Cytogenetic and immunogenotypic alterations of blast crisis cells in chronic myelogenous leukemia independently linked to immunophenotypic expression

Deletions of immunoglobulin heavy chain and T cell receptor gene regions are uniquely associated with lymphoid blast transformation of chronic myeloid leukemia

Background

Chronic myelogenous leukemia (CML) results from the neoplastic transformation of a haematopoietic stem cell. The hallmark genetic abnormality of CML is a chimeric BCR/ABL1 fusion gene resulting from the Philadelphia chromosome rearrangement t(9;22)(q34;q11). Clinical and laboratory studies indicate that the BCR/ABL1 fusion protein is essential for initiation, maintenance and progression of CML, yet the event(s) driving the transformation from chronic phase to blast phase are poorly understood.

Results

Here we report multiple genome aberrations in a collection of 78 CML and 14 control samples by oligonucleotide array comparative genomic hybridization. We found a unique signature of genome deletions within the immunoglobulin heavy chain (IGH) and T cell receptor regions (TCR), frequently accompanied by concomitant loss of sequences within the short arm regions of chromosomes 7 and 9, including IKZF1, HOXA7, CDKN2A/2B, MLLT3, IFNA/B, RNF38, PAX5, JMJD2C and PDCD1LG2 genes.

Conclusions

None of these genome losses were detected in any of the CML samples with myeloid transformation, chronic phase or controls, indicating that their presence is obligatory for the development of a malignant clone with a lymphoid phenotype. Notably, the coincidental deletions at IGH and TCR regions appear to precede the loss of IKZF1 and/or p16 genes in CML indicating a possible involvement of RAG in these deletions.

The spectrum of adult B-lymphoid leukemias with BCR-ABL: molecular diagnostic, cytogenetic, and clinical laboratory perspectives

The Philadelphia (Ph) chromosome is characteristic of chronic myelogenous leukemia (CML), but it is also the most frequent cytogenetic abnormality in precursor B-lymphoblastic leukemia (ALL) of adults. The vast majority of CML patients have a BCR-ABL translocation that yields a 210 kD (p210) oncoprotein, whereas adult Ph-positive ALL cases can present with either a p190 or a p210 oncoprotein, or both. Considering that 30% of the patients with CML that progress to blast crisis will have a lymphoblastic presentation, adults presenting with a p210 ALL may have either a de novo ALL or CML presenting for the first time in lymphoblastic phase. To identify the distinguishing features, cases of p190-ALL, p210-ALL, and lymphoblastic CML were compared. In spite of significant overlap between the three entities, a number of features were found to aid in their differentiation. p210-ALL patients present at a younger age with blasts that frequently show loss of expression of CD34, whereas p190-ALL patients present with marked increase in peripheral blast percentage. Interestingly, bone marrow findings characteristic of a myeloproliferative disorder are specific, but are not sensitive for lymphoblastic CML. This study suggests that despite the similarities between these leukemias, p190-ALL, p210-ALL, and lymphoblastic phase CML likely represent three distinct diseases.

Phenotype of blasts in chronic myeloid leukemia in blastic phase-Analysis of bone marrow trephine biopsies and correlation with cytogenetics

We identified different phenotypic subsets among 62 cases of chronic myeloid leukemia (CML) in blast crisis (BC) (26% B-lymphoblastic, and 74% various myeloblastic subsets) on bone marrow trephines and correlated the blast-phenotype with cytogenetics. Five of myeloid-BC had an associated 3q26 abnormality and two of these showed a megakaryoblastic-phenotype. While myeloid-BC was associated with additional copies of Philadelphia (Ph) (29%) (p=0.08), numerical abnormalities (51%) (p=0.007), trisomy-8 (29%) (p=0.08) and 17p-loss (22%), none of lymphoid-BC showed these abnormalities. Among myeloid-BC, CD34-negative cases were more often associated with trisomy-8, 17p-loss and numerical abnormalities, and the CD117-negative subset with additional copies of Ph (p<0.05).

