Cytogenetic and molecular genetic analysis of three cases with small supernumerary marker chromosomes

Objective To delineate the origin and structure of 3 cases of small supernumerary marker chromosomes (sSMCs) through cytogenetic and molecular genetic analysis. Methods Conventional G, C and N banding were carried out to analyze the chromosomal karyotypes. Chromosomal microarray analysis (CMA) and fluorescence in situ hybridization (FISH) were used to delineate the origin and structure of the sSMCs. Results In case 1, chromosomal karyotype of peripheral blood sample was 47,XY,+mar. This de novo sSMC was a dual-satellited dicentric inverted duplicated marker chromosome, for which CMA yielded a normal result. It was predicted to not increase the risk of offspring. In case 2, the fetal chromosomal karyotype was 47,XY,+mar[17]/46,XY[33]. Chromosomal banding suggested that this de novo segment contained euchromatin, and the result of CMA was arr[hg19] 5p12q11.1(45 694 574-49 475 697) × 3. FISH showed the sSMC to be a fragment derived from 5p12 containing the HCN1 gene. Case 3 was found to have a fetal karyotype of 45,XY,-13[25]/46,XY,r(13)[18]/46,XY,-13,+mar[7]. Both parents had refused further examination. Conclusion Conventional chromosomal banding combined with molecular methods can delineate the origin and structure of the sSMCs, which can help with prediction of their pathogenicity and facilitate genetic counseling.