Genetic studies of collagen types XV and XVIII : type XV collagen deficiency in mice results in skeletal myopathy and cardiovascular defects, while the homologous endostatin precursor type XVIII collagen is needed for normal development of the eye

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Overlapping genomic clones coding for the α1 chain of mouse type XV collagen (<em>Col15a1</em>) were isolated. The gene was found to be 110 kb in length and to contain 40 exons. Analysis of the proximal 5'-flanking region showed properties characteristic of a housekeeping gene promoter, and functional analysis identified cis-acting elements for both positive and negative regulation of <em>Col15a1</em> gene expression. The general exon-intron pattern of the mouse <em>Col15a1</em> gene was found to be highly similar to that of its human homologue, and comparison of 5'-flanking sequences indicated four conserved domains. The genomic area encoding the end of the N-terminal non-collagenous domain nevertheless showed marked divergence from the human form. Due to the lack of two exons coding for the N-terminal collagenous domain and a codon divergence in one exon, the mouse β1(XV) chain contains seven collagenous domains whereas the human equivalent contains nine.</p><p>In order to understand the biological role of this protein, a null mutation in the <em>Col15a1</em> gene was introduced into the germ line of mice. Despite the wide tissue distribution of type XV collagen, the null mice developed and reproduced normally and were indistinguishable from their wild-type littermates. After three months of age, however, microscopic analysis revealed progressive histological changes characteristic of myopathic disorder, and treadmill exercise resulted in greater skeletal muscle injury than in the wild-type mice. Irrespective of potential anti-angiogenic properties of type XV collagen-derived endostatin, the number of vessels appeared normal. Nevertheless, ultrastructural analyses revealed markedly abnormal capillaries and endothelial cell degeneration in the heart and skeletal muscle. Perfused hearts showed a diminished inotropic response, and exercise resulted in cardiac injury, changes that mimic early or mild heart disease. Thus type XV collagen appears to function as a necessary structural component for stabilizing cells with surrounding connective tissue in skeletal muscle cells and microvessels.</p><p>Mice lacking the type XV collagen homologue, type XVIII collagen, showed delayed regression of blood vessels in the vitreous body of the eye and abnormal outgrowth of the retinal vessels. This suggests that collagen XVIII plays a role in regulating vascular development in the eye. Moreover, type XVIII collagen was found to be important at the surface between the inner limiting membrane and the collagen fibrils of the vitreous body. <em>Col18a1</em> deficient mice serve as an animal model for the recessively inherited Knobloch syndrome, characterized by various eye abnormalities and occipital encephalocele. The results presented in this thesis indicate diverse biological roles for the closely related collagen types XV and XVIII.</p></div>