Prenatal detection and characterization of a small supernumerary marker chromosome (sSMC) derived from chromosome 22 with apparently normal phenotype.

To present prenatal findings and molecular cytogenetic characterization of a small supernumerary marker chromosome (sSMC) derived from chromosome 22 with apparently normal phenotype. An amniocentesis was performed at 15 weeks' gestation and a small marker chromosome in the female fetus of a twin pregnancy was noted. A second amniocentesis was performed at 18 weeks; G-banding analysis on amniotic cells confirmed the small marker chromosome found in the female fetus. Both parents and the male twin fetus had normal karyotypes. Spectral karyotyping (SKY), Fluorescence in situ hybridization (FISH) analyses with chromosomal specific whole chromosome painting probe (WCP 22) and alphoid satellite DNA probe (D22Z4) were used to identify the origin of the sSMC. The make-up of the sSMC was characterized by further FISH studies with chromosome region specific probes. The twin babies were delivered normally at 35 weeks' gestation. The female neonate with sSMC did not show any dysmorphic features, except for a type II atrial septum defect (ASD) at birth. She was found to be developing and growing normally at her 2-year follow-up. Conventional G-banding study confirmed the presence of a sSMC with bi-satellites. SKY and FISH with D22Z4 probes showed that the marker originated from chromosome 22. FISH studies using 4 locus-specific DNA probes in the 22q11.2 region (N25 probe to detect the D22S75 locus within the velocardiofacial syndrome/DiGeorge syndrome (VCFS/DGS) critical region, a clone to detect the Bid locus just distal to the cat eye syndrome (CES) critical region and two clones 77H2 and 109L3 to detect the proximal end of the CES critical region, (CECR2 and CECR7), did not reveal any hybridization signal with the marker chromosome. The karyotype of the fetus was 47,XX,+ mar. ish der(22) (SKY+,D22Z42 + ,CECR7-,CECR2-, BID-,D22S75-). The supernumerary marker chromosome in this case was a de novo inv dup(22)(q11.2) and contained a duplicated proximal long arm region < 400 kb from the centromere; it did not appear to affect the phenotype of the child.