Whole exome sequencing identifies a splicing mutation in NSUN2 as a cause of a Dubowitz-like syndrome.

Abstract

Dubowitz syndrome (DS) is an autosomal recessive disorder characterized by the constellation of mild microcephaly, growth and mental retardation, eczema and peculiar facies. Over 140 cases have been reported, but the genetic basis is not understood. We enrolled a multiplex consanguineous family from the United Arab Emirates with many of the key clinical features of DS as reported in previous series. The family was analyzed by whole exome sequencing. RNA splicing was evaluated with reverse-transcriptase PCR, immunostaining and western blotting was performed with specific antibodies, and site-specific cytosine-5-methylation was studied with bisulfite sequencing. We identified a homozygous splice mutation in the NSUN2 gene, encoding a conserved RNA methyltransferase. The mutation abolished the canonical splice acceptor site of exon 6, leading to use of a cryptic splice donor within an AluY and subsequent mRNA instability. Patient cells lacked NSUN2 protein and there was resultant loss of site-specific 5-cytosine methylation of the tRNA(Asp GTC) at C47 and C48, known NSUN2 targets. Our findings establish NSUN2 as the first causal gene with relationship to the DS spectrum phenotype. NSUN2 has been implicated in Myc-induced cell proliferation and mitotic spindle stability, which might help explain the varied clinical presentation in DS that can include chromosomal instability and immunological defects.