Cytogenetic and molecular analysis of small supernumerary marker chromosomes in prenatal diagnosis
Objective To investigate the detection,formation mechanism and genetic counseling of small supernumerary marker chromosomes(sSMC)in prenatal diagnosis,so as to provide a reference for the clinical diagnosis.Methods This study retrospective analyzed the clinical data of 10938 cases from January 2015 to December 2022 who underwent prenatal diagnosis of fetal chromosome karyotyping at the prenatal diagnosis center of Shaoxing Women and Children's Health Care Hospital.Tradi-tional G banding,C banding and other techniques were used to combine with molecular detection techniques such as CMA and FISH to determine the composition,fragment size and mosaic of sSMCs.Results A total of 13 cases with sSMCs were detected,seven of which were mosaicism,and the indication for prenatal diagnosis was mainly a high risk of trisomy 21,followed by ad-vanced age.According to the structural classification of the sSMC,inverted duplication structure in 8 cases,3 were small ring structure,one was min chromosome and one with complex rearrangement,the abnormality rates were 61.5％(8/13),23.0％(3/13),7.7％(1/13)and 7.7％(1/13),respectively.Eight cases had further analyzed with CMA testing,of which 4 were pathogenic in-volving 5p12q11.1,11q23 and 22q1 1 duplication and inv dup(18)(p 11.32p1 1.21),inv dup(15)(q11.2q12),all terminated preg-nancy.One case of 15q1 1.2 duplication with unknown clinical significance and continued pregnancy.Three had no copy number variation and large segments homozygosity,and 2 cases except one pregnant woman who required induction of labor continued pregnancy.One case of sSMC was diagnosed as i(Yp)by karyotype and FISH test which both ends contained the SRY genes,and the pregnant woman choose to induce labor.Conclusion The mechanism of sSMC occurrence is complex.Traditional karyotyp-ing test combined with molecular detection techniques including CMA and FISH can clarify the character and origin of sSMC,and combined with other comprehensive factors such as proportion of mosaicism and UPD,may provides a relatively accurate assessment genetic counseling of sSMC carriers in prenatal diagnosis,in order to avoid the birth of abnormal children.
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