Clinical relevance of immunoglobulin mutation analysis

[Molecular analysis of immunoglobulin genes in the tumor B cells in splenic marginal zone lymphoma]

AIM

To determine the immunoglobulin variable heavy chain (gVH) genes and their somatic mutations and to compare these data with the clinical and laboratory parameters of patients and the outcomes of the disease.

SUBJECTS AND METHODS

The investigation enrolled 24 patients (9 men and 15 women whose age was 32 to 77 years (median age, 60 years) with splenic marginal zone B-cell lymphoma (SMZBCL). The latter was diagnosed on the basis of histological and immunohistochemical examinations of a bone marrow trephine biopsy specimen, immunophenotyping of peripheral lymphoid cells or bone marrow aspirates.

RESULTS

The mutational status of the IgVH genes was analyzed in all the 24 patients. It was found that the tumor cells contained mutated IgVH genes (the VH1-family genes participated in most cases) in 15 (62.5%) patients with SMZBCL and unmutated ones in 9 (37.5%). There was a tendency towards more common tumor progression in patients with unmutated IgVH genes than in those with mutated ones.

CONCLUSION

The presence of SMZBCL cases with both mutated and unmutated IgVH genes and the higher frequency of the VH1-2 gene are likely to indicate the molecular heterogeneity of the origin of this lymphoma.

Ig gene diversification and selection in follicular lymphoma, diffuse large B cell lymphoma and primary central nervous system lymphoma revealed by lineage tree and mutation analyses

Follicular lymphoma (FL), diffuse large B cell lymphoma (DLBCL) and primary central nervous system lymphoma are B cell malignancies. FL and DLBCL have a germinal center origin. We have applied mutational analyses and a novel algorithm for quantifying shape properties of mutational lineage trees to investigate the nature of the diversification, somatic hypermutation and selection processes that affect B cell clones in these malignancies and reveal whether they differ from normal responses. Lineage tree analysis demonstrated higher diversification and mutations per cell in the lymphoma clones. This was caused solely by the longer diversification times of the malignant clones, as their recent diversification processes were similar to those of normal responses, implying similar mutation frequencies. Since previous analyses of antigen-driven selection were shown to yield false positives, we performed a corrected analysis of replacement and silent mutation patterns, which revealed selection against replacement mutations in the framework regions, responsible for the structural integrity of the B cell receptor, but not for positive selection for replacements in the complementary determining regions. Most replacements, however, were neutral or conservative, suggesting that if at all selection operates in these malignancies it is for structural B cell receptor integrity but not for antigen binding.

Site-specific translocation and evidence of postnatal origin of the t(1;19) E2A-PBX1 fusion in childhood acute lymphoblastic leukemia

The t(1;19) translocation yields a fusion between E2A and PBX1 genes and occurs in 5% of acute lymphoblastic leukemia in children and adults. We used chromosomal translocations and Ig heavy chain (IGH)/T cell antigen receptor (TCR) rearrangements to develop an understanding of the etiology and natural history of this subtype of leukemia. We sequenced the genomic fusion between E2A and PBX1 in 22 preB acute lymphoblastic leukemias and two cell lines. The prenatal origin of the leukemia was assessed in 15 pediatric patients by screening for the clonotypic E2A-PBX1 translocation in neonatal blood spots, or Guthrie cards, obtained from the children at the time of birth. Two patients were determined to be weakly positive for the fusion at the time of birth, in contrast to previously studied childhood leukemia fusions, t(12;21), t(8;21), and t(4;11), which were predominantly prenatal. The presence of extensive N-nucleotides at the point of fusion in the E2A-PBX1 translocation as well as specific characteristics of the IGH/TCR rearrangements provided additional evidence for a postnatal, preB cell origin. Intriguingly, 16 of 24 breakpoints on the 3.2-kb E2A intron 14 were located within 5 bp, providing evidence for a site-specific recombination mechanism. Breakpoints on the 232-kb PBX1 intron 1 were more dispersed but highly clustered proximal to exon 2. In sum, the translocation breakpoints displayed evidence of unique temporal, ontological, and mechanistic formation than the previously analyzed pediatric leukemia translocation breakpoints and emphasize the need to differentiate cytogenetic and molecular subgroups for studies of leukemia causality.

The configuration of the immunoglobulin genes in B cell chronic lymphocytic leukemia

B cell chronic lymphocytic leukemia (CLL) lacks a consistent genetic abnormality. However, immunoglobulin V(H) gene segment mutation analysis has provided insights into the pathogenesis of these diseases and allowed the development of powerful prognostic markers. Immunoglobulin gene chromosomal translocations are rare in CLL and involve a distinct subset of genes including BCL3, BCL11A and CCND2. BCL2 translocations in CLL appear to arise via a different mechanism from comparable translocations seen in B cell non-Hodgkin lymphoma.

Analysis of the IgV(H) somatic mutations in splenic marginal zone lymphoma defines a group of unmutated cases with frequent 7q deletion and adverse clinical course

This study aimed to correlate the frequency of somatic mutations in the IgV(H) gene and the use of specific segments in the V(H) repertoire with the clinical and characteristic features of a series of 35 cases of splenic marginal zone lymphoma (SMZL). The cases were studied by seminested polymerase chain reaction by using primers from the FR1 and J(H) region. The results showed unexpected molecular heterogeneity in this entity, with 49% unmutated cases (less than 2% somatic mutations). The 7q31 deletions and a shorter overall survival were more frequent in this group. Additionally a high percentage (18 of 40 sequences) of SMZL cases showed usage of the V(H)1-2 segment, thereby emphasizing the singularity of this neoplasia, suggesting that this tumor derives from a highly selected B-cell population and encouraging the search for specific antigens that are pathogenically relevant in the genesis or progression of this tumor.