SOX10 mutations mimic isolated hearing loss.

Ninety genes have been identified to date that are involved in non-syndromic hearing loss, and more than 300 different forms of syndromic hearing impairment have been described. Mutations in SOX10, one of the genes contributing to syndromic hearing loss, induce a large range of phenotypes, including several subtypes of Waardenburg syndrome and Kallmann syndrome with deafness. In addition, rare mutations have been identified in patients with isolated signs of these diseases. We used the recent characterization of temporal bone imaging aspects in patients with SOX10 mutations to identify possible patients with isolated hearing loss due to SOX10 mutation. We selected 21 patients with isolated deafness and temporal bone morphological defects for mutational screening. We identified two SOX10 mutations and found that both resulted in a non-functional protein in vitro. Re-evaluation of the two affected patients showed that both had previously undiagnosed olfactory defects. Diagnosis of anosmia or hyposmia in young children is challenging, and particularly in the absence of magnetic resonance imaging (MRI), SOX10 mutations can mimic non-syndromic hearing impairment. MRI should complete temporal bones computed tomographic scan in the management of congenital deafness as it can detect brain anomalies, cochlear nerve defects, and olfactory bulb malformation in addition to inner ear malformations.

SOX10 mutations in patients with Waardenburg-Hirschsprung disease.

Waardenburg syndrome (WS; deafness with pigmentary abnormalities) and Hirschsprung's disease (HSCR; aganglionic megacolon) are congenital disorders caused by defective function of the embryonic neural crest. WS and HSCR are associated in patients with Waardenburg-Shah syndrome (WS4), whose symptoms are reminiscent of the white coat-spotting and aganglionic megacolon displayed by the mouse mutants Dom (Dominant megacolon), piebald-lethal (sl) and lethal spotting (ls). The sl and ls phenotypes are caused by mutations in the genes encoding the Endothelin-B receptor (Ednrb) and Endothelin 3 (Edn3), respectively. The identification of Sox10 as the gene mutated in Dom mice (B.H. et al., manuscript submitted) prompted us to analyse the role of its human homologue SOX10 in neural crest defects. Here we show that patients from four families with WS4 have mutations in SOX10, whereas no mutation could be detected in patients with HSCR alone. These mutations are likely to result in haploinsufficiency of the SOX10 product. Our findings further define the locus heterogeneity of Waardenburg-Hirschsprung syndromes, and point to an essential role of SOX10 in the development of two neural crest-derived human cell lineages.

How Tissue Mechanical Properties Affect Enteric Neural Crest Cell Migration.

Neural crest cells (NCCs) are a population of multipotent cells that migrate extensively during vertebrate development. Alterations to neural crest ontogenesis cause several diseases, including cancers and congenital defects, such as Hirschprung disease, which results from incomplete colonization of the colon by enteric NCCs (ENCCs). We investigated the influence of the stiffness and structure of the environment on ENCC migration in vitro and during colonization of the gastrointestinal tract in chicken and mouse embryos. We showed using tensile stretching and atomic force microscopy (AFM) that the mesenchyme of the gut was initially soft but gradually stiffened during the period of ENCC colonization. Second-harmonic generation (SHG) microscopy revealed that this stiffening was associated with a gradual organization and enrichment of collagen fibers in the developing gut. Ex-vivo 2D cell migration assays showed that ENCCs migrated on substrates with very low levels of stiffness. In 3D collagen gels, the speed of the ENCC migratory front decreased with increasing gel stiffness, whereas no correlation was found between porosity and ENCC migration behavior. Metalloprotease inhibition experiments showed that ENCCs actively degraded collagen in order to progress. These results shed light on the role of the mechanical properties of tissues in ENCC migration during development.

Expression of the SOX10 gene during human development.

SOX10, a new member of the SOX gene family, is a transcription factor defective in the Dom (Dominant megacolon) mouse and in the human Shah-Waardenburg syndrome. To help unravel its physiological role during human development, we studied SOX10 gene expression in embryonic, fetal, and adult human tissues by Northern blot and in situ hybridization. As in mice, the human SOX10 gene was essentially expressed in the neural crest derivatives that contribute to the formation of the peripheral nervous system, and in the adult central nervous system. Nevertheless, it was more widely expressed in humans than in rodents. The spatial and temporal pattern of SOX10 expression supports an important function in neural crest development.

The SOX10 transcription factor: evaluation as a candidate gene for central and peripheral hereditary myelin disorders.

