Association of Increased Thymidine Uptake Relative to Tumor Cell Proliferation in Low-Grade Follicular Lymphoma and DNA Repair.

Abstract 3236

Poster Board III-173

We have observed that the uptake of the thymidine analog ¹⁸F-fluorothymidine (FLT) is highly disproportional to cellular proliferation in untreated low-grade follicular lymphoma (FL). Since the uptake of thymidine/thymidine analogs is also increased with enhanced DNA repair synthesis, we investigated whether the “excess” uptake of FLT in FL may be related to DNA repair. We stained tumor samples from 20 patients each with grade 1 FL and DLBCL for Ki-67, a marker of cell proliferation and DNA replication but not DNA repair and 2 DNA replication and repair biomarkers: proliferating cell nuclear antigen (PCNA) and replication protein A (RPA). Median %Ki-67-positive cells (Ki-67 index) was 10% (range; 5-20%) in FL compared to 80% (range 60-90%) in DLBCL (P<4×10^-20). In contrast, median %positive cells for PCNA and RPA were 90% and 100% in FL and 100% and 100% in DLBCL. In both FL and DLBCL, PCNA showed a characteristic staining pattern with 3+ or 4+ staining of the proliferating Ki-67-positive cells vs. 1+ to 2+ staining of quiescent cells. Similar results were obtained when staining for thymidine kinase I (TK1). Interestingly, similar staining pattern to that seen in lymphoma samples was seen in germinal center but not mantle zone cells of normal tonsils and reactive lymph nodes indicating that the DNA repair seen in both FL and DLBCL is likely related to somatic hypermutations (SHM) generated by error-prone DNA repair known to occur in FL, most DLBCLs and normal germinal center cells. Comparison of FLT uptake in FL and DLBCL indicated that > 70% of FLT uptake in FLs with a Ki-67 index of ≤10% was due to DNA repair. In contrast, contribution of DNA repair to overall FLT uptake in DLBCL with a Ki-67 index of ≥ 80% was <10% presumably due to the 4-fold lower fraction of quiescent compared with proliferating cells with high replicative DNA synthesis. This is the first demonstration of a high contribution of SHM/DNA repair to overall uptake of thymidine/thymidine analogs in low-grade FL. Our data further suggest that FLT use for assessing response to cytostatic therapy in low-grade FL will be confounded by this high contribution and highlights the pitfalls associated with the use of PCNA or RPA to assess proliferative activity in FL and DLBCL.