Analysis of Expressed and Non-Expressed IGK Locus Rearrangements in Chronic Lymphocytic Leukemia

Abstract

Immunoglobulin κ (IGK) locus rearrangements were analyzed in parallel on cDNA/genomic DNA in 188 κ-and 103 λ-chronic lymphocytic leukemia (CLL) cases. IGKV-KDE and IGKJ-C-intron-KDE rearrangements were also analyzed on genomic DNA. In κ-CLL, only 3 of 188 cases carried double in-frame IGKV-J transcripts: in such cases, the possibility that leukemic cells expressed more than one κ chain cannot be excluded. Twenty-eight κ-CLL cases also carried nonexpressed (nontranscribed and/or outof-frame) IGKV-J rearrangements. Taking IGKV-J, IGKV-KDE, and IGKJ-C-intron-KDE rearrangements together, 38% of κ-CLL cases carried biallelic IGK locus rearrangements. In λ-CLL, 69 IGKV-J rearrangements were detected in 64 of 103 cases (62%); 24 rearrangements (38.2%) were in-frame. Four cases carried in-frame IGKV-J transcripts but retained monotypic light-chain expression, suggesting posttranscriptional regulation of allelic exclusion. In all, taking IGKV-J, IGKV-KDE, and IGKJ-C-intron-KDE rearrangements together, 97% of λ-CLL cases had at least 1 rearranged IGK allele, in keeping with normal cells. IG repertoire comparisons in κ-versus λ-CLL revealed that CLL precursor cells tried many rearrangements on the same IGK allele before they became λ producers. Thirteen of 28 and 26 of 69 non-expressed sequences in, respectively, κ-or λ-CLL had < 100% homology to germline. This finding might be considered as evidence for secondary rearrangements occurring after the onset of somatic hypermutation, at least in some cases. The inactivation of potentially functional IGKV-J joints by secondary rearrangements indicates active receptor editing in CLL and provides further evidence for the role of antigen in CLL immunopathogenesis.

IGHV gene insertions and deletions in chronic lymphocytic leukemia: “CLL‐biased” deletions in a subset of cases with stereotyped receptors

“…It has been proposed that B and T cells have developed special mechanisms to detect and destroy non-functional transcripts (nonsense-mediated decay) in order to avoid production of truncated Ig and T cell receptor polypeptide chains [43,44]. Nonetheless, nonfunctional Ig transcripts (as in case GRE-D1 of the present series) have also been reported previously in CLL [45], as well as autoimmunity [46] or immunodeficiency [47], suggesting potential dysfunction of nonsense-mediated decay or other surveillance processes in the context of neoplasia/inflammation.…”

B cell receptor light chain repertoires show signs of selection with differences between groups of healthy individuals and SLE patients

“…Such transcripts include aberrantly rearranged and unrearranged V L genes (Delpy et al, 2004; Staudt and Lenardo, 1991). B cells were sorted on the basis of κ and λ isotypes (CD19+κ+λ-CD138- and CD19+κ-λ+CD138-), and the differences between these two populations are of interest because rearrangement of IGL genes and expression of the λ L-chain repertoire occurs after IGK rearrangement (Belessi et al, 2005; Brauninger et al, 2001; Hieter et al, 1981; Klein et al, 2005; Korsmeyer et al, 1981; Korsmeyer et al, 1982; van der Burg et al, 2001). This temporal difference could result in different selective forces acting on B cells bearing different isotypes.…”

“…This restriction can be attributed to the order and timing of recombination. Initial recombination occurs at IGK , and only if recombination of IGK fails will IGL undergo recombination (Belessi et al, 2005; van der Burg et al, 2001). While B cells have the potential to undergo many recombination events, the actual number of attempts at recombination is limited and exhaustive recombination does not occur (Louzoun et al, 2002; Mehr et al, 1999).…”

“…This observation suggests that the κ locus continues to prepare for recombination despite a B cell having a functional λ L-chain. If the κ-locus is deleted before recombination begins at λ (as is typical of λ-expressing cells) (Belessi et al, 2005; Brauninger et al, 2001; Hieter et al, 1981; Klein et al, 2005; Korsmeyer et al, 1981; Korsmeyer et al, 1982; van der Burg et al, 2001), then expression of any Vκ would be unexpected. However, deletion at the κ locus is primarily Cκ deletion.…”

Somatic Hypermutational Status and Gene Repertoire of Immunoglobulin Rearrangements in Chronic Lymphocytic Leukemia

“…TheratioofexpressedκandλlightchainsinCLLBlymphocytesmirrorsthatofnormalB cells (2:1) [97].AsisthecasewithIGHrearrangements,roughly50%ofIGK/IGLrearrangements belongtothemutatedsubtypeand,inmostcases,IGHandIGK/IGLrearrangements are of the same mutational status [98].AskewedusageofIGKV/IGLVandIGKJ/IGLJsubgroups and individual genes has been reported, but the interpretations of whether their rela-tivefrequenciesdifferfromthoseofnormalBcellsarediscrepant,probablyduetodifferent normal control datasets used for comparison. Similar to IGHV, the distribution of individual IGKV and IGLV genes between mutated and unmutated rearrangements is asymmetrical and,forsomegenes,CLL-biased.Inaddition,certainIGKV-IGKJandIGLV-IGLJcombinations are over-represented and CLL-related [97][98][99]. Importantly, non-stochastic pairing of heavy and light chains has been detected and shown to depend on VH CDR3 motifs [100].…”