Trisomy first, translocation second, uniparental disomy and partial trisomy third: a new mechanism for complex chromosomal aneuploidy.

A 2-year-old, short, microcephalic and developmentally retarded boy revealed a pattern of multiple minor anomalies, hypospadias and a dysplastic right kidney. Maternal age at delivery was 41 years. His karyotype showed two cell lines, one apparently normal, the other with a 1p+ chromosome. FISH examinations showed that the segment attached to 1p was from chromosome 16, and molecular investigations disclosed maternal heterodisomy 16, except for the segment (16)(pter-->p13.1) for which there was mosaicism between trisomy and uniparental disomy (UPD). Most likely, the zygote was trisomic for chromosome 16 due to a maternal meiosis I nondisjunction; a somatic rearrangement would have then occurred at an early postzygotic stage whereby a segment of the paternal chromosome 16 was translocated onto 1p. Subsequently, the paternal chromosomes 16 and 16p- had been lost in the normal and the translocation cell line, respectively. The chromosome aberration was detected secondary to the disclosure of maternal UPD 16 because of the demonstration of a paternal band at several loci on distal 16p. This case shows that chromosome aberrations may be formed in a more complicated manner than primarily assumed. Hence, the phenotype might also be due to underlying factors such as UPD or undetected mosaicism in addition to the more obvious implications of the chromosome rearrangement itself (e.g. partial trisomy).