Question

Explain the following observations in detail. What is happening? Why does this happen?

1. Hirschsprung disease - Mutation in GDNF or its receptor. Child can’t void solid wastes properly.

2.In embryos that are snail-/-, NCCs fail to migrate.

Answer 1

Hirschsprung disease (HSCR) is a congenital disorder characterized by the absence of enteric nervous plexuses in hind gut. Ten to forty percent of HSCR patients carry a dominant loss-of-function mutation in the gene encoding the receptor tyrosine kinase RET, a receptor for glial cell line-derived neurotrophic factor (GDNF). Although several mutations have also been found in the GDNF gene of HSCR patients, their impact on GDNF function is unknown. In this study, we have characterized the effect of these mutations on the ability of GDNF to bind and activate its receptors. Although none of the four mutations analyzed appeared to affect the ability of GDNF to activate RET, two of them resulted in a significant reduction in the binding affinity of GDNF for the binding subunit of the receptor complex, GFR(alpha)1. Our results indicate that, although none of the GDNF mutations identified so far in HSCR patients are per se likely to result in HSCR, two of these mutations (i.e. D150N and I211M) may, in conjunction with other genetic lesions, contribute to the pathogenesis of this disease.The main signs and symptoms of Hirschsprung disease are constipation or intestinal obstruction, usually appearing shortly after birth. Most often, an infant or a child with Hirschsprung disease will have other symptoms, including growth failure, swelling of the abdomen, unexplained fever, or vomiting.

SIP1 and SNAIL (SNAIL1 and SNAIL2 (SLUG) are known transcriptional repressors that control Cadherin activation [Comijn et al., 2001]. Micro-deletions or mutations in SIP1 transcriptional repressor have been identified in ∼200 patients with Mowat-Wilson syndrome (OMIM 235730) [Dastot-Le Moal et al., 2007; Saunders et al., 2009] an autosomal dominant disorder characterized by distinct facial [Saunders et al., 2009] dysmorphisms, microcephaly, mental retardation, and epilepsy [Mowat et al., 1998; Mowat et al., 2003; Horn et al., 2004; Zweier et al., 2005; Garavelli et al., 2009]. SIP1 normally represses E-cadherin expression [Comijn et al., 2001] and in mice carrying null mutations in Sip1, E-Cadherin levels stay abnormally high [Van de Putte et al., 2007]. Consequently, cells at the neural plate boundary cannot undergo an EMT and instead remain tethered to the epithelium. The end result is that NCC migration is delayed and affected mice exhibit abnormal morphology of the neural plate [Van de Putte et al., 2007].

The transcription factor Snail also represses E-cadherin gene transcription, through histone deacetylase-mediated chromatin remodeling [Peinado et al., 2004a; Peinado et al., 2004b]. Over-expression of Snail1 (which should down regulate E-cadherin function) is sufficient to induce EMT [Ikenouchi et al., 2003]. Snail1-null mice are early embryonic lethals. Conditional loss of Snail1 in murine neural crest cells has shown that Snail1 is not required for neural crest cell delamination and migration [Murray and Gridley, 2006a; Murray and Gridley, 2006b]. Redundant functions of Snail1 and Slug in cranial neural crest cells most likely account for a lack of phenotypic presentation in Snail1 conditional knock-out mice [Murray et al., 2006]. To date, no mutations in SNAIL have been associated with congenital malformations although the role of this gene in promoting epithelial-to-mesenchymal transitions in metastatic cancers is an area of active research [Herfs et al., 2010; Larriba et al., 2010; Wu and Zhou, 2010].