Accessory small chromosomes in amniotic fluid cells

Mosaic trisomy of a small r(1) with an abnormal phenotype

Cytogenetic studies of a mildly dysmorphic 10-year-old male with mild developmental delay and learning difficulties revealed mosaicism for a supernumerary ring chromosome in approximately half of the cells. The karyotype of this patient was established as 47,XY,+r[15]/46,XY[15].ish r(1)(D1Z7+,wcp1-). Although the presence of euchromatic material was shown by C banding, the lack of hybridization with the whole chromosome paint 1 (wcp1) probe suggests that few unique sequences are contained in the ring and that these sequences likely explain the child's dysmorphic features and developmental delay. A review of the literature, including the present case, suggests that the significance of euchromatin in supernumerary r(1) as determined by both C banding and fluorescence in situ hybridization (FISH) with chromosome 1 painting probes can be used as a prognostic indicator for potential severity of the clinical phenotype.

Supernumerary microchromosomes identified as inverted duplications of chromosome 15: a report of three cases

Supernumerary bisatellited microchromosomes detected in three unrelated patients were identified as inverted duplications of chromosome 15. Each of these chromosomes contained a small euchromatic interstitial band presumably derived from the proximal portion of region 15q1. The clinical significance of this material was difficult to assess. Two of our cases were ascertained as the result of routine amniotic fluid studies. One of the affected fetuses had an unusual form of mosaicism 46,XY/48,XY, + inv dup(15), + inv dup(15), but no apparent developmental abnormalities. The inv dup (15) of the second fetus was familial in origin; no phenotypic abnormalities or evidence of mosaicism were detected in the carrier parent. The third inv dup(15) was found in a 20.5-month-old boy referred for developmental retardation. The clinical findings in this case were similar to those seen in patients with large inv dup(15)'s and did not suggest Prader-Willi syndrome.

Familial supernumerary microchromosome mosaicism: phenotypic effects and an attempt at characterization

A healthy normally intelligent man and his two daughters who had mild developmental delay and different malformations were determined to have an acrocentric supernumerary microchromosome (MC). The father and both daughters were noted to be mosaic for the MC, perhaps suggesting mitotic instability of the MC cell line with loss over time. The MC was negative staining on G-banding and showed absence of centromeric heterochromatin on C-banding. A centromere was identified with silver staining; no NORs were identified. The MC approximated the size of the short arm of the Y chromosome and showed cytologic behavior similar to that of a Y chromosome. H-Y antigen and Y-DNA gene probe studies failed to confirm a Yp origin.

Properties and significance of a small marker chromosome in amniotic fluid cells

We report the presence of a small supernumerary metacentric chromosome in an amniotic fluid cell culture. The extra chromosome was inherited from the mosaic mother, and was characterized by the presence of four satellite regions, two on each side of the centromere. The pregnancy proceeded to term, and our findings were confirmed in the clinically normal offspring. The problems posed by the presence of the marker chromosome, in terms of both cytogenetic diagnosis and genetic counseling, are discussed in the light of similar cases from the literature.