Tetrasomy 18p in one non-identical twin born to healthy parents: A case report

Tetrasomy 18p is an extremely rare disorder with estimated incidence 1:140,000-180,000. The most common clinical presentations include developmental delay, intellectual disability, and several dysmorphic features such as the triangular face, low-set ears, depressed nasal bridge or smooth philtrum. The disease's hallmark is a small supernumerary isochromosome 18p consisting of two copies of the same arm of chromosome 18. Males and females are affected equally. Here, we present the case of 20 months old girl with tetrasomy 18p, the only affected child out of the dizygotic twins born to healthy parents. Routine cytogenetic testing revealed the presence of the marker chromosome in the girl's sample. Combination of Multicolor Banding fluorescent in situ hybridization and array-based Comparative Genomic Hybridization confirmed the isochromosome 18p in this girl. Quantitative Fluorescence PCR determined the maternal origin of the isochromosome 18p. The presented case of tetrasomy18p in one of the non-identical twins supports the hypothesis that tetrasomy 18p arises de novo and are of maternal origin. We conclude that the formation of isochromosome 18p is a random event that can occur in pregnancies of cytogenetically normal parents and maternal age is probably the main risk factor in etiology of the tetrasomy 18p.