Novel biallelic SASS6 variants associated with primary microcephaly and fetal growth restriction.

Primary microcephaly is characterized by a head circumference prenatally or at birth that falls below three standard deviations from age-, ethnic-, and sex-specific norms. Genetic defects are one of the underlying causes of primary microcephaly. Since 2014, five variants of the SASS6 gene have been identified as the cause of MCPH 14 in three reported families. In this study, we present the genetic findings of members of a nonconsanguineous Chinese couple with a history of microcephaly and fetal growth restriction (FGR) during their first pregnancy. Utilizing trio whole-exome sequencing, we identified compound heterozygous variants involving a frameshift NM_194292.3:c.450_453del p.(Lys150AsnfsTer7) variant and a splice region NM_194292.3:c.1674+3A>G variant within the SASS6 gene in the affected fetus. Moreover, reverse transcriptase-polymerase chain reaction from RNA of the mother's peripheral blood leukocytes revealed that the c.1674+3A>G variant led to the skipping of exon 14 and an inframe deletion. To the best of our knowledge, the association between FGR and SASS6-related microcephaly has not been reported, and our findings confirm the pivotal role of SASS6 in microcephaly pathogenesis and reveal an expanded view of the phenotype and mutation spectrum associated with this gene.

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 are 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.

A primary microcephaly-associated sas-6 mutation perturbs centrosome duplication, dendrite morphogenesis, and ciliogenesis in Caenorhabditis elegans.

The human SASS6(I62T) missense mutation has been linked with the incidence of primary microcephaly in a Pakistani family, although the mechanisms by which this mutation causes disease remain unclear. The SASS6(I62T) mutation corresponds to SAS-6(L69T) in Caenorhabditis elegans. Given that SAS-6 is highly conserved, we modeled this mutation in C. elegans and examined the sas-6(L69T) effect on centrosome duplication, ciliogenesis, and dendrite morphogenesis. Our studies revealed that all the above processes are perturbed by the sas-6(L69T) mutation. Specifically, C. elegans carrying the sas-6(L69T) mutation exhibit an increased failure of centrosome duplication in a sensitized genetic background. Further, worms carrying this mutation also display shortened phasmid cilia, an abnormal phasmid cilia morphology, shorter phasmid dendrites, and chemotaxis defects. Our data show that the centrosome duplication defects caused by this mutation are only uncovered in a sensitized genetic background, indicating that these defects are mild. However, the ciliogenesis and dendritic defects caused by this mutation are evident in an otherwise wild-type background, indicating that they are stronger defects. Thus, our studies shed light on the novel mechanisms by which the sas-6(L69T) mutation could contribute to the incidence of primary microcephaly in humans.

Novel SASS6 compound heterozygous mutations in a Chinese family with primary autosomal recessive microcephaly.

Primary autosomal recessive microcephaly (MCPH) is a rare hereditary disease characterized by congenitally small with brain circumference of the head below 3 standard deviations (SD). By far, 18 MCPH genes have been reported to be associated with the disease. SASS6 gene functioned in assembly of centrioles that the majority of MCPH genes present at the centrosome. There was only research reporting a homozygous missense mutation in SASS6 gene detected in a consanguineous Pakistani family. By conducting Whole-exome sequencing (WES) and Sanger sequencing on the family trio, we identified two novel splice site mutations c.127-13A>G and c.1867+2T>A in compound heterozygous hereditary form in the SASS6 gene. The two mutations were confirmed to alter mRNA splicing by RT-PCR assay. Our finding supported the role of SASS6 in the pathogenesis of microcephaly, expanding mutation spectrums and contributing to understanding of molecular mechanisms of MCPH.