Status of the Immunoglobulin Heavy Chain and Light Chain Genes in Chronic Lymphocytic Leukemia and Related Disorders
Authors: Nakahashi Hirotaka , Karasawa Masamitsu , Saito Akio , Toyama Kohtaro , Mitsui Takeki , Hashimoto Yoko , Koiso Hiromi , Yokohama Akihiko , Uchiumi Hideki , Saitoh Takayuki , Handa Hiroshi , Tsukamoto Norifumi , Murakami Hirokazu , Nojima Yoshihisa 
Published: April 2020

Abstract
The incidence of chronic lymphocytic leukemia (CLL) is low in Asian countries including Japan, while it is the most common type of leukemia in western countries. It has been evident that the immunoglobulin heavy chain variable region (IGHV) gene mutation status can predict the prognosis in CLL; unmutated IGHV genes correlate with a worse prognosis than mutated genes. Over-representation of selected IGHV genes is noted in western CLL, in particular IGHV1-69, IGHV4-34, IGHV3-7, and IGHV3-21. Although their relative frequencies vary between cohorts, the most frequent gene in western countries is IGHV1-69, which is found in about 10–20% of all CLL patients. Several studies have shown very unusual Ig characteristics in CLL: highly restricted IGHV gene usage and very similar antigen-binding sequences (stereotyped antigen receptors), suggesting a role for antigen selection during the development and maintenance of the disease. For the purpose of clarifying the characteristics of CLL in the Japanese population, we analyzed both IGHV and Ig light chain (κ-chain, IGK and λ-chain, IGL) genes in 81 CLL cases and compared the findings with cases of 52 leukemic chronic lymphoproliferative disorders (CLPD) including 6 hairy cell leukemia (HCL), 1 prolymphocytic leukemia (PLL), 31 indolent lymphoma in leukemic phase (15 mantle cell lymphoma (MCL), 7 follicular lymphoma (FL), 5 splenic marzinal zone lymphoma (SMZL), and 4 lymphoplasmacytic lymphoma (LPL)) and 14 cases that could not be classified further. Of the 81 Japanese CLL patients, 17 (21.3%) had unmutated IGHV, and 63 (78.7%) had mutated IGHV. The number of CLL with mutated IGHV was at a higher frequency compared to previous reports from western countries. It may be partly explained by the fact that the commonly unmutated IGHV1-69 type was rare (1.2%), but the commonly mutated IGHV4-34 type was frequent (27.2%) in the Japanese CLL patients. We previously reported that IGHV1-69 CLL is rare in Japan (1/44), which is confirmed by the present study of newly diagnosed cases (0/37). Moreover, only 1 of 65 CLL patients was reported to use IGHV1-69 in China. These findings suggest that IGHV1-69 is extremely rare in Asia. Similar to reports from Scandinavian countries, IGHV3-21 cases showed biased λ-chain expression (5/6), but were not associated with overuse of IGLV3-21 (V2-14) in our cohort. Recently, studies of B-cell antigen receptors (BCRs) in patients with CLL identified that subsets of cases expressed almost identical BCRs. We also found a pair of CLL patients who had the same IGHV4-39, IGHD6-13, IGHJ5 (heavy chain) and IGKV1-39 (O12), IGKJ1 or 2 segment with remarkably similar H and L CDR3 sequences. The use of IGHV, IGKV and IGLV was significant different when compared between CLL and leukemic CLPD. IGHV4-34, which was the most preferentially used in CLL patients (21/81, 26.0%), was used rarely in CLPD patients (4/52, 7.7%, p = 0.007). Of the 4 CLPD patients with IGHV4-34, 3 were MCL (CD5+) and 1 was unclassified CLPD (CD5 −). As leukemic cells of all CLL cases were CD5+, only 1 of the 25 patients using IGHV4-34 had CD5 negative cells. In normal B-cell development, naive IGHV4-34 B-cells are positively selected and mostly restricted to the follicular mantle zone but these cells are largely excluded from the germinal centers. This mechanism may be relevant to IGHV4-34 usage being underrepresented in CLPD other than MCL, which mainly consisted of GC- or post-GC-derived lymphomas/leukemias. In CLPD patients, only 1 patient with SMZL used the IGHV1-69 gene. Interestingly, IGHV1-69 was associated with IGHD5-24, IGHJ3, IGKV3-20 and IGLKJ1, which have been previously identified to comprise one of the stereotypical BCR gene subsets in patients with CLL. In the CLL patients, IGKV3-11 (L6) and IGLV3-21 (V2-14) were the most frequent IGKV (7/43) and IGLV (11/35), respectively. However, in the CLPD patients, IGKV3-11 and IGLV3-21 were used by none (0/26, p = 0.03) and only 1 MCL patient (1/22, p = 0.002), respectively. To date little data has been obtained on CLL in Japan and other Asian countries, where the susceptibility to CLL is very low. Thus, it is important to investigate genetic and environmental differences between Asian and western countries to identify risk factors that give rise to this disease. In addition, a comparison of the disease features of CLL with other lymphoproliferative disorders will further elucidate the clinical and pathogenetic characteristics of CLL.