In severe forms of 2° hyperparathyroidism nodular formations within diffusely hyperplastic tissue are a frequent finding. This observation probably corresponds to the occurrence of a monoclonal type of cell proliferation within a tissue which initially exhibits polyclonal growth. Such clonal, benign tumoral growth was initially shown by Arnold et al using chromosome X-inactivation analysis method , and subsequently confirmed by other groups . In fact, one could also speak of multiclonal proliferation since several different clones may coexist in the same patient, and sometimes even in a single parathyroid gland (Figure 11). Acquired mutations of tumor enhancer or tumor suppressor genes are almost certainly involved in the development of such cell clones but precise knowledge about acquired genetic abnormalities remains limited . To identify new locations of parathyroid oncogenes or tumor suppressor genes important in this disease, Imanishi et al performed both comparative genomic hybridization (CGH) and genome-wide molecular allelotyping on a large group of uremia-associated parathyroid tumors . One or more chromosomal changes were present in 24% of tumors, markedly different from the values in common sporadic adenomas (28% and 72%, respectively), whereas no gains or losses were found in 76% of tumors. Two recurrent abnormalities were found, namely gain of chromosome 7 (9% of tumors) and gain of chromosome 12 (11% of tumors). Losses on chromosome 11, the location of the MEN1 tumor suppressor gene, occurred in only one uremia-associated tumor (2%), as compared to 34% in adenomas. The additional search for allelic losses with polymorphic microsatellite markers led to the observation of recurrent allelic loss on 18q (13% of informative tumors). Lower frequency loss was detected on 7p, 21q, and 22q. Interestingly, the cyclin D1 oncogene, activated and overexpressed by clonal gene rearrangement or other mechanisms in 20-40% of parathyroid adenomas, has not been found to be overexpressed in uremia-associated tumors.

Another interesting question was that of a possible involvement of somatic genes playing a major role in the normal reguation of parathyroid function, such as the CaR and VDR genes. The expression of these two genes was found to be decreased in the hyperplastic parathyroid tissue of uremic patients. The decrease was particularly marked in nodular areas, as compared to diffuse areas of parathyroid gland hyperplasia (Figure 4). Moreover, in uremic rats the decrease in CaR expression was inversely related to the degree of parathyroid cell proliferation . However, the search for mutations or deletions of the VDR gene or the CaR gene in uremic hyperparathyroidism has remained unsuccessful . The question remains unsolved at present whether the downregulation of CaR and VDR expression is a primary event or whether it is secondary to hyperplasia.

Whether benign parathyroid tumors may evolve towards malignant forms is still subject to debate. Since in dialysis patients parathyroid carcinoma is a rare event, malignant transformation of clonal parathyroid neoplasms must be exceptional.

Thus genome-wide allelotyping and CGH have directly confirmed the presence of monoclonal parathyroid neoplasms in uremic patients with refractory secondary hyperparathyroidism whereas the candidate gene approach has led to only modest results. Somatic inactivation of the MEN1 gene does contribute to the pathogenesis of uremia-associated parathyroid tumors, but its role in this disease appears to be very limited, and there is probably no role for DNA changes of the CaR and VDR genes. Recurrent DNA abnormalities suggest the existence of new oncogenes on chromosomes 7 and 12, and tumor suppressor genes on 18q and 21q, involved in uremic hyperparathyroidism. An analysis of hyperplastic parathyroid tissue from uremic patients by high-density oligonucleotide microarray technique led to the finding of 16 overexpressed and 132 repressed genes while subtraction library method produced 34 overexpressed and 40 repressed genes. Differentially expressed genes between diffuse and nodular tissue samples included genes related to DNA stability and repair, RNA stability and degradation, protein synthesis and processing, cell growth, and tumorigenesis and cell cycle . The patterns of somatic DNA alterations indicate that markedly different molecular pathogenetic pathways exist for clonal outgrowth in severe uremic hyperparathyroidism, as compared to common sporadic parathyroid adenomas. It must be pointed out that our group did not find a correlation between the presence of microscopically evident nodules and the clonal character of resected parathyroid tissue; appearances of several glands with histologic patterns of diffuse hyperplasia also were unequivocally monoclonal in the absence of detectable nodular formations, suggesting that the current criteria for pathological diagnosis do not reflect the genetic differences among these two pathohistological types.