Chaiyasap Pongsathorn, Kulawonganunchai Supasak, Srichomthong Chalurmpon, Tongsima Sissades, Suphapeetiporn Kanya, Shotelersuk Vorasuk
Interdepartment of Biomedical Sciences, Faculty of Graduate School, Chulalongkorn University, Bangkok, Thailand.
Interdepartment of Biomedical Sciences, Faculty of Graduate School, Chulalongkorn University, Bangkok, Thailand; Genome Institute, National Center for Genetic Engineering and Biotechnology, Khlong Nueng, Khlong Luang, Pathum Thani, Thailand.
PLoS One. 2014 Jun 20;9(6):e100191. doi: 10.1371/journal.pone.0100191. eCollection 2014.
Congenital heart defects (CHD) occur in 40% of patients with trisomy 21, while the other 60% have a structurally normal heart. This suggests that the increased dosage of genes on chromosome 21 is a risk factor for abnormal heart development. Interaction of genes on chromosome 21 or their gene products with certain alleles of genes on other chromosomes could contribute to CHD. Here, we identified a pair of monozygotic twins with trisomy 21 but discordant for a ventricular septal defect and epilepsy. Twin-zygosity was confirmed by microsatellite genotyping. We hypothesized that some genetic differences from post-twinning mutations caused the discordant phenotypes. Thus, next generation sequencing (NGS) technologies were applied to sequence both whole genome and exome of their leukocytes. The post-analyses of the sequencing data revealed 21 putative discordant exonic variants between the twins from either genome or exome data. However, of the 15 variants chosen for validation with conventional Sanger sequencing, these candidate variants showed no differences in both twins. The fact that no discordant DNA variants were found suggests that sequence differences of DNA from leukocytes of monozygotic twins might be extremely rare. It also emphasizes the limitation of the current NGS technology in identifying causative genes for discordant phenotypes in monozygotic twins.
21三体综合征患者中40%患有先天性心脏病（CHD），而另外60%的患者心脏结构正常。这表明21号染色体上基因剂量的增加是心脏发育异常的一个危险因素。21号染色体上的基因或其基因产物与其他染色体上某些基因的等位基因相互作用可能导致CHD。在此，我们鉴定了一对患有21三体综合征的同卵双胞胎，他们在室间隔缺损和癫痫方面表现不一致。通过微卫星基因分型确认了双胞胎的同卵性。我们假设双胞胎后发生的突变导致的一些基因差异造成了不一致的表型。因此，应用下一代测序（NGS）技术对他们白细胞的全基因组和外显子组进行测序。对测序数据的后续分析从基因组或外显子组数据中揭示了双胞胎之间21个推定的不一致外显子变异。然而，在选择用传统桑格测序进行验证的15个变异中，这些候选变异在双胞胎中均未显示出差异。未发现不一致的DNA变异这一事实表明，同卵双胞胎白细胞DNA的序列差异可能极其罕见。这也强调了当前NGS技术在鉴定同卵双胞胎不一致表型的致病基因方面存在局限性。