The SOX10 transcription factor is involved in development of neural crest derivatives and fate determination in glial cells. SOX10 mutations have been found in patients with intestinal aganglionosis and depigmentation with deafness (Waardenburg-Hirschsprung). Associated neurological signs have been reported in some cases, including a patient exhibiting a central and peripheral myelin deficiency. Therefore, we screened for SOX10 mutations in a large cohort of patients with peripheral and central myelin disorders. 56 were affected by classical demyelinating Charcot-Marie-Tooth disease without identified mutations in the genes encoding PNS myelin proteins (PMP22, P0), connexin 32 and the zinc-finger transcription factor, EGR2. 88 patients with undetermined leukodystrophy were selected from a large European prospective study. Associated clinical, magnetic resonance imaging and electrophysiological signs were consistent with a defect in CNS myelination in 83 and with an active degeneration of the CNS myelin in 5. No abnormalities in the proteolipid protein gene (PLP) were found. The absence of SOX100 mutation in this large cohort of patients suggests that this gene is not frequently involved in peripheral or central inherited myelin disorders.

Human Connexin 32, a gap junction protein altered in the X-linked form of Charcot-Marie-Tooth disease, is directly regulated by the transcription factor SOX10.

Mutations in SOX10, a transcription modulator crucial in the development of the enteric nervous system (ENS), melanocytes and glial cells, are found in Shah-Waardenburg syndrome (WS4), a neurocristopathy that associates intestinal aganglionosis, pigmentation defects and sensorineural deafness. Expression of MITF and RET, two genes that play important roles during melanocyte and ENS development, respectively, are controlled by SOX10. The observation that some WS4 patients present with myelination defects of the central and peripheral nervous systems correlates with the recent finding that P(0), a major component of the peripheral myelin, is another transcriptional target of SOX10. These phenotypic features suggest that SOX10 could regulate expression of other genes involved in the myelination process as well. Thus, we tested the ability of SOX10 to regulate expression of MBP, PMP22 and Connexin 32, three major proteins of the peripheral myelin. Our study shows that this factor, in synergy with EGR2, strongly activates Cx32 expression in vitro by directly binding to its promoter. In agreement with this finding, SOX10 and EGR2 mutants identified in patients with peripheral myelin defects fail to transactivate the Cx32 promoter. Moreover, we show that a mutation of the Cx32 promoter previously described in a patient with the X-linked form of Charcot-Marie-Tooth (CMTX) disease impairs SOX10 function. In addition to providing new insights into the molecular mechanisms underlying some of the peripheral myelin defects observed in CMTX disease, these results further extend the spectrum of genes that are regulated by SOX10.

Human homology and candidate genes for the Dominant megacolon locus, a mouse model of Hirschsprung disease.

Hirschsprung disease (HSCR) is a congenital disorder of the enteric nervous system characterized by the absence of enteric ganglia. Three genes for HSCR have been identified: the RET proto-oncogene, the gene coding for the endothelin B receptor (EDNRB), and the endothelin 3 gene (EDN3). In mice, natural and in vitro-induced mutations affecting the Ret, Ednrb, and Edn3 genes generate a phenotype similar to human HSCR. Another model of HSCR disease is the Dominant megacolon (Dom), a spontaneous mouse mutation for which the target gene has not yet been identified. The Dom mutation has been mapped to the middle-terminal region of mouse chromosome 15, between D15Mit68 and D15Mit2. Using new or known polymorphisms for conserved human/mouse genes, we established the homology between the Dom locus and human chromosome 22q12-q13. Two genes, Smstr3 and Adsl, not previously mapped in the mouse genome, were located on mouse Chromosome 15. Three genes (Smstr3, Lgals1, and Pdgfb) are possible Dom candidates, as they do not recombine with the Dom mutation in a 252 Dom/+ animal backcross.

Functional analysis of Sox10 mutations found in human Waardenburg-Hirschsprung patients.

The Sry-related protein Sox10 is selectively expressed in neural crest cells during early stages of development and in glial cells of the peripheral and central nervous systems during late development and in the adult. Mutation of the Sox10 gene leads to neural crest defects in the Dominant megacolon mouse mutant and to combined Waardenburg-Hirschsprung syndrome in humans. Here, we have studied the four Sox10 mutations found to date in Waardenburg-Hirschsprung patients both in the context of the rat and the human cDNA. Unlike the rat Sox10 protein, which failed to show transcriptional activity on its own, human Sox10 displayed a weak, but reproducible, activity as a transcriptional activator. All mutant Sox10 proteins, including the one that only lacked the 106 last amino acids were deficient in this capacity, indicating that the carboxyl terminus of human Sox10 carries a transactivation domain. Whereas all four mutants failed to transactivate, only two failed to synergistically enhance the activity of other transcription factors. Synergy required both the ability to bind to DNA and a region in the amino-terminal part of Sox10. Those mutants that failed to synergize were unable to bind to DNA. Analysis of the naturally occurring Sox10 mutations not only helps to dissect Sox10 structure, but also allows limited predictions on the severity of the disease.

Peripheral neuropathy with hypomyelination, chronic intestinal pseudo-obstruction and deafness: a developmental "neural crest syndrome" related to a SOX10 mutation.

We describe the case of a girl with an unusual congenital phenotype, combining peculiar peripheral nerve lesions with hypomyelination, chronic intestinal pseudoobstruction, and deafness. She was found to have a de novo heterozygous frameshift mutation in the gene encoding the SOX10 transcription factor. The likely role of SOX10 in determining the fate of Schwann cells during early embryogenesis may explain the peripheral nervous system developmental disorder observed in this patient.

Interaction among SOX10, PAX3 and MITF, three genes altered in Waardenburg syndrome.

Waardenburg syndrome (WS) is an autosomal dominant disorder with an incidence of 1 in 40 000 that manifests with sensorineural deafness and pigmentation defects. It is classified into four types depending on the presence or absence of additional symptoms. WS1 and WS3 are due to mutations in the PAX3 gene whereas some WS2 cases are associated with mutations in the microphthalmia-associated transcription factor (MITF) gene. The WS4 phenotype can result from mutations in the endothelin-B receptor gene (EDNRB), in the gene for its ligand, endothelin-3 (EDN3), or in the SOX10 gene. PAX3 has been shown to regulate MITF gene expression. The recent implication of SOX10 in WS4 prompted us to test whether this transcription factor, known to cooperate in vitro with PAX3, is also able to regulate expression from the MITF promoter. Here we show that SOX10, in synergy with PAX3, strongly activates MITF expression in transfection assays. Analyses revealed that PAX3 and SOX10 interact directly by binding to a proximal region of the MITF promoter containing binding sites for both factors. Moreover, SOX10 or PAX3 mutant proteins fail to transactivate this promoter, providing further evidence that the two genes act in concert to directly regulate expression of MITF. In situ hybridization experiments carried out in the dominant megacolon (DOM:) mouse, confirmed that SOX10 dysfunction impairs MITF: expression as well as melanocytic development and survival. These experiments, which demonstrate an interaction between three of the genes that are altered in WS, could explain the auditory-pigmentary symptoms of this disease.

Mutation of the Sry-related Sox10 gene in Dominant megacolon, a mouse model for human Hirschsprung disease.

The spontaneous mouse mutant Dominant megacolon (Dom) is a valuable model for the study of human congenital megacolon (Hirschsprung disease). Here we report that the defect in the Dom mouse is caused by mutation of the gene encoding the Sry-related transcription factor Sox10. This assignment is based on (i) colocalization of the Sox10 gene with the Dom mutation on chromosome 15; (ii) altered Sox10 expression in the gut and in neural-crest derived structures of cranial ganglia of Dom mice; (iii) presence of a frameshift in the Sox10 coding region, and (iv) functional inactivation of the resulting truncated protein. These results identify the transcriptional regulator Sox10 as an essential factor in mouse neural crest development and as a further candidate gene for human Hirschsprung disease, especially in cases where it is associated with features of Waardenburg syndrome.

Loss-of-function mutations in SIP1 Smad interacting protein 1 result in a syndromic Hirschsprung disease.

Hirschsprung disease (HD) has been described in association with microcephaly, mental retardation and characteristic facial features, delineating a syndrome possibly caused by mutations localized at chromosome 2q22--q23. We have analyzed a de novo translocation breakpoint at 2q22 in one patient presenting with this syndrome, and identified a gene, SIP1, which is disrupted by this chromosomal rearrangement. SIP1 encodes Smad interacting protein 1, a new member of the delta EF1/Zfh-1 family of two-handed zinc finger/homeodomain transcription factors. We determined the genomic structure and expression of the human SIP1 gene. Further analysis of four independent patients showed that SIP1 is altered by heterozygous frameshift mutations causing early truncation of the protein. SIP1, among other functions, seems to play crucial roles in normal embryonic development of neural structures and neural crest. Its deficiency, in altering function of the TGF beta/BMP/Smad-mediated signalling cascade, is consistent with some of the dysmorphic features observed in this syndrome, in particular the enteric nervous system defect that underlies HD.

A molecular analysis of the yemenite deaf-blind hypopigmentation syndrome: SOX10 dysfunction causes different neurocristopathies.

The Yemenite deaf-blind hypopigmentation syndrome was first observed in a Yemenite sister and brother showing cutaneous hypopigmented and hyperpigmented spots and patches, microcornea, coloboma and severe hearing loss. A second case, observed in a girl with similar skin symptoms and hearing loss but without microcornea or coloboma, was reported as a mild form of this syndrome. Here we show that a SOX10 missense mutation is responsible for the mild form, resulting in a loss of DNA binding of this transcription factor. In contrast, no SOX10 alteration could be found in the other, severe case of the Yemenite deaf-blind hypopigmentation syndrome. Based on genetic, clinical, molecular and functional data, we suggest that these two cases represent two different syndromes. Moreover, as mutations of the SOX10 transcription factor were previously described in Waardenburg-Hirschsprung disease, these results show that SOX10 mutations cause various types of neurocristopathy.