Composite low grade B-cell lymphomas with two immunophenotypically distinct cell populations are true biclonal lymphomas - A molecular analysis usinglaser capture microdissection

Low grade B-cell lymphomas comprise several well defined, clinically and im munophenotypically distinct disease entities. Composite lymphomas showing p henotypic characteristics of more than one of these tumor subtypes in the s ame site are rare, and both common and separate clonal origins of the two t umor parts have been reported for cases studied by molecular methods. We de scribe the detailed immunohistochemical and molecular findings in three cas es with features of composite low grade B-cell non-Hodgkin's lymphoma (B-NH L). All three neoplasms contained morphologically distinct but interwoven c ompartments of different cell, types, which exhibited discordant expression of several markers, including CD5, CD10, CD43, and cyclin D1, According to their morphology and phenotypes, they were classified as mantle cell lymph oma and follicular lymphoma (Case 1), follicular lymphoma and small lymphoc ytic lymphoma (Case 2), and mantle cell lymphoma and chronic lymphocytic le ukemia/small lymphocytic lymphoma (Case 3). PCR analysis of DNA obtained fr om whole tissue sections failed to reveal evidence for biclonality in any o f the cases. We therefore isolated cell populations with different antigen expression patterns by laser capture microdissection and analyzed them by p olymerase chain reaction amplification and sequencing of clonal immunoglobu lin heavy chain gene rearrangements and oncogene rearrangements. Sequence a nalysis revealed unrelated clonal rearrangements in each of the two tumor p arts in all three cases, suggesting distinct clonal origins. In addition, C ase 1 showed a bcl-2 rearrangement present only in the follicular lymphoma part. Our findings suggest that low grade B-NHL with two distinct morpholog ical and immunophenotypic patterns in the same anatomical site are frequent ly biclonal. This is in keeping with current classification schemes, which recognize subtypes of low grade B-NHL as separate disease entities. Further more, our analysis demonstrates the power of laser capture microdissection in revealing molecular microheterogeneity in complex neoplasms.