Rare e14a3 (b3a3) BCR-ABL fusion in chronic myeloid leukemia in India: the threats and challenges in monitoring minimal residual disease (MRD)

Objective: The primary objective of this work was to confirm the occurrence of rare BCR ABL fusion variant involving the a3 region of the ABL gene in a patient positive for t(9;22) translocation but negative for common major and minor breakpoint cluster regions and the challenges and threats that it poses in a routine laboratory setting which use commercial kits for monitoring the minimal residual disease.

Methods: A patient with elevated white blood cell count was subjected to classical cytogenetics, FISH as well as RT-PCR testing using commercial kits as well as published primers and in house testing protocol. PCR amplicon generated from in the process was sequenced and analyzed.

Results: The translocation event in chromosome 9 and 22 could be successfully detected. BCR/ABL dual color, dual fusion probe generated a classical balanced translocation scenario within the nucleus of affected cells and presented a '1O1G2F' signal pattern. RT-PCR with probes from commercial kit designed to detect common breakpoints within the M- and m-BCR regions involving e13a2, e14a2 and e1a2 fusion variants respectively failed to generate any signal. Further investigation revealed presence of the rare e14a3 (b3a3) fusion.

Discussion: This is the first report of rare e14a3 fusion in the BCR ABL gene in a CML patient from India. The observation indicates the need for interrogating rare BCR ABL fusions when common breakpoint cluster regions are absent such that minimal residual disease (MRD), critical for disease monitoring, can be performed and false positive remission cases can be avoided. It also emphasizes the utility and significance of cytogenetics and FISH techniques in primary diagnosis of CML and use of RT-PCR based assays only for generating secondary information within special reference to MRD.

Conclusion: The rare e14a3 (b3a3) fusion of the BCR ABL gene is present in Indian population as demonstrated from this first report and clinical laboratories using commercial kit that do not cover such rare fusions are likely to generate false result thereby declaring complete molecular remission in CML patients under therapy while conducting MRD assay using RT-PCR technology.