Recurrent immature Ig gene rearrangements in TP53-mutated B-precursor adult ALL

During the premature stage of lymphopoiesis, immunoglobulin (Ig) and T-cell receptor (TCR) genes undergo rearrangements — key mechanisms of the physiological process. Ig/TCR rearrangements have been recognized as leukemic clonal attributors, which can be used in measurable (minimal) residual disease (MRD) assessment to predict response to therapy and prognosis. Approximately 10% of adult patients diagnosed with acute lymphoblastic leukemia (ALL) are found to have TP53 anomalies at the time of diagnosis, a predictor of poor prognosis.

The aim of a study by Silvia Salmoiraghi and colleagues was to analyze the association of Ig/TCR genetic rearrangements with TP53 status in adult patients with Philadelphia chromosome-negative ALL. The results have been recently published in the journal Genes.¹

Methods:

• DNA samples were obtained from bone marrow or peripheral blood (containing 30% blast cells) at the time of diagnosis in 171 patients with ALL participating in the NILG-ALL 09/00 clinical trial.
• These samples were analyzed, with the cohort consisting of 114 patients with B-precursor ALL (negative for t(9;22) or BCR/ABL1 transcripts) and 57 with T-lineage adult ALL.
• Clonal Ig/TCR rearrangements were recognized via conventional or next-generation (NGS)-based methods.
• Polymerase chain reaction (PCR) amplification and Sanger sequencing verified TP53 mutations with an allele burden of > 20%.
• Quantitative PCR (referencing the hTERT gene) was employed for TP53 copy number evaluation in 158 DNA samples.

Results

The median age of patients was 34.6 years with 57% being male. Central nervous system (CNS) involvement was seen in 6%, and 53% had hepatosplenomegaly. Patient cytogenic profiles were normal in 56%, inconclusive in 26%, and adverse in 18%.

Implementation of an NGS-based technique led to the discovery of a greater number of molecular variations of TP53 mutations. 16 TP53 gene mutations were identified in 14 patients (8%) which were more frequent in patients with B-ALL than those with T-ALL (10% versus 5%).

As was shown previously,² relapse occurred earlier (within 15 months after achieving complete remission) and were more frequent (13 out of 14 patients relapsed) in patients with TP53 mutation status versus other patients. In keeping with this, leukemia-free survival and overall survival were poorer in these patients. Univariate and multivariate analysis identified age, CNS involvement, and white blood cell level at time of diagnosis as key factors predicting survival.²

A strong association was seen between incomplete Ig heavy chain (IgH) rearrangement and TP53-positive mutational status: 100% (10 out of 10) of patients with TP53-positive B-precursor ALL expressed DH/JH rearrangement compared with only 36% for the other subtype (p < 0.0001). Remarkably, while all patients with TP53-mutated B-lineage ALL expressed a DH/JH clonal variation, this was not the case for patients with T-ALL (0 out of 3).

Based on the European Group for the Immunological Characterization of Leukemias (EGIL) classification, patients with TP53-positive mutational status were more frequently categorized as pro-B ALL compared to non-TP53 mutated ALLs (46% versus 25%, p = 0.05).

Furthermore, in half of the patients (5 out of 10) with TP53-mutated B-ALL, DH/JH clonal rearrangement could be assessed for MRD at two consecutive timepoints during the study. MRD positivity was detected in at least at one sample for each of these patients, which suggests a link between TP53 mutation and high relapse risk.

Conclusion

The current study was able to confirm that TP53-positive mutation status is strongly associated with incomplete DH/JH rearrangement of the Ig receptor. Although the sample size was not large enough to draw more definitive conclusions, it is important to note that both events were also linked to poor outcome. As a result of this, TP53 mutation status screening at the time of diagnosis, as well as incomplete DH/JH rearrangements, may be viewed as an important part of the MRD work-up in patients with B-precursor ALL.

Though the rationale for the coexistence of TP53 mutations and immature Ig rearrangement needs further exploration, it can be deduced that the TP53 mutation develops during early B-cell maturation stages, or perhaps also at the stage of preleukemic transformation, similar to acute myeloid leukemia.