ESPE Abstracts

Introduction: The ZSWIM6 gene was first described with a unique de-novo heterozygous variant causing acromelic frontonasal dysostosis, characterized by craniofacial, brain, and limb malformations. Another de-novo heterozygous variant was later described causing severe intellectual disability, hypotonia amd seizures but without craniofacial or limb malformations. These findings suggest a clear genotype-phenotype correlation for ZSWIM6. Gene interactions point to this gene being a transcription factor and phenotypic variation highlights its potential function in neuronal and skeletal development. However, specific molecular mechanisms of the gene remain entirely unknown.

Patients and Methods: Two siblings from 1^st cousin consanguinity from Kurdistan, Iraq born SGA with a specific facial phenotype (prominent forehead including supraorbital ridges, broad nose, depressed nasal bridge), persistent severe short stature, microcephaly, and developmental delay were examined using Whole Exome Sequencing (WES) methods. The variant found and two other previously published de-novo heterozygous variants in gene (p.Arg913Ter, Arg1163Trp) were prepared by side-directed mutagenesis. ZSWIM6 gene expression was validated in human embryonic HEK293 cells via Western blot analysis. Furthermore, its transcriptional activity was assessed using a dual luciferase reporter assay, wherein it was compared with selected target genes hypothesized to possess function links.

Results: A novel homozygous variant in the ZSWIM6 gene (c.3119G>A, p.Arg1040His) was found in our sibling pair who exhibit some clinical manifestations consistent with previously published phenotypes. However, notable distinctions can be found (lack of craniofacial or limb malformations, milder intellectual disability, lack of truncal hypotonia, seizures, autistic features). Cellular expression was confirmed in all three in-vitro variants of the ZSWIM6 protein. The dual luciferase assay revealed that ZSWIM6 is an activator of the HECW2 and ZIC2 genes, which are key components of neurological and skeletal development. Furthermore, we found that the individual ZSWIM6 variants induced distinct transcriptional activity of the resulting protein product.

Conclusion: Homozygous variants in ZSWIM6 are shown to cause a new phenotype of severe syndromic short stature in two siblings examined by WES. The function of ZSWIM6 as an important transcription factor, involved in the sonic-hedgehog pathway, was proven for the first time. Further testing is required to elucidate specific pathophysiological pathways. The combination of WES and in vitro functional studies is crucial in uncovering new genetic causes and mechanisms in such conditions.