Cytogenetic and molecular genetic evolution of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is genetically characterized by the presence of the reciprocal translocation t(9;22)(q34;q11), resulting in a BCR/ABL gene fusion on the derivative chromosome 22 called the Philadelphia (Ph) chromosome. In 2-10% of the cases, this chimeric gene is generated by variant rearrangements, involving 9q34, 22q11, and one or several other genomic regions. All chromosomes have been described as participating in these variants, but there is a marked breakpoint clustering to chromosome bands 1p36, 3p21, 5q13, 6p21, 9q22, 11q13, 12p13, 17p13, 17q21, 17q25, 19q13, 21q22, 22q12, and 22q13. Despite their genetically complex nature, available data indicate that variant rearrangements do not confer any specific phenotypic or prognostic impact as compared to CML with a standard Ph chromosome. In most instances, the t(9;22), or a variant thereof, is the sole chromosomal anomaly during the chronic phase (CP) of the disease, whereas additional genetic changes are demonstrable in 60-80% of cases in blast crisis (BC). The secondary chromosomal aberrations are clearly nonrandom, with the most common chromosomal abnormalities being +8 (34% of cases with additional changes), +Ph (30%), i(17q) (20%), +19 (13%), -Y (8% of males), +21 (7%), +17 (5%), and monosomy 7 (5%). We suggest that all these aberrations, occurring in >5% of CML with secondary changes, should be denoted major route abnormalities. Chromosome segments often involved in structural rearrangements include 1q, 3q21, 3q26, 7p, 9p, 11q23, 12p13, 13q11-14, 17p11, 17q10, 21q22, and 22q10. No clear-cut differences as regards type and prevalence of additional aberrations seem to exist between CML with standard t(9;22) and CML with variants, except for slightly lower frequencies of the most common changes in the latter group. The temporal order of the secondary changes varies, but the preferred pathway appears to start with i(17q), followed by +8 and +Ph, and then +19. Molecular genetic abnormalities preceding, or occurring during, BC include overexpression of the BCR/ABL transcript, upregulation of the EVI1 gene, increased telomerase activity, and mutations of the tumor suppressor genes RB1, TP53, and CDKN2A. The cytogenetic evolution patterns vary significantly in relation to treatment given during CP. For example, +8 is more common after busulfan than hydroxyurea therapy, and the secondary changes seen after interferon-alpha treatment or bone marrow transplantation are often unusual, seemingly random, and occasionally transient. Apart from the strong phenotypic impact of addition of acute myeloid leukemia/myelodysplasia-associated translocations and inversions, such as inv(3)(q21q26), t(3;21)(q26;q22), and t(15;17)(q22;q12-21), in CML BC, only a few significant differences between myeloid and lymphoid BC are discerned, with i(17q) and TP53 mutations being more common in myeloid BC and monosomy 7, hypodiploidy, and CDKN2A deletions being more frequent in lymphoid BC. The prognostic significance of the secondary genetic changes is not uniform, although abnormalities involving chromosome 17, e.g., i(17q), have repeatedly been shown to be ominous. However, the clinical impact of additional cytogenetic and molecular genetic aberrations is most likely modified by the treatment modalities used.

Cytogenetics of chronic myeloid leukaemia

The standard Philadelphia (Ph) translocation t(9;22), its variants and a proportion of Ph-negative cases are positive for the BCR-ABL fusion gene, as determined by molecular analysis. Extensive deletions of chromosome 9 and 22 derived sequences around the translocation breakpoints on the derivative 9 are seen in 10-30% of patients at diagnosis and may confer a worse prognosis. Additional cytogenetic changes can occur in the few months before or during disease progression and are often specific for blast morphology; however, the molecular basis of the most common additional cytogenetic abnormalities is largely unknown. Cytogenetics is important for monitoring patient response to treatment but is increasingly being replaced by the more sensitive and less invasive techniques of RT-PCR and FISH.