Prenatal diagnosis of a de novo small supernumerary marker chromosome 16

In about 0.05% of newborn children, small supernumerary marker chromosomes (sSMC) are detected. The majority of these sSMC is proven to be de novo. The clinical outcome of an sSMC is difficult to predict as they can have different phenotypic consequences because of differences in the euchromatic content of DNA, differences in the degree of mosaicism or existence of an uniparental disomy (UPD) of the chromosomes homologous to the sSMC. We report here on the prenatal detection of a small supernumerary marker chromosome from chromosome 16. Prenatal cytogenetic analysis of cultured amniocytes was performed at week 17 of gestation. In all cells analysed a male chromosome complement with a paracentric inversion of the short arm of the X chromosome was found. This finding was confirmed by FISH analysis. Furthermore, in 18 out of 25 cells a supernumerary marker chromosome was detected. CenM-FISH and subcenM-FISH revealed that the marker chromosome consists of heterochromatic and euchromatic regions of chromosome 16 (min(16)(: p11.21->q11.1:)), leading to a partial trisomy of 16p11.21->16q11.1. With microsatellite analysis a maternal uniparental disomy for chromosome 16 was proven. After 39 weeks of gestation a boy of 2960g was spontaneously delivered with an Apgar score of?/10/10. Clinical examination revealed besides a posterior plagiocephaly, no indication for malformations or abnormalities of internal organs so far. Since only few cases of prenatal diagnosed de novo supernumerary marker derived from chromosome 16 have been reported, clinical consequences of the chromosomal findings can not be definetely predicted. Thus, careful follow-up examinations are indicated. Among the cases that were published, only one case seems to have comparable breakpoints to the case we present here. For this casuistic, a 3 month old girl with macroglossia, dysmorphic features, mild gross motor delay and asymmetric lower extremities was described.