Identification and Characterization of Monoclonal Lymphoid Populations by Analysis of Complementary RNA of Clonespecific DNA Sequences

The detection of a monoclonal lymphoid population in a clinical specimen is highly suggestive for a neoplastic lymphoproliferative disease. There are only few disorders where mono- or oligoclonality is of benign origin and is not associated with malignancy. The detection of a clonal lymphocytic population is thus of great diagnostic interest. Traditionally, the molecular detection of clonal populations is performed by Southern blot analysis of rearranged T-cell receptor or immunoglobulin genes. However, Southern blot analysis cannot be performed on small biopsies of tissue, because of the requirement for sufficient high quality DNA. T-cell receptor (TCR) gamma genes rearrange in the majority of T-lineage acute lymphocytic leukemias (ALL). Southern blot analysis show rearrangements in about 60% of pre-B lineage ALL, but not in mature B-ALL [1–3]. Immunoglobulin heavy chain IgH genes rearrange in Band pre-B ALL but not in T-ALL [4]. Considerable diversity is created in the junctional region of rearranging variable (V) and joint (J) genes by imprecise recombination and by the insertion of random nucleotide sequences (N regions) [5–7]. Immunoglobulin heavy chain genes additionally recombine various diversity (D) genes [8]. The highest variation is found in the third complementarity determining region (CDR3) [9]. The unique junctional sequence of rearranging genes is a clonespecific “molecular fingerprint” of an individual lymphocytic clone. Rearranging immunoglobulin genes undergo secondary rearrangements probably predominantly by an exchange of the VH gene leading to a subclone formation in about one third of all cases with ALL [10, 11].