Chronic Lymphocytic Leukemia Subset Expressing Mutated IGHV3-23 Has Peculiar Clinical and Biological Features.

In the last years, the B cell receptor (BCR) has become a key molecule in chronic lymphocytic leukemia (CLL), given the correlation between mutational status of immunoglobulin heavy chain variable (IGHV) genes and disease prognosis. Recently, a fraction of CLL has been shown to preferentially express specific IGHV genes, often in a non-random combination with homologous heavy chain complementarity-determining region-3 (HCDR3) and peculiar light chains. Some of these stereotyped BCR mark CLL subsets with peculiar clinical behavior regardless of IGHV mutations. These data suggest a role for BCR in defining the clinical and biological features of CLL, also beyond the mutational status of IGHV genes.

A HCDR3-driven clustering of 1,426 IG sequences (1,398 patients) was performed using ClustalX(1.83). Time to treatment (TTT) intervals, Rai staging, IGHV mutational status, CD38, ZAP-70, and karyotype abnormalities evaluated by FISH were available for 617 patients. Gene expression profiling (GEP) and quantitative real-time PCR experiments (QRT-PCR) were performed on purified CLL cells.

IGHV3-23 was totally absent in 71 identified stereotyped clusters despite being the second most frequently used IGHV gene, such distribution was significantly skewed (p<0.0001), compared with the distribution of IGHV genes belonging to stereotyped BCR clusters observed in our series. Although 109/134 IGHV3-23 were mutated (M), alignment of IGHV sequences revealed a high degree of conservation in the context of the 13 AA positions involved in superantigen binding by IGHV3 subgroup genes, suggesting that the majority of M IGHV3-23 cases maintained the capacity to mediate superantigen recognition and binding. Median TTT (73 months) of 43 M IGHV3-23 CLL was significantly shorter than median TTT (253 months, p=0.0153) of 333 M CLL, as well as of 326 M CLL in which 7 cases belonging to the bad prognosis IGHV3-21/IGLV3-21 cluster were excluded (253 months, p=0.0082). Multivariate Cox proportional hazard analyses selected IGHV3-23 usage (p=0.029), Rai stage (p<0.0001) and FISH group (p<0.0001) as independent markers of disease progression for 376 M CLL, and for the cohort in which 7 M CLL from the IGHV3-21/IGLV3-21 cluster were excluded. Comparing 5 M IGHV3-23 and 22 M non-IGHV3-23 CLL for their differential GEP, 212 genes were selected, 108 up-regulated and 104 down-regulated in M IGHV3-23 CLL. Using the “Gene-Ontology Tree Machine” platform, a set of growth/tumor suppressor genes (PDCD4, TIA1, RASSF5), all down-regulated in M IGHV3-23 CLL, was constantly found in several gene-ontology categories related to apoptosis. QRT-PCR confirmed a significant down-regulation of these genes in 15 M IGHV3-23 compared to 35 M non-IGHV3-23 CLL. Given the notion that PDCD4 and TIA1 are among the genes under control of miR-15a and miR-16-1 a “Gene Set Enrichment Analysis” carried out on the 212 differentially expressed genes, confirmed that M IGHV3-23 samples were significantly deprived in genes whose expression is under control of miR-15a and miR-16-1. Accordingly, QRT-PCR experiments performed on 15 M IGHV3-23 and 35 M non-IGHV3-23 CLL revealed significant higher levels of both miR-15a (p=0.0007) and miR-16-1 (p=0.0031) in M IGHV3-23 cases. No difference was found in the distribution of patients with 13q14 deletion between M IGHV3-23 CLL and M non-IGHV3-23 CLL (p=0.19). Considering the cases used for microRNA expression experiments (data available in 47/50 cases), 8/15 M IGHV3-23 CLL bore the 13q14 deletion in more than 20% of nuclei, against 19/32 cases in the group of M non-IGHV3-23 CLL (p=0.94).

Expression of IGHV3-23 marks a subset of M CLL with a worse prognosis; such a peculiar clinical behavior may be related to superantigen stimulation combined with down-regulation of specific growth/tumor suppressor genes and up-regulation of miR-15a and miR-16-1.

No relevant conflicts of interest to declare.