Enfermedad diverticular no complicada.

This work is important because it advances our understanding of the role chromatin, and specifically H2A.Z, play in regulation of developmental gene expression and developmental signalling pathways such as the canonical Wnt pathway. Greater understanding of developmental processes will provide foundations for the solutions to problems relating to early developmental processes, such as the detection and treatment of congenital defects and strategies to prevent reproduction in genetically modified or feral animals harmful to the environment.

It may be that the aetiologies of many developmental defects that are inadequately understood may involve chromatin’s role in gene regulation. Genetic networks for

developmental signalling genes within C. elegans found a selection of genes with a

large number of connections (hub genes) (Lehner et al., 2006). All hub genes identified are chromatin remodelling components, indicating a central role for chromatin in

developmental signalling pathways. Though this study was performed in C. elegans

these genes have orthologues in other metazoans suggesting that the hub gene characteristic of chromatin remodelling is conserved (Lehner et al., 2006). Interestingly, the expression patterns of remodelling genes are enriched in a stage and tissue specific

manner during X. laevis development indicating an important role development (Linder

et al., 2004). Hub genes are likely to cause susceptibility to genetic diseases for two reasons. Firstly, they are important to diverse functions including signalling, DNA repair, extracellular matrix, and cytoskeletal functions (Lehner et al., 2006). Secondly, it is proposed that the hub genes modulate gene expression to ensure proper biological outcomes despite mutations in signalling molecules or other problems in signalling pathways such as the Wnt or PCP pathways (Lehner et al., 2006). Incorporation of H2A.Z is reliant on one of these chromatin remodelling complexes, SRCAP, and

therefore could be essential to proper regulation of numerous genes and resilience against congenital defects (Ruhl et al., 2006).

6.3.1 Gastrulation, neurulation, and neural tube defects

Neural tube defects (NTDs) are the second most common congenital defects in humans, after heart defects (Frey and Hauser, 2003), affecting approximately 1 in 1000 live births (Padmanabhan, 2006). NTDs are a significant cause of mortality in the first year of human life (Padmanabhan, 2006). Care of NTD patients also has financial and quality of life costs (Padmanabhan, 2006). Neural tube defects in humans have their root in cell movement and cell fate determinations very early in development, however exact aetiologies remain unknown (Keller, 2002; Padmanabhan, 2006) though the role of dietary folic acid in preventing NTDs is well established (Berry et al., 1999). Better understanding of the events that lead to NTDs should improve prevention, detection, and perhaps treatment.

Work already performed in X. laevis indicates that perturbing H2A.Z’s function during

early development leads to improper gastrulation, defective mesoderm development,

and neural tube defects (Ridgway et al., 2004a). The use of X. laevis was critical to the

discovery of this link between H2A.Z and defective neural tube development. The characteristics of other commonly used developmental models would have prevented

the discovery. D. melanogaster is an invertebrate. In zebrafish the movements of neural

tube closure are unlike those of higher vertebrates, zebrafish with a disrupted PCP pathway do not show NTDs (Papan and Campos-Ortega, 1994; Ueno and Greene, 2003). The mouse may be useful in further work, however as a eutherian is technically difficult as an experimental embryonic model.

NTDs occur where there is a failure of neural tube closure, there are two contexts where failure of convergent extension results in these defects (Wallingford and Harland,

2001). Convergent extension is involved during gastrulation for establishing proper mesodermal structures (Keller, 2002). The notochord then induces neural differentiation in overlying ectoderm tissue (Murdoch et al., 2001). In the induced neural plate

convergent extension is required for neural tube closure at neurula stages in mice and X.

laevis (Copp et al., 2003; Kibar et al., 2001; Murdoch et al., 2001; Wallingford et al., 2002). PCP pathway components have been shown to be required for neural tube closure in both organisms (Copp et al., 2003; Kibar et al., 2001; Murdoch et al., 2001; Wallingford et al., 2002).

In embryos where H2A.Z function is perturbed (Ridgway et al., 2004a), improper

gastrulation disrupts normal neurulation. X. laevis H2A.Zdn embryos phenocopy the

most severe human NTD, where the craniorachischisis, cranial neural tube fails to close from the midbrain down the length of the spinal cord (Ueno and Greene, 2003; Wallingford and Harland, 2001). Craniorachischisis accounts for 10-20% of human NTDs (Berry et al., 1999; Murdoch et al., 2001; Seller, 1987). Mesodermal convergent

extension failure in X. laevis results in neural folds too separated to fuse; therefore

defects in mesodermal convergent extension are also causes for some neural tube defects (Keller, 2002). A similar phenotype has been seen in mice when the PCP

pathway is disrupted (Kibar et al., 2001; Wang et al., 2006). In mouse, Dv12 (a Dsh

homologue) is required for lengthening and narrowing of the neural plate consistent

with convergent extension, and dominant negative Dvl2s produce NTDs because the

wider gap between the neural folds prevents neural tube closure (Wang et al., 2006). While the blastopore appears to close in these mouse embryos notochord cell localisation is perturbed (Wang et al., 2006), indicating that convergent extension of

mesodermal tissues occurs in mammals as observed in X. laevis. X laevis embryos with

tissue with blocked PCP signalling onto embryos with normal mesoderm (Wallingford

and Harland, 2001). However, even when the morphological movements of X. laevis

models of NTD do not precisely reiterate the human aetiology the differences may be informative regarding the molecular control of the genes and signalling pathways

involved. I have shown that H2A.Z mRNA is enriched in mesoderm and notochord, not

the neural plate. H2A.Z is required for establishing convergent extension at gastrulation and maintaining it in the developing notochord. The developing neural tissue also undergoes convergent extension. H2A.Z may not have a role in neural convergent extension (it is not detectably enriched in this tissue) perhaps due to different details for the same convergent extension pathway(s) in the two tissue types or related to the

interplay of the PCP and Ca2+ mediated pathways during convergent extension.

6.3.2 Wider Implications

The pathways that determine developmental gene expression in the early embryo are also central to numerous biological processes throughout an organism’s life, including wound healing and the progression of cancer (Sato et al., 2004; Sparmann and van Lohuizen, 2006; Ueno and Greene, 2003; van Es et al., 2003). Embryonic development is therefore a model for the basic processes of multicellular life. Interestingly, a macroarray-based analysis identified H2A.Z as up regulated during tail regeneration in X. laevis larvae (Tazaki et al., 2005) indicating that the histone variant is utilised in later regulation of differentiation pathways. A better understanding of the regulation of developmental gene expression would improve our understanding of numerous biological processes and illnesses. Applications of this understanding would include new treatments based on the growth of specific differentiated cells to repair damaged tissues, and anti-cancer drugs with greater specificity for cancerous cells.