardiologists, and neurologists. Genetic counseling is recommended for affected families, though the condition arises sporadically as a post-zygotic event and recurrence risk is considered very low.

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