Immunological and Molecular Classification of Human Leukemias

Abstract

The combination of immunological and molecular analyses represents the most accurate tool presently available for a more precise classification of human leukemia cells. In this study we report our findings, using this combined approach, in the different forms of acute and chronic leukemia. All cases of common acute lymphoblastic leukemia (cALL) studied showed (with only one exception) a heavy chain Ig gene rearrangement; a light chain gene reorganization was found in 30% of the cases. The B-cell origin was further confirmed by the expression of at least one B-cell associated antigen (0KB2, B4, Leul2, BA-1, Bl). The molecular analysis was of particular value in the rare cases of non-T, non-B, non-common or “null” ALL (TdT+, HLA-Dr+). All 9 cases so far tested showed in fact a heavy chain Ig gene rearrangement; 3 cases also had a light chain gene reorganization. These findings, coupled to the expression in most cases of other B-cell antigens, indicates that most if not all cases of TdT+, HLA-Dr+ “null” ALL are characterized by the proliferation of early Bcells. A germ-line configuration has instead been so far found in all cases of T-ALL studied. Similarly, in all but one of the 25 cases of acute myeloid leukemia studied the Ig genes were in a germ-line configuration. In one case, however, an unexpected heavy and light (k) Ig gene rearrangement was found. Interestingly a heavy chain gene rearrangement was documented in both cases of TdT+ AML studied.

In chronic leukemias the analysis of the configuration of the Ig genes has helped to clarify the B-cell origin of hairy-cell leukemia; in our series all 6 cases had a novel configuration of the heavy and light Ig genes. Furthermore, molecular analysis may be relevant in the non infrequent cases of B-cell chronic lymphocytic leukemia (B-CLL) in which the membrane bound Ig are not visible and in which DNA assessments may help to demonstrate the monoclonal B-cell proliferation as well as the Ig light chain involved.

In conclusion, molecular studies, coupled to extensive immunotyping are a most valid combination for the identification of the cell lineage and monoclonality of individual cases of difficult classification. The reproducibilty of the assay is confirmed by the very low percentage of non-B cell leukemias showing an unexpected gene rearrangement.

The recognition of the heterogeneity of acute and chronic leukemias has represented an important achievement in our understanding of the biological properties of the neoplastic cells and of their relationship with the putative normal counterparts. A more precise characterization of the pathological cells, mainly with the aid of immunological techniques both with conventional assays and more recently with monoclonal antibodies (McAb), has enabled a further subdivision of the lymphoid leukemias in different clinical entities which display well-defined features. In many cases this has also led to relevant prognostic and therapeutical implications. Over the last few years significant progress has been made in the classification of lymphoid leukemias following the combined use of McAb and of molecular analyses at the DNA level. In particular, the recognition that in the process of B-cell differentiation the first event appears to be a novel organization in the configuration of the immunoglobulin (Ig) chain genes, which gives rise first to a heavy chain gene rearrangement, and then to a rearrangement of the light chain genes, k preceding λ (1–3), has introduced a most valuable tool in the definition of the cell lineage of several lymphoproliferative disorders (4–9). Thus, it has been demonstrated that the majority, if not all, cases of common acute lymphoblastic leukemia (cALL) are characterized by the neoplastic expansion of immature B-

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cells (4,5,9). This has also been confirmed on the basis of the expression of Bcell associated antigens either on primary or on phorbol diester (TPA)-induced leukemic cells (9,10).

The evidence of the rearranged configuration of the Ig genes has also been of great value in clarifying the B-cell origin of hairy-cell leukemia (HCL) (11), the cell lineage of which had for long time remained controversial, despite its reactivity with several anti-B McAb (12–14).

In the present study we report our experience on the pattern of Ig gene arrangement in patients with different forms of acute and chronic leukemia; particular attention will be focused on the relevance of molecular analyses in the definition of the cell lineage of non-T acute leukemia.