B-cell precursor ALL results from a leukemic transformation of a lymphoid precursor at an early stage of B-cell differentiation and it is therefore not surprising that over 80% of both adult and pediatric cases carry an
IGH rearrangement and around 40% of them carry an
IGK rearrangement
[43][44]. As might be expected following the same logic, over 90% of T-ALL cases harbor a
TRB rearrangement
[45], over 80% a
TRG and almost 70% (adults) or 40% (children) a
TRD rearrangement
[43][44]. Exceptionally, however, there are also ALL cases with all IG/TR loci in germline configuration—those are most probably derived from very immature progenitor cells.
Although cross-lineage rearrangements have not been detected in human thymocytes and their frequency in B-cells is very low (<0.5%)
[46][47], these so-called illegitimate rearrangements have been identified in leukemic cells besides the lineage-consistent rearrangements:
IGH rearrangements in 22% of T-ALL cases
[35] and TR rearrangements in 80–90% of patients with B-ALL
[48][49]. Cross-lineage rearrangements in B-ALL have several special characteristics, compared to regular rearrangements:
TRB rearrangements contain particularly the most downstream Vb gene segments and solely the Jb2 segment,
TRG rearrangements involve Jg1 segments in 70% of cases, and 80% of
TRD rearrangements are represented by incomplete Vd2-Dd3 or Dd2-Dd3 junctions, which are rare in T-cells
[44][48][50][51][52].
TRB rearrangements are virtually absent in pro-B-ALL and in infants, and patients with complete
TRB gene rearrangements show a more mature IG/TR profile (higher frequency of
IGK,
TRG, and Vd2–Ja rearrangements)
[53]. Remarkably, the frequency of cross-lineage Vd2-Dd3 rearrangements significantly decreases with age at diagnosis, while cross-lineage
TRG rearrangements are rarely found in patients below 2 years of age
[27][41]. In T-ALL, the cross-lineage
IGH rearrangements are rather immature, as they are characterized by a high frequency of incomplete D-J rearrangements and frequent usage of most downstream Dh6-19 and Dh7-27 and most upstream Jh1 and Jh2 gene segments
[35]. Cross-lineage rearrangements are rare in mature B- and T-cell malignancies, probably due to the absence of recombinase activity
[54][55][56]. This corresponds with the reported decreasing incidence of cross-lineage
TRG rearrangements in more mature B-ALLs: pro-B (57%), common (47%), pre-B (42%), and mature-B (0%) ALL
[49]. Also, in more mature T-ALLs with biallelic
TRD deletions and completed
TRA rearrangements the
IGH gene rearrangements are virtually absent
[35]. In contrast to CLL, a mature B-cell malignancy, high incidence of non-coding/out-of-frame rearrangements was observed in ALL, suggesting that antigen selection pressure does not play a crucial role in ALL
[57].
In CLL, so called stereotyped B-cell receptors are a common phenomenon. Their complementarity-determining region 3 (CDR3) sequences are closely similar (share structural features like V-gene, length, amino acid composition) among unrelated cases, suggesting that stimulation by (auto)antigens may play a role in CLL pathogenesis
[58][59]. The
IGH/
IGK/
IGL repertoires in CLL are biased and differ from repertoires in normal B-cells
[60][61][62][63]. Additionally, certain V-segments (
IGHV3-21 and
IGHV1-69) are associated with poor outcome
[57]. Furthermore, presence of somatic mutations in variable heavy chain genes defines two CLL subtypes associated with a different clinical course. About half of CLL cases have more than 98% identity to the closest germline V-gene (“unmutated”), which corresponds to inferior outcome compared to patients with “mutated” CLL (less than 98% identity)
[64][65].