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.
There is considerable interest in the use of next-generation sequencing to help diagnose unidentified genetic conditions, but it is difficult to predict the success rate in a clinical setting that includes patients with a broad range of phenotypic presentations.
The authors present a pilot programme of whole-exome sequencing on 12 patients with unexplained and apparent genetic conditions, along with their unaffected parents. Unlike many previous studies, the authors did not seek patients with similar phenotypes, but rather enrolled any undiagnosed proband with an apparent genetic condition when predetermined criteria were met.
This undertaking resulted in a likely genetic diagnosis in 6 of the 12 probands, including the identification of apparently causal mutations in four genes known to cause Mendelian disease (TCF4, EFTUD2, SCN2A and SMAD4) and one gene related to known Mendelian disease genes (NGLY1). Of particular interest is that at the time of this study, EFTUD2 was not yet known as a Mendelian disease gene but was nominated as a likely cause based on the observation of de novo mutations in two unrelated probands. In a seventh case with multiple disparate clinical features, the authors were able to identify homozygous mutations in EFEMP1 as a likely cause for macular degeneration (though likely not for other features).
This study provides evidence that next-generation sequencing can have high success rates in a clinical setting, but also highlights key challenges. It further suggests that the presentation of known Mendelian conditions may be considerably broader than currently recognised.
Otocephaly or dysgnathia complex is characterised by mandibular hypoplasia/agenesis, ear anomalies, microstomia, and microglossia; the molecular basis of this developmental defect is largely unknown in humans.
This study reports a large family in which two cousins with micro/anophthalmia each gave birth to at least one child with otocephaly, suggesting a genetic relationship between anophthalmia and otocephaly. OTX2, a known microphthalmia locus, was screened in this family and a frameshifting mutation was found. The study subsequently identified in one unrelated otocephalic patient a sporadic OTX2 mutation. Because OTX2 mutations may not be sufficient to cause otocephaly, the study assayed the potential of otx2 to modify craniofacial phenotypes in the context of known otocephaly gene suppression in vivo. It was found that otx2 can interact genetically with pgap1, prrx1, and msx1 to exacerbate mandibular and midline defects during zebrafish development. However, sequencing of these loci in the OTX2-positive families did not unearth likely pathogenic lesions, suggesting further genetic heterogeneity and complexity.
Identification of OTX2 involvement in otocephaly/dysgnathia in humans, even if loss of function mutations at this locus does not sufficiently explain the complex anatomical defects of these patients, suggests the requirement for a second genetic hit. Consistent with this notion, trans suppression of otx2 and other developmentally related genes recapitulate aspects of the otocephaly phenotype in zebrafish. This study highlights the combined utility of genetics and functional approaches to dissect both the regulatory pathways that govern craniofacial development and the genetics of this disease group.
Significant advancements in understanding the molecular pathophysiology of laterality determination were recently made. However, there are large gaps in our knowledge of the initial processes that lead to laterality defects, such as heterotaxy syndrome (HS, also known as situs ambiguous) and situs inversus totalis (SIT). The former refers to abnormal distribution of visceral organs, and the latter refers to a complete laterality inversion of both abdominal and thoracic viscera.
In order to identify a mutated gene in SIT and HS patients, the authors performed homozygosity mapping in a consanguineous family with laterality disorders identified in two siblings.
A homozygous deleterious mutation in the CCDC11 gene was identified in the patients. The mutation resulted in an abnormally smaller protein in the patient's skin fibroblasts. The parents and five healthy siblings were heterozygous for the mutation, which was not present in 112 anonymous controls.
Few genes have been associated with both SIT and HS, usually accompanied by other abnormalities. The authors suggest that CCDC11 is associated with autosomal recessive laterality defects of diverse phenotype resulting in SIT in one individual family member who is otherwise healthy, and in complex laterality anomalies (HS) in another member. This report underscores the importance of CCDC11 in laterality determination.