Development of real-time quantitative polymerase chain reaction assays to track treatment response in retinoid resistant acute promyelocytic leukemia.

Molecular detection of minimal residual disease (MRD) has become established to assess remission status and guide therapy in patients with PML-RARA+ acute promyelocytic leukemia (APL). However, there are few data on tracking disease response in patients with rarer retinoid resistant subtypes of APL, characterized by PLZF-RARA and STAT5b-RARA. Despite their relative rarity (<1% of APL) we identified 6 cases (PLZF-RARA, n=5; STAT5b-RARA, n=1), established the respective breakpoint junction regions and designed real-time quantitative polymerase chain reaction (RQ-PCR) assays to detect leukemic transcripts. The relative level of fusion gene expression in diagnostic samples was comparable to that observed in t(15;17)-associated APL, affording assay sensitivities of ~1 in 104-105. Serial samples were available from 2 PLZF-RARA APL patients. One showed persistent PCR positivity, predicting subsequent relapse, and remains in CR2, ~11 years post-autograft. The other, achieved molecular remission (CRm) with combination chemotherapy, remaining in CR1 at 6 years. The STAT5b-RARA patient failed to achieve CRm following frontline combination chemotherapy and ultimately proceeded to allogeneic transplant on the basis of a steadily rising fusion transcript level. These data highlight the potential of RQ-PCR detection of MRD to facilitate development of more individualized approaches to the management of rarer molecularly-defined subsets of acute leukemia.