Characterization of somatic mutations in nonneoplastic cells

Acquired genetic alterations, of which many are visible on the chromosome level, are believed to be necessary for tumorigenesis, and most tumors display characteristic patterns of recurrent structural and numerical aberrations. However, two of the most frequently occurring chromosome mutations in malignant and benign solid tumors, i.e., trisomy 7 and rearrangements of 12q13-15, have recently been reported as characteristic aberrations in reactive lesions, in particular in osteoarthritis (OA), suggesting that these mutations are important in nonneoplastic disease processes as well. In the present thesis, cytogenetic and molecular genetic techniques have been used to address different biological aspects of these two mutations. In the first study, multiple biopsies from OA-patients were independently short-term cultured and cytogenetically analyzed. The homogenous chromosome aberration pattern observed in all cell cultures from each patient ruled out the possibility that clonal chromosomal aberrations in OA-affected tissues are in vitro artifacts. In the second study, the clonal derivation of cells with +7 was investigated by performing X chromosome inactivation analysis of independent trisomy 7-enriched cell populations from OA-patients. The results suggested a polyclonal origin of cells with +7, and this conclusion was, in the third study, indirectly supported by the scattered and wide distribution of trisomy 7-carrying cells in tissue sections from patients with pigmented villonodular synovitis, OA, and different forms of synovitis. These combined data challenge the notion that the finding of clonal chromosome aberrations should be regarded as evidence for a monoclonal cell proliferation, characteristic for neoplasia. The study also showed that cells with +7 in OA have a mesenchymal origin, which together with the previously reported findings of trisomic clones being derived from epithelial, myeloid or endothelial progenitors, settle that gain of trisomy 7 occurs in different cell types in different settings. A further observation was that the frequency of +7 seemed to increase with the pathological state of the tissue. However, the findings of the fourth study suggested that the incidence of +7, in both nonneoplastic synovia and the majority of solid tumors, might increase with age rather than being associated with particular diseases. In the fifth and the sixth studies, the tumor-associated gene HMGIC was found to be involved in some rearrangements of the 12q13-15 region in OA-affected tissues. Moreover, transcription and translation of this gene frequently occurred in synovia, but the expression was not associated with breakpoints of the 12q13-15 region. These findings point out that disruption of HMGIC, as well as expression of the gene, are not restricted to true neoplasms.