Sox10 mutation disrupts neural crest development in Dom Hirschsprung mouse model

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.

Brachyury-related transcription factor Tbx2 and repression of the melanocyte-specific TRP-1 promoter

Previous work has demonstrated that two key melanocyte-specific elements termed the MSEu and MSEi play critical roles in the expression of the melanocyte-specific tyrosinase-related protein 1 (TRP-1) promoter. Both the MSEu and MSEi, located at position -237 and at the initiator, respectively, bind a melanocyte-specific factor termed MSF but are also recognized by a previously uncharacterized repressor, since mutations affecting either of these elements result in strong up-regulation of TRP-1 promoter activity in melanoma cells. Here we demonstrate that repression mediated by the MSEu and MSEi also operates in melanocytes. We also report that both the MSEu and MSEi are recognized by the brachyury-related transcription factor Tbx2, a member of the recently described T-box family, and that Tbx2 is expressed in melanocyte and melanoblast cell lines but not in melanoblast precursor cells. Although Tbx2 and MSF each recognize the TRP-1 MSEu and MSEi motifs, it is binding by Tbx-2, not binding by MSF, that correlates with repression. Several lines of evidence tend to point to the brachyury-related transcription factor Tbx2 as being the repressor of TRP-1 expression: both the MSEu and MSEi bind Tbx2, and mutations in either element that result in derepression of the TRP-1 promoter diminish binding by Tbx2; the TRP-1 promoter, but not the tyrosinase, microphthalmia, or glyceraldehyde-3-phosphate dehydrogenase (G3PDH) promoter, is repressed by Tbx2 in cotransfection assays; a high-affinity consensus brachyury/Tbx2-binding site is able to constitutively repress expression of the heterologous IE110 promoter; and a low-affinity brachyury/Tbx2 binding site is able to mediate Tbx2-dependent repression of the G3PDH promoter. Although we cannot rule out the presence of an additional, as yet unidentified factor playing a role in the negative regulation of TRP-1 in vivo, the evidence presented here suggests that Tbx2 most likely is the previously unidentified repressor of TRP-1 expression and as such is likely to represent the first example of transcriptional repression by a T-box family member.

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.

Ku70: a candidate tumor suppressor gene for murine T cell lymphoma

We present evidence that inactivation of the Ku70 gene leads to a propensity for malignant transformation both in vitro and in vivo. In vitro, Ku70-/- mouse fibroblasts displayed an increased rate of sister chromatid exchange and a high frequency of spontaneous neoplastic transformation. In vivo, Ku70-/- mice, known to be defective in B but not T lymphocyte maturation, developed thymic and disseminated T cell lymphomas at a mean age of 6 months with CD4+CD8+ tumor cells. These findings directly demonstrate that Ku70 deficiency facilitates neoplastic growth and suggest a novel role of the Ku70 locus in tumor suppression.

Animal models for dysmorphology

Areas where studies of animal models have been vital to the understanding of human malformation syndromes include the dissection of complex genetic mechanisms such as imprinting, the creation of animal models of known human conditions, and the provision of further clues involving gene families and pathways.

Origin and early development of Schwann cells

Cellular events leading to the generation of Schwann cells from the neural crest have recently been clarified and it is now possible to outline a relatively simple model of the Schwann cell lineage in the rat and mouse. Neural crest cells have to undergo three main developmental transitions to become mature Schwann cells. These are the formation of Schwann cell precursors from crest cells, the formation of immature Schwann cells from precursors and, lastly, the postnatal and reversible generation of non-myelin- and myelin-forming Schwann cells. Axonal signals involving neuregulins are important regulators of these events, in particular of the survival, proliferation, and differentiation of Schwann cell precursors. Transcription factors likely to be involved in the developmental transitions are beginning to be identified. These include Oct-6, Krox-20, and Pax-3 but also members of the basic helix-loop-helix family, Sox 10, and the cAMP response element binding protein CREB.

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.

Hirschsprung's disease genes and the development of the enteric nervous system

Hirschsprung's disease or aganglionic megacolon causes chronic, congenital obstipation at an incidence of 1 per 5000 live births. Two approaches have been vital to the present understanding of the pathogenesis and genetic background of the disease: disease linkage analyses and mouse models of aganglionic megacolon. Because the increasing number of transgenic or natural mouse strains with congenital megacolon has led to mutation screening in Hirschsprung's disease patients, almost every second patient could now receive a genetic explanation for his/her disease. The known disease genes include tyrosine kinase receptor Ret, endothelin receptor B and its ligand endothelin 3. In addition, mutations have been found in the gene encoding the glial cell line-derived neurotrophic factor, the ligand for Ret, but these may only have a modifier effect. The mouse models have also provided insight into the developmental mechanisms of the normal intestinal innervation. We combine here the present clinical data on the gene mutations in Hirschsprung's disease with the experimental molecular biology data, and formulate a hypothesis on the pathogenesis of this multigenic-multifactorial disease.

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.