Molecular basis of chronic lymphocytic leukemia: correlation between the immunoglobulin heavy chain mutational status and the expression of apoptotic genes

Chronic lymphocytic leukemia (CLL), the most common type of leukemia in Western countries, manifests as clonal expansion of mature CD5+ CD19+ CD23+ sIgMlow B lymphocytes and it is characterized by an extremely heterogeneous clinical course. Leukemic, as well as normal B lymphocytes, express immunoglobulin antigenic receptor at their surface. The structure of its heavy chain (IGH) variable region is being formed during B-cell differentiation, through rearrangements between IGHV, IGHD and IGHJ genes. After antigen encouner, B lymphocytes undergo the process of affinity maturation in germinal centers of secondary lymphoid follicles, durring which IGHV-IGHD-IGHJ rearrangements accumulate somatic hypermutations. It has been shown that the mutational status of rearranged immunoglobulin heavy variable genes (IGHV) represents the most reliable molecular marker in CLL, which defines two CLL subsets: M-CLL and U-CLL. The patients without or with a small percentage of IGHV mutations (UCLL) usually have more agressive disease and inferior prognosis in comparison to patients expressing mutated IGH rearrangements (M-CLL). Biased IGHV gene repertoire between MCLL and U-CLL clones, population differences in IGHV gene usage, as well as the expression of highly homologous, ˝stereotyped˝ rearrangements, strongly imply the role of antigenic stimulation in pathogenesis of CLL. CLL typifies the malignancy caused by defective apoptosis. Genetic alterations and aberrant expression of numerous proteins involved in extrinsic and intrinsic pathways of apoptosis regulation have been described in CLL. Reduced apoptotic capacity of CLL clones is, in part, caused by disturbances in the expression of Bcl2 family proteins. In this study, we analyzed IGHV mutational status and gene repertoire of IGHV-IGHD-IGHJ rearrangements in CLL patients. Furthermore, the expression of Bcl2 family genes, the key regulators of intrinsic apoptotic pathway, has been studied, in order to elucidate their role in resistance of CLL B lymphocytes to apoptosis. The expression levels of Bcl2, Bax and Bcl2L12 genes were measured by qRT-PCR, and association of their expression with clinical and molecular prognostic factors (IGHV mutational status, expression of CD38 and lipoprotein lipase) was analized. Bcl2L12 is a novel member of Bcl2 family of apoptotic proteins, whose pro- or anti-apoptotic function has not been elucidated yet. Besides the expression analysis of Bcl2L12 gene in CLL, one of the aims of this diss