Genetic changes in lymphoid leukemia

Abstract

It is today generally accepted that the vast majority of malignant tumors arise as a result of genetic changes in critical genes involved in the tightly regulated control of cell growth and cell death. Despite the remarkable progress in the identification of such genes during the past two decades, many cancer genes probably remain to be identified. This thesis has focused on genetic changes in two B-cell malignancies, chronic lymphocytic leukemia (CLL) and hairy cell leukemia (HCL), where the pathogenesis still is largely undefined. More specifically genetic changes on chromosome 13 and chromosome 5, commonly observed in CLL and HCL, respectively, were studied. Identifying and characterizing the genes located a the site of these alterations may uncover the underlying mechanisms involved in the transformation of these malignancies.

Deletions involving 13q14 are found in more than half of all CLL cases, indicating the loss of a tumor suppressor gene. However, no specific inactivating mutations have been found in any gene in the region, suggesting that a non-classical tumor suppressor mechanism may be involved. Detailed analysis of the candidate genes RFP2/LEU5 and DLEU2, located Weill the deleted 13q14 region, lead to the hypothesis that the DLEU2 gene encodes a non-coding antisense RNA to RFP2/LEU5, since one DLEU2 exon directly overlaps the first exon of the RFP2/LEU5 gene in the opposite orientation. Furthermore, RFP2/LEU5 was found to be a bicistronic gene encoding two separate proteins, leu5 and dltet, thus adding to the complexity of the region. The fact that these complex structures are largely conserved in mouse further underlines the significance of the unusual organization of the critical CLL deletion region.

The frequent findings of chromosome 5 aberrations in HCL patients, often including the 5q13.3 and 5p13.1 bands, suggest the location of a putative pathogenic gene in these regions. A constitutional inversion found in one HCL patient, involving the same band', was used as the starting point in an attempt to identify gene(s) involved in the transformational events of HCL. The constitutional breakpoints at 5q13.3 and 5p13.1 were first mapped and cloned, and thereafter candidate genes were defined. Interestingly, one of the genes, the p53-induced ENC-1, was found to be highly expressed in HCL cells, suggesting a link between increased expression of ENC-1 and HCL. In addition, a novel putative cis-acting antisense gene (LIFR-AS) to the Leukemia Inhibitory Factor Receptor (LIFR) gene, was identified only 3 kb from the 5p13 breakpoint. None of the other candidate genes showed a differential expression or genetic alterations in HCL cells.

An improved insight into the genetic changes involved in the development of Cis- and HCL will not only increase our overall understanding concerning transformation but may also be used for refined diagnostics, prognostication and the design of future therapies.