Supplementary Material for: Somatic Mutations Are Not Observed by Exome Sequencing of Lymphocyte DNA from Monozygotic Twins Discordant for Congenital Hypothyroidism due to Thyroid Dysgenesis

Background/Aims: Congenital primary hypothyroidism (CH) is a rare pediatric disorder estimated to occur in about 1:2,500 live births. Approximately half of these cases entail ectopic thyroid tissue, which is believed to result from a migration defect during embryogenesis. Approximately 3% of CH cases are explained by mutation(s) in known genes, most of which are transcription factors implicated in the embryology of the thyroid gland. Surprisingly, monozygotic (MZ) twins are usually discordant for CH due to thyroid dysgenesis, suggesting that most cases are not caused by transmitted genetic variation. One possible explanation is somatic mutation in genes involved in thyroid migration occurring after zygotic twinning. Such mutations should be observed only in the affected twin. Methods: To test the hypothesis of somatic mutation, we performed whole exome sequencing of DNA from three pairs of MZ twins discordant for CH with ectopic glands. Results: We found no somatic mutations exclusive to any of the three affected twins or in any of the unaffected twins. Conclusion: Either somatic mutations are not significant for the etiology of CH or else such mutations lie outside regions of the genome accessible by exome sequencing technology.背景与研究目的：先天性原发性甲状腺功能减退症（Congenital primary hypothyroidism, CH）是一种罕见的儿科疾病，据估计其活产儿发病率约为1:2500。该病症约半数病例存在异位甲状腺组织，该表现被认为是胚胎发育阶段甲状腺迁移缺陷所致。约3%的CH病例可通过已知基因的突变得到解释，其中大多数为参与甲状腺胚胎发育的转录因子编码基因。令人意外的是，因甲状腺发育不全导致的CH患者中，同卵双生子（monozygotic, MZ）通常患病表型不一致，这提示大多数病例并非由经遗传传递的遗传变异所致。一种可能的解释为：在合子分裂后发生的、与甲状腺迁移相关的基因体细胞突变（somatic mutation），此类突变应仅可在患病双生子体内检出。 方法：为验证体细胞突变假说，我们对3对CH患病不一致且伴异位甲状腺的同卵双生子的DNA样本开展了全外显子组测序（whole exome sequencing）。 结果：我们未在3例患病双生子或未患病双生子中发现仅存在于单一个体的体细胞突变。 结论：要么体细胞突变并非CH病因学中的关键影响因素，要么此类突变位于外显子组测序技术无法覆盖的基因组区域。