Genetic diseases of the extracellular matrix: more than just connective tissue disorders.

The rapidly increasing knowledge about the molecular biology of the extracellular matrix has changed the concepts for the pathomechanisms of heritable connective tissue diseases. The spectrum of genetic matrix disorders is much broader than previously thought and now also includes diseases of organs such as the kidney, eye, and muscles. In addition, evidence is emerging that certain "acquired" diseases may be inherited, and that defects in signal transduction and patterning genes contribute to the pathology of connective tissue disorders. The phenotypes of genetic matrix disorders are determined by basic biological characteristics of the extracellular matrix. (a) The extracellular matrix occurs ubiquitously and is important for organ development and functions. (b) Matrix macromolecules are often large oligomers that polymerize into suprastructures at several hierarchic levels. They form insoluble fibrils or filaments that are further assembled into tissue suprastructures, for example, bundles or networks of fibrils. (c) Matrix suprastructures share characteristics with metal alloys. Tissue-specific mixtures of matrix molecules form specific arrays that differ from those of the pure components. Therefore the phenotypes of matrix diseases reflect a cascade of pathological events disturbing alloy formation, such as abnormal protein synthesis and folding, defective fibrillogenesis, and bundling, all capable of leading to abnormal cell-matrix interactions.