Genetic variation in MKL2 and decreased downstream PCTAIRE1 expression in extreme, fatal primary human microcephaly

Abstract

The genetic mechanisms driving normal brain development remain largely unknown. We performed genomic and immunohistochemical characterization of a novel, fatal human phenotype including extreme microcephaly with cerebral growth arrest at 14–18 weeks gestation in three full sisters born to healthy, non-consanguineous parents. Analysis of index cases and parents included familial exome sequencing, karyotyping, and genome-wide single nucleotide polymorphism (SNP) array. From proband, control and unrelated microcephalic fetal cortical tissue, we compared gene expression of RNA and targeted immunohistochemistry. Each daughter was homozygous for a rare, non-synonymous, deleterious variant in the MKL2 gene and heterozygous for a private 185 kb deletion on the paternal allele, upstream and in cis with his MKL2 variant allele, eliminating 24 CArG transcription factor binding sites and MIR4718. MKL1 was underexpressed in probands. Dysfunction of MKL2 and its transcriptional coactivation partner, serum response factor (SRF), was supported by a decrease in gene and protein expression of PCTAIRE1, a downstream target of MKL2:SRF heterodimer transcriptional activation, previously shown to result in severe microcephaly in murine models. While disruption of the MKL2:SRF axis has been associated with severe microcephaly and disordered brain development in multiple model systems, the role of this transcription factor complex has not been previously demonstrated in human brain development.

Fetal brain disruption sequence versus fetal brain arrest: A distinct autosomal recessive developmental brain malformation phenotype

“…They concluded that dysfunction of MKL2 and its transcriptional coactivation partner, serum response factor (SRF), was supported by a decrease in gene and protein expression of PCTAIRE1, a downstream target of MKL2:SRF heterodimer transcriptional activation, previously shown to result in severe microcephaly in murine models. However, these results do not definitively establish causality of the role of MKL:SRF disruption in normal human brain development [Ramos et al, ]. Mutation analysis of MKL2 in our patients showed negative results.…”

“…These imaging findings show a considerable overlap with the condition described here. However, several characteristic anomalies such as intrauterine growth retardation, multiple cardiac anomalies, cleft lip and palate [Ramos et al, ] were absent in our patients and those reported by Alexander et al [] and Schram et al []. Exome sequencing for the three sibs described by Ramos et al [] showed a homozygous rare, non‐synonymous, deleterious variant in MKL2 .…”

“…However, several characteristic anomalies such as intrauterine growth retardation, multiple cardiac anomalies, cleft lip and palate [Ramos et al, ] were absent in our patients and those reported by Alexander et al [] and Schram et al []. Exome sequencing for the three sibs described by Ramos et al [] showed a homozygous rare, non‐synonymous, deleterious variant in MKL2 . They concluded that dysfunction of MKL2 and its transcriptional coactivation partner, serum response factor (SRF), was supported by a decrease in gene and protein expression of PCTAIRE1, a downstream target of MKL2:SRF heterodimer transcriptional activation, previously shown to result in severe microcephaly in murine models.…”

“…Recently Ramos et al [] pointed out to FBDS in three sisters with severe lethal congenital microcephaly. The brain MRI of the three sibs showed complete lissencephaly with failure of normal opercularization, thin cerebral mantle and cerebellar hypoplasia.…”

Exon Array Biomarkers for the Differential Diagnosis of Schizophrenia and Bipolar Disorder

“…MKL2 is a proto-oncogene which is widely distributed, highly expressed in brain regions such as the dentate gyrus (Allen Institute Brain Science [70]), and associated with microcephaly [71]. The MKL2 gene consists of multiple exons many of which are downregulated in SZ and not significantly changed in BD.…”