Periventricular nodular heterotopia is caused by defective neuronal migration that results in heterotopic neuronal nodules lining the lateral ventricles. Mutations in filamin A (FLNA) or ADP-ribosylation factor guanine nucleotide-exchange factor 2 (ARFGEF2) cause periventricular nodular heterotopia, but most patients with this malformation do not have a known aetiology. Using comparative genomic hybridization, we identified 12 patients with developmental brain abnormalities, variably combining periventricular nodular heterotopia, corpus callosum dysgenesis, colpocephaly, cerebellar hypoplasia and polymicrogyria, harbouring a common 1.2Mb minimal critical deletion in 6q27. These anatomic features were mainly associated with epilepsy, ataxia and cognitive impairment. Using whole exome sequencing in 14 patients with isolated periventricular nodular heterotopia but no copy number variants, we identified one patient with periventricular nodular heterotopia, developmental delay and epilepsy and a de novo missense mutation in the chromosome 6 open reading frame 70 (C6orf70) gene, mapping in the minimal critical deleted region. Using immunohistochemistry and western blots, we demonstrated that in human cell lines, C6orf70 shows primarily a cytoplasmic vesicular puncta-like distribution and that the mutation affects its stability and subcellular distribution. We also performed in utero silencing of C6orf70 and of Phf10 and Dll1, the two additional genes mapping in the 6q27 minimal critical deleted region that are expressed in human and rodent brain. Silencing of C6orf70 in the developing rat neocortex produced periventricular nodular heterotopia that was rescued by concomitant expression of wild-type human C6orf70 protein. Silencing of the contiguous Phf10 or Dll1 genes only produced slightly delayed migration but not periventricular nodular heterotopia. The complex brain phenotype observed in the 6q terminal deletion syndrome likely results from the combined haploinsufficiency of contiguous genes mapping to a small 1.2Mb region. Our data suggest that, of the genes within this minimal critical region, C6orf70 plays a major role in the control of neuronal migration and its haploinsufficiency or mutation causes periventricular nodular heterotopia.
Bibliographical noteFunding Information:
We demonstrated that the minimal critical deleted region in 6q27 contains three genes that are expressed in human as well as rodent brain. Silencing Phf10 and Dll1, through in utero electroporation, only caused slightly delayed neuronal migration, while silencing C6orf70 reproduced PNH, suggesting that this gene plays a major role in the control of neuronal migration. This hypothesis was supported by the finding of a p.Ile250Asn-p.Ile207Asn mutation in a patient with isolated bilateral PNH and epilepsy.
This work was supported by funding from the Sixth Framework Programme of the EU, project grant LSH-CT-2006-037315 (EPICURE) (to R.G., A.R. and C.C.), the European Research Projects on Rare Diseases (E-Rare-2, TUB-GENCODEV, 11-027) (to R.G.), the Oxford NIHR Biomedical Research Centre Oxford and FWF grants P24367-B24 and I914-B13 (to D.A.K), INSERM (to A.R. and C.C.), the Health Research Council of New Zealand (to S.P.R.) and the Cure Kids New Zealand (to S.P.R.). A.C. is supported by a fellowship from FRM (Fondation pour la Recherche Medicale) and E.P.P. is a postdoctoral researcher supported by Else Kröner-Fresenius-Stiftung foundation (2010_A145). RJL and GM are supported by the Murdoch Children’s Research Institute and the Victorian State Government Operational Infrastructure Support Program.
- 6q terminal deletion syndrome
- Brain malformations
- C6orf70 gene
- Periventricular nodular heterotopia