Trisomy 18 mosaicism in a mildly retarded boy with postnatal overgrowth

Area of Science:

• Genetics
• Pediatrics
• Developmental Biology

Background:

• Trisomy 18 mosaicism is a chromosomal abnormality where cells have an extra copy of chromosome 18 in some, but not all, cells.
• This condition can lead to a range of developmental and physical challenges.
• Previous reports on trisomy 18 mosaicism often describe normal or subnormal intelligence.

Observation:

• A 6-year-old boy presented with mild intellectual disability and nonspecific dysmorphic features.
• Clinical observations included microcephaly, micropenis with cryptorchidism, and notably, postnatal overgrowth.
• Analysis revealed trisomy 18 in 44% of peripheral lymphocytes, confirmed by fluorescent in situ hybridization.

Findings:

• The patient exhibited trisomy 18 mosaicism with a unique combination of features.
• This case is significant due to the presence of postnatal overgrowth, a rare finding in reported trisomy 18 mosaicism cases.
• The patient's intellectual disability aligns with some previous reports, but the overgrowth is atypical.

Implications:

• This case expands the phenotypic spectrum of trisomy 18 mosaicism.
• It underscores the importance of genetic testing in children with developmental delays and unusual physical characteristics.
• Further research is needed to understand the variable expressivity and long-term outcomes of trisomy 18 mosaicism, particularly concerning growth patterns.