Slowed conduction and thin myelination of peripheral nerves associated with mutant rho Guanine-nucleotide exchange factor 10

Abstract

Slowed nerve-conduction velocities (NCVs) are a biological endophenotype in the majority of the hereditary motor and sensory neuropathies (HMSN). Here, we identified a family with autosomal dominant segregation of slowed NCVs without the clinical phenotype of HMSN. Peripheral-nerve biopsy showed predominantly thinly myelinated axons. We identified a locus at 8p23 and a Thr109Ile mutation in ARHGEF10, encoding a guanine-nucleotide exchange factor (GEF) for the Rho family of GTPase proteins (RhoGTPases). Rho GEFs are implicated in neural morphogenesis and connectivity and regulate the activity of small RhoGTPases by catalyzing the exchange of bound GDP by GTP. Expression analysis of ARHGEF10, by use of its mouse orthologue Gef10, showed that it is highly expressed in the peripheral nervous system. Our data support a role for ARHGEF10 in developmental myelination of peripheral nerves.

Figure 1

Haplotype analysis of chromosome 8p23 STR markers. a, Pedigree of the family with slowed NCVs. Squares = male; circles = female; unblackened = unaffected; blackened = reduced nerve conduction phenotype; slashed = deceased; diagonal arrow = proband. Blood samples were obtained after informed consent and approval of the Committee for Medical Ethics, University of Antwerp. A genomewide scan was performed using 382 STR markers from the ABI Prism Linkage Mapping, Set MD-10 (Applied Biosystems). Conclusive linkage was found with D8S264. Additional STR markers were used in the fine mapping: three known markers (D8S504, AF009208, and AF009213) and four novel STR markers (designated “STR1,” “STR2,” “STR3,” and “STR4”) (table A [online only]). Genotypes are represented by allele numbers; “0 0” = failed genotype. For each STR marker, a disease haplotype was constructed (box). The presence of the Thr109Ile mutation is indicated by a plus sign (+), the absence by a minus sign (−). The meiotic recombination events, indicated with an arrow, assign the disease locus to the chromosome segment contained between D8S504 and STR2. b, Electron microscopy. Transverse section of superficial fibular nerve shows thinly myelinated fibers (arrows). Scale bar = 10 μm.

Figure 2

Positional cloning and expression analysis of ARHGEF10. a, Linked region on chromosome 8p23. Meiotic recombinants with STR2 and D8S504 defined a critical region of 1.5 Mb. Shaded boxes indicate the five known genes in the region. b, Genomic structure of human ARHGEF10 and mouse Gef10, aligned. The 5′ and 3′ UTRs are light gray; the Dbl domain is dark gray. An arrow indicates the position of the Thr109Ile mutation. c–e, Expression analysis of mouse Gef10 on cDNA of motor and sensory neurons isolated from VH and DRG of mice embryos, age 13 d. Expression analysis was performed with oligonucleotide primer pairs spread over the proximal, middle, and distal part of the gene. Subsequent direct-DNA sequencing analysis of PCR fragments was performed. M = 100-bp size marker (Invitrogen); Br = mouse brain cDNA (Clontech); VH = VH cDNA; DRG = DRG cDNA; C− = blanco. c, cDNA primer1 and primer2, splicing of exon 4 in the smaller fragment in brain, VH, and DRG. d, cDNA primer3 and primer4, skipping of exon 21 in DRG. e, cDNA primer5 and primer6, additional exon identified, located between exon 22 and 23 in VH. (Primer sequences shown in table D [online only].)

Figure 3

Expression pattern of Gef10 mRNA in mouse embryo. a and b, Frontal view on an E8.5 embryo (a) and blowup of the head region (b) hybridized with the GEF10 probe. GEF10 mRNA is expressed in the neuroepithelium (ne) of brain vesicles and optic stalk (os). c and d, Dorsal (c) and lateral (d) views on an E9.5 embryo. High levels of GEF10 mRNA are observed in the thin roof of the rhombencephalon (trr), the neuroepithelium lining the brain ventricles, and, to a lesser extend, in the neural tube (nt). e–h, Bright-field (e) and corresponding dark-field image (f) of a transversal section at the level of the epithalamus of E12.5 embryo. Transcripts of GEF10 are present in the ependymal layer (el) of neural tube and brain vesicles, as well as in the trigeminal ganglia (tg) of the head. GEF10 transcripts are also detected in the endolymphatic sac (es) and semicircular canal (ssc). g, Dark-field image of a transversal section through the neural tube. Note high expression in the ependymal layer of the neural tube and the DRG. h, Transversal section through the region of the eye at E12.5, revealing very high GEF10 expression levels in the neural layer of the retina (nlr). can(XI) = cranial accessory (XI) nerve; tg(V) = trigeminal (V) ganglion; vcg(VIII) = vestibulocochlear (VIII) ganglion; ov = otic vesicle; l = lens; lge = lateral ganglionic eminence; lb = limb bud. The whole-mount and radioactive in situ hybridizations were performed as reported elsewhere (Wilkinson ; Dewulf et al. 1995).