Feasiblity and Implication of Bidirectional Sequencing Using a Multiplex Framework 2 Region Primer for Somatic Mutation Analysis of the Immunoglobulin (IgH) Heavy Chain in Chronic Lymphocytic Leukemia (CLL).

Background: A somatic mutation rate of >2% of the immunoglobulin (IgH) heavy chain gene, in comparison to germline DNA, is a positive prognostic indicator in chronic lymphocytic leukemia (CLL). Genomic DNA is amplified using the Biomed-2 multiplex PCR primer set, sequenced in the reverse direction only using a 3′ J_H primer, and compared to the germline sequence in VBase, a comprehensive directory of all human germline variable region sequences compiled from over a thousand published sequences. It is unknown whether forward sequencing using the multiplex 5′ Biomed-2 primers in a single reaction is feasible or whether doing so improves the accuracy of the mutational analysis.

Methods: DNA was extracted from peripheral blood or bone marrow mononuclear cells from patients with CLL using the QIAGEN QIAspin DNA mini-kit. The FR2 region of the IgH gene was amplified using tube B of the Biomed-2 multiplex PCR primer set, separated in 2% agarose gel, excised, and prepared for sequencing using the QIAGEN QIAquick gel extraction kit. Bidirectional DNA sequencing was performed with the full set of multiplex 5′ FR2 primers combined in one reaction for the forward sequence and in a second reaction with the Biomed-2 3′ J_H primer for the reverse sequence. A consensus sequence was obtained and mutation rates were calculated based on the consensus sequence and the reverse sequence separately.

Results: The FR2 region was amplified from 30 CLL patients. 20 samples produced bidirectional consensus DNA sequences, 2 sequenced only with multiplex FR2 primers, 6 sequenced only with a J_H primer, and 2 produced no sequences. Compared with unidirectional J_H sequencing, bidirectional sequencing did not reassign any patient in the germline IgH group to the mutated category. In 15 patients, the uni- and bi-directional sequencing were identical. In 4 patients, a single basepair difference was noted. A random permutation test yields a two-sided p-value as 12.5%, indicating that no significant difference was detected between the uni- and bi-directional sequencing. Multiplex bidirectional sequencing did provide sequence information within the CDR3 region at the 3′ terminus of the amplimer which is partially truncated when using the J_H primer alone.

Conclusion: Bidirectional sequencing does not provide a somatic IgH mutation rate that differs significantly from that obtained from the reverse sequence alone and does not likely influence the IgH mutational prognostic assignment. Nonetheless, the bidirectional consensus sequence adds bases in the CDR3 region which can be used for research applications such as the generation of patient-specific primers for PCR. It is also noteworthy that multiplex PCR using the Biomed-2 primers is feasible.