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）是一类病因多样且并不罕见的疾病。随着全外显子组测序（whole-exome sequencing，WES）在临床与研究领域的广泛应用，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引发极为罕见的14型原发性小头畸形。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。
病例报告：本研究报道一个中国家系中复发性先天性小头畸形病例。两名受累胎儿均在妊娠中期早期出现小头畸形，且伴随胼胝体发育不全。针对首例受累胎儿，三人组全外显子组测序（trio WES）检测发现，SASS6基因存在2个复合杂合候选变异：c.1139T>C（p.L380P）与c.1223C>G（p.T408S）。第二名受累胎儿同样继承了这两个变异，而经桑格测序（Sanger sequencing）验证，表型正常的子代未携带任一变异。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）指南，这两个变异均被归类为意义未明变异。
结论：本研究报道了SASS6基因的新型双等位变异，该基因编码SAS6中心粒组装蛋白，与产前起病的常染色体隐性遗传性小头畸形相关。本研究推测，这两个位置紧密相邻的复合杂合变异可能通过增强整体卷曲螺旋结构的不稳定性，进而引发本病例的表型特征。