Functional study of the type XVIII collagen

Author(s)

Oscar Takeo Suzuki

• Document type: Doctoral Thesis
• Press: São Paulo. , gráficos, ilustrações, tabelas.
• Institution: Universidade de São Paulo (USP). Instituto de Biociências (IBIOC/SB)
• Defense date: 2006-08-04

Examining board members

Maria Rita dos Santos e Passos Bueno; Luciana Amaral Haddad; Helena Bonciani Nader; Luis Eduardo Soares Netto; Telma Maria Tenorio Zorn

• Advisor: Maria Rita dos Santos e Passos Bueno
• Field of knowledge: Biological Sciences - Genetics
• Indexed in: Banco de Dados Bibliográficos da USP-DEDALUS; Biblioteca Digital de Teses e Dissertações - USP
• Location: Universidade de São Paulo. Biblioteca do Instituto de Biociências; IB/D-1219

Abstract

Knobloch syndrome (KS) is an autosomal recessive disorder characterized by ophthalmological defects and presence of an occipital encephalocele. Clinical variability is present, however, all patients present high grade myopia, vitreoretinal degeneration and in most cases, retinal detachment; the occipital defect is also variable. Studies show that the KS is caused by mutations in COL18A1, the gene that codes for type XVIII collagen. This collagen is an extracellular matrix proteoglycan and has been the focus of a great number of studies due to its C-terminal domain, endostatin. Endostatin is a 20 kDa fragment that is proteolytically cleaved and possesses a high antiangiogenic activity. Type XVIII collagen is known to be expressed in three isoforms, different among themselves in the N-terminal region. These isoforms have distinct expression patterns, but are present in most basement membranes. Besides endostatin, type XVIII collagen also presents other domains with unknown functions: a thrombospondin domain, found in all isoforms; a frizzled domain, present in the longest isoform. The clinical variability spectrum in KS and the molecular mechanisms that lead to the phenotype are still uncertain. The aim of this study was to identify novel mutations in COL18A1 in additional KS families, to develop biochemical diagnostic tests that could allow the screening of a larger number of patients and to evaluate the effect of naturally found variants in the function of endostatin. We also performed a two-hybrid screening in order to identify proteins that can interact with the thrombospondin domain. The characterization of seven novel mutations in KS patients allowed us to better determine the clinical variability of KS. This work shows for the first time the presence of neuronal migration defects in some KS patients. The lack of detected pathogenic mutations in three families led us to propose the genetic heterogeneity of this syndrome. We demonstrate the possibility to use immunohistochemistry in skin biopsies as a diagnosis method. Our results also show the altered properties of T48 endostatin in its interaction with some extracellular matrix proteins. The N104 variant, that has been previously associated with prostate cancer, do not present any change in its interaction to the tested molecules. Finally, the two-hybrid system was not a good method to detect interacting proteins with the thrombospodin domain of collagen XVIII. (AU)