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）并非罕见的多病因疾病。随着全外显子测序（whole-exome sequencing, WES）在临床与科研领域的广泛应用，与MCPH相关的致病基因数量正持续增长。SASS6基因的双等位基因突变可引发一种极为罕见的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：本文报道一个中国家系中复发的先天性小头畸形病例。两名受累胎儿于妊娠中期早期即出现小头畸形，且伴随胼胝体发育不全。针对首例受累胎儿实施的三人组全外显子测序，检测到SASS6基因上的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，而正常子代经Sanger测序验证未携带上述变异。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，上述两种变异均被归类为意义未明变异（variant of uncertain significance, VUS）。
结论：本文报道了编码SAS6中心粒组装蛋白的SASS6基因上的新型双等位基因突变，该变异与常染色体隐性遗传的产前起病型小头畸形相关。我们推测，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，进而导致本病例的表型特征。