Multiplex Quantitative Real-Time PCR for the Detection of t(14;18) Translocations with Breakpoints within 5 Different Regions of the BCL-2 Gene: MBR, 3′MBR, mcr, 5′mcr, icr.

Follicular lymphomas (FL) are associated with the chromosomal translocation t(14;18)(q32;q21). Most breakpoints of chromosome 18 (60%) occur in the major breakpoint region (MBR) of the BCL-2 gene. Further breakpoints have been detected in the minor cluster region (mcr), less frequent breakpoints are found in regions called 3′-MBR, 5′-mcr and icr. On chromosome 14 most breakpoints are located within one of the six JH-genes. Therefore, BCL-2 translocations with breakpoints within the MBR and mcr are generally detected by PCR using combinations of different BCL-2 primers with one JH-consensus primer. We have developed a multiplex quantitative real-time PCR strategy that that can be used to detect t(14;18) translocations with breakpoints located within all regions mentioned above. To minimize the costs for expensive probes we used the JH-consensus sequence as a target for one “consensus probe” (fluorescent labelled minor groove binder probe) for all assays in combination with 6 different JH intron primers. To reduce the size of amplified PCR fragments 12 BCL-2 primers were chosen in combination with 6 JH intron primers for the detection of all 5 breakpoint regions. It is very important to choose short DNA target sequences for amplification: (a) to establish a real-time PCR with a high amplification efficacy; (b) to be able to amplify target sequences also from partially degraded DNA isolated from formaldehyde-fixed paraffin-embedded tissue sections; (c) to achieve a high sensitivity to detect 1–3 copies per assay.

Peripheral bood mononuclear cells (PBMNC) and formalin fixed, paraffin embedded lymph node tissue obtained from 139 FL patients at the time of diagnosis (LN and PBMNC, n = 54; LN only, n = 3; PBMNC only, n = 82) were tested by multiplex quantitative real-time PCR. 80 breakpoints were identified within the MBR (61%) region. For comparison, 78/80 breakpoints were also detected by our standard real-time PCR assay with one BCL-2-MBR- primer and one JH consensus primer in combination with a fluorescent probe located within the BCL-2 sequence [Doelken et al., BioTechniques, 1998]. Two additional translocations with breakpoints located 5′ of the target sequence of the standard PCR were found by using two additional MBR primers. In addition, five mcr breakpoints (5%), one breakpoint in the 3′MBR region and one breakpoint in the icr region were found. Based on these results the prevalence of breakpoints in various regions of the BCL-2 gene in FL patients is: MBR = 61% (80/139); mcr = 5% (5/139); 3′MBR = 1% (1/139); icr = 1% (1/139); 5′mcr = 0%). Furthermore, based on quantitative PCR results the t(14;18) translocations detected in this study were undoubtedly lymphoma associated and did not belong to t(14;18)-positive non-lymphoma B cell clones found in healthy persons.

By applying this multiplex quantitative real-time PCR strategy t(14;18) translocations with breakpoints in five different breakpoint clusters can be detected in about 70% of patients with follicular lymphoma. The assays can be used for a fast and reliable quantitative detection of t(14;18) translocations on DNA isolated from fresh lymph nodes or pathological specimens as well as blood samples at the time of diagnosis. In almost all cases quantitative results will allow a distinction whether the translocation found is lymphoma associated or not, which will in turn allow a quantitative MRD analysis on follow-up samples during and after treatment.