Abnormalities of chromosome 7q have rarely been reported in chronic B-cell lymphoproliferative disorders.1 Chromosomal translocation leading to overexpression of the CDK6 gene, which is located in 7q21, has recently been described in splenic marginal zone lymphoma (SMZL).2,3 CDK6 together with CDK4 are serine/threonine kinases that positively regulate the G1 phase progression in association with D-type cyclin. Overexpression of CDK6 is considered to contribute to leukemogenesis through dysregulation of cell proliferation, thereby favoring progression through the G1 phase. It seems likely that CDK6 overexpression is implicated in lymphomas with low proliferative index and, as we show here, may also be involved in other low-grade malignancies such as B-cell chronic lymphocytic leukemia (B-CLL). We have previously described the molecular characterization of a t(7;14)(q21;q32) observed in a case of B-CLL. The t(7;14) involved the immunoglobulin heavy chain (IgH) locus in 14q32 and a transposon-like element located 29 kbp upstream of the CDK6 coding sequence.4 However, no material was available for CDK6 expression analysis in this case. To analyze CDK6 involvement in B-CLL, we have examined the status of the CDK6 gene in bone marrow or blood samples from a further 8 adult patients with B-CLL carrying cytogenetic abnormalities (deletions, duplications, or translocations) involving bands 7q21 to 7q22 (1.5% of our B-CLL series).
B-CLL diagnosis was established according to the World Health Organization Classification of Tumours and was characterized by positivity for CD5 and CD23 expression. All the tumor samples were screened, where possible, by Southern blot, fluorescence in situ hybridization (FISH), reverse-transcriptase real-time quantitative polymerase chain reaction (RT-RQ-PCR), and immunoblotting. In 3 patients, involvement of the CDK6 gene was found by at least 2 different methods. The relevant clinical, biologic, and molecular data for these patients are given in Table 1. FISH and Southern blot analysis showed that an interesting common feature was that in all 3 cases the CDK6 gene was rearranged by reciprocal translocations involving either Igκ, Igλ, or IgH locus and not with only the Igκ light chain locus as has previously been reported in other series.2,3
Marked overexpression of CDK6 was observed in all 3 patients with CDK6 rearrangement compared with normal peripheral blood lymphocytes and with B-CLL without CDK6 involvement (Figure 1). This indicates that FISH and CDK6 expression analysis can be successfully used to identify patients with CDK6 involvement. As previously reported,2 CDK6 overexpression can result from aberrant variable-diversity-joining (VDJ) or variable-joining (VJ) recombination, leading to the juxtaposition of the CDK6 gene with the Ig gene enhancer during B-cell differentiation. In addition, our findings demonstrate the involvement of the CDK6 gene in B-CLL, underlying that those abnormalities are not restricted to SMZL.