Introduction: NLABRs are frequently observed in cancer free-subjects. We recently observed that NLABR-positive clones can persist up to 60 days (Ladetto et al, J Clin Oncol 2003). However the long-term kinetics and potential pre-neoplastic role of NLABR-carrying cells are unknown. To define the natural history of NLABR-positive clones, long term monitoring of cancer-free subjects carrying these lesions has been performed.

Methods: 118 subjects undergoing periodical blood examinations for warfarin therapy were screened for the bcl-2/IgH translocation. PCR-positive subjects underwent subsequent monitoring at least once every three months. NLABR-positive clones were monitored using both nested and real time-PCR according to previously published approaches (Ladetto et al Exp Hematol 2001). Sequence homology of NLABRs has always been confirmed by direct sequencing of nested PCR products.

Results: 15 NLABR-positive subjects were identified out of 118 (12.7%) subjects. NLABR-positive subjects were monitored for a median time of 13 months (mos) (range 3–30 mos) for a total number of 60 timepoints. In eight subjects (53%), NLABRs detected at study initiation were not detected again in follow-up samples. These eight subjects have been monitored for median period of 12 mos (range 3–28 mos). Follow-up samples in this group were usually PCR-negative, although transient PCR-positivity due to unrelated NLABRs were noticed in two samples. In seven subjects (47%), the same NLABR observed at study initiation was detected one or more times at follow-up. In four subjects, NLABRs detected at diagnosis were amplified in every available follow-up sample (three to seven samples were available for each subject). In three, NLABRs detected at diagnosis were amplified only in a fraction of follow-up samples while the remaining were PCR-negative. Overall, persistent NLABRs were followed on these subjects for a median time of 15 months (range 3–30). The median burden of persistent NLABRs assessed by real-time PCR was 33 rearrangements (rg)/106 diploid genomes (dg) (range <10–760), while the median burden of short-lived NLABRs was <10rg/106 dg (range <10–330). The number of NLABR-positive cells appeared to be rather stable in subjects with persistent NLABR-positive clones. In none of these subjects we could detect differences greater than 1 log among available follow-up samples. Subjects having mixed PCR-positive and PCR-negative results had a smaller tumor burden compared to those constantly PCR-positive. This is consistent with the presence of a small though persistent clonal population. Studies on selected populations showed that NLABR-positive cells were CD19-positive.

Discussion: NLABR-positive clones are long-lived cell populations in approximately 50% of cases. Based on this finding it is reasonable to hypothesize the existence of a follicular lymphoma (FL)-related lymphoproliferation of undetermined significance. Since NLABRs occurs in more than than 10% of healthy subjects, this condition is expected to be highly prevalent in the general population (as observed in MGUS and CLUS) and of potential relevance for the pathogenesis of FL.

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