Oligoclonality and Clonal Evolution

An oligoclonal pattern is defined as the presence of more bands than would be seen from a single monoclonal cell population, specifically, more than two bands per PCR. An oligoclonal pattern can be seen when there is expansion of several B-cell clones in reactive processes or in immuno-compromised individuals with a reduced B-cell repertoire, as well as in specimens with very few B cells. Oligoclonal patterns must be interpreted with care. In addition, rare patients will have two separate monoclonal cell populations; however, specific documentation of two separate B-cell populations by flow cytometry or immunophenotyping must be demonstrated to support the interpretation of two monoclonal populations.

During the course of the disease, the monoclonal population may develop additional IGH rearrangements that may alter the size of bands seen by IGH PCR or Southern blot (clonal evolution). If IGH PCR is used for MRD testing, clonal evolution may change or eliminate the diagnostic PCR band(s) and lead to false-negative results. IGLK gene rearrangements involving the kappa-deleting element (Kde) appear to be more stable than IGH gene rearrangements, possibly because they usually delete the two enhancers for the IGLK gene.45 However, these IGLK-Kde rearrangements are present in only a minority of BCL,lim-iting their clinical utility.

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