The relationship between number of interphase nors and nor-bearing chromosomes in non-Hodgkin's lymphoma.

The ribosomal genes (located on the acrocentric chromosomes 13-15, 21-22) may be identified by their silver stained gene products, i.e. NOR related proteins. The NOR bearing chromosome activity can be observed at metaphase with the potential for all ten chromosomes to be positively stained. On the other hand, during interphase they fuse so that eventually only a single silver positive structure is seen in resting normal cells. Investigations of histopathological sections of non-Hodgkin's lymphoma (NHL) have demonstrated a correlation between the numbers of interphase NORs and the grade of tumour. There is generally a higher number of interphase in high-grade, and a lower number in low-grade tumours. This histopathological and cytogenetic study of 13 patients with NHL shows that the higher numbers of interphase NORs in the high-grade tumours is not necessarily a reflection of increased numbers of NOR-bearing chromosomes. Examples were found of high-grade neoplasms, showing the expected high numbers of interphase NORs, but not an increased number of NOR-bearing chromosomes. Conversely, some low-grade tumours, with the expected low number of interphase NORs, had increased numbers of NOR-bearing chromosomes. Our conclusion is that the interphase NOR number is related to factors other than chromosome numbers. We suggest that NOR numbers at interphase may be related to cell turnover. This is supported by previous investigations using DNA flow cytometry and the monoclonal antibody Ki67.