Expression of T-Lineage Affiliated Transcripts and TCR Rearrangements in Hypogranular Variant Acute Promyelocytic Leukemia (APL): Implications for the Cellular Target of the t(15;17).

APL is generally considered to be the most differentiated subtype of acute myeloid leukemia (AML) and to derive from late myeloid-restricted progenitors. This is, however, difficult to reconcile with the observation that approximately 25% of APL cases, particularly those with the hypogranular variant (M3v) form, express the T-lineage affiliated marker, CD2. To gain further insight into the extent of T-lineage deregulation in primary APL blasts and its implications for the likely target progenitor for the t(15;17), we quantified the expression of pre-Tα(pTα), T earlyα(TEA) and RAG1 transcripts by RQ-PCR (expressed as ΔCt ABL-target, whereby ΔCt =−2 is approximately 3 log higher than ΔCt =−12) and determined TCRδ and γ status in 36 cases of PML-RARA positive APL, including 21 classical hypergranular (M3) and 15 M3v forms. pTαforms part of the pre-TCR necessary for TCRαβ lineage differentiation and TEA represents a sterile transcript involved in the early stages of TCRαrearrangement.

Both classical and M3v APL expressed pTαat higