Characterization of a de novo Supernumerary Neocentric Ring Chromosome Derived from Chromosome 7

Supernumerary ring chromosomes (SRC) are usually derived from regions adjacent to the centromere. Their identification may be challenging, particularly in case of low mosaicism. Here, we report on a patient who was referred for major in utero growth retardation, severe developmental delay, facial dysmorphism, cleft palate, and hypospadias. The karyotype showed a small SRC in mosaic. The combination of FISH, M-FISH and array-CGH was necessary for a complete characterization of this SRC. M-FISH revealed that the SRC originated from chromosome 7. Array-CGH performed with a 400K oligonucleotide array showed a gain in region 7q22.1q31.1 present in low mosaic. This result was confirmed by FISH using BAC probes specific for chromosome 7. The SRC was a neocentric ring derived from 7q22.1q31.1 and was found in only 8% of the cells. This is the first patient carrying a mosaic neocentric SRC derived from the long arm of chromosome 7. Our study emphasizes the need to combine different techniques and to use adapted bioinformatic tools for low-mosaicism marker identification. It also contributes to the delineation of the partial trisomy 7q phenotype.