Supplementary Material for: Case report: prenatal recurrent microcephaly and corpus callosum abnormalities in a Chinese family with novel biallelic SASS6 mutations

Introduction: Primary microcephaly (MCPH) is not an uncommon disorder with multiple etiologies. There is a growing number of MCPH-related genes discovered due to the extensive application of whole-exome sequencing (WES) in clinical and research settings. Biallelic mutations in the SASS6 gene cause an extremely rare MCPH, type 14. To date, only two families with SASS6 gene-related microcephaly have been reported. Case description: We report a case of recurrent congenital microcephaly in a Chinese family. The two affected fetuses presented with microcephaly early in the second trimester with agenesis of the corpus callosum. In the first affected fetus, trio WES detected two compound heterozygous candidate variants c.1139T>C(p.L380P) and c.1223C>G (p.T408S) in the SASS6 gene. Another affected fetus also inherited both variants, while the normal child carried neither variant through Sanger sequencing analysis. Both variants were classified as a variant of uncertain significance according to the current American College of Medical Genetics and Genomics guidelines. Conclusion: We reported novel biallelic variants in the SASS6 gene, encoding the SAS6 centriolar assembly protein, associated with prenatal onset of autosomal recessive microcephaly. We postulate that the pathomechanism of the compound heterozygous variants in close proximity could potentiate the overall coiled instability leading to the phenotypic features of our case.

引言：原发性小头畸形（Primary microcephaly, MCPH）是一种病因多样且并非罕见的疾病。随着全外显子组测序（whole-exome sequencing, WES）在临床与科研场景中的广泛应用，与MCPH相关的致病基因数量正不断增加。SASS6基因的双等位基因突变可导致极为罕见的14型原发性小头畸形（MCPH14）。截至目前，全球仅报道过2个与SASS6基因相关的小头畸形家系。
病例描述：本研究报道1个中国家系中复发性先天性小头畸形病例。该家系中的2名受累胎儿均于妊娠中期早期出现小头畸形，且伴有胼胝体发育不全。在首例受累胎儿中，三联体全外显子组测序（trio WES）检测到SASS6基因上存在2个复合杂合候选变异：c.1139T>C（p.L380P）与c.1223C>G（p.T408S）。另一名受累胎儿同样继承了这两个变异，而经桑格测序（Sanger sequencing）分析验证，该家系中的表型正常子代未携带上述任何一个变异。根据当前美国医学遗传学与基因组学学会（American College of Medical Genetics and Genomics, ACMG）的指南，上述两个变异均被归类为意义未明变异。
结论：本研究报道了编码SAS6中心粒组装蛋白的SASS6基因上的新型双等位变异，该变异与常染色体隐性遗传性小头畸形的产前发病相关。本研究推测，这两个紧密相邻的复合杂合变异可能通过增强整体卷曲结构的不稳定性，从而引发本病例的表型特征。