Implementing the Ion Torrent™ Oncomine™ Leader-J assay for testing IGHV somatic hypermutation status in CLL samples

The BCR IGH is coded for by multiple genes, including VDJ genes. Only productive and functional IG rearrangements are retained in B-cells. IGH gene rearrangements in B-cells undergo additional somatic mutations on exposure to antigen as part of the germinal center reaction in lymphoid tissues. SHM occurs in antigen-activated germinal center B cells and in the setting of CLL, SHM status is used to define distinct prognostic subgroups. By convention, the SHM status in CLL is reported on the dominant clonal population and is defined as >2% (mutated status) from the closest germline IGHV reference sequence. As per the ERIC guidelines, leader primers should be used for SHM assessment so that the whole IGHV region sequence is obtained. In order to conform to these guidelines Oncolab diagnostics took part in an early access Beta testing programme of the new Oncomine® Leader –J assay and compared our findings to our current FR1 assay. This was a comparison of the well established FR1 Invivoscribe ® Lymphotrack assay and the leader-J assay which was performed as part of an early access Beta testing programme. The Leader-J assay showed excellent concordance for variable mutation rate, SHM status. Leader primers crucial, especially in borderline mutated cases with one borderline case needing to be corrected from mutated to unmutated when using the leader-J assay. A cut-off of 98% for SHM is arbitrary in terms of clinical outcome with improved prognosis as the IGHV identity becomes increasingly different from the germline. SHM status remains important for motivation of therapy especially in unmutated cases. A slightly higher failure rate is reported when using a leader assay and an FR1 assay should be run as second line in these cases. This Leader-J assay performed well with excellent correlation to our current assay, is an easy to use and robust assay which provides accurate results across different sample types and allows multiplexing with improved TATs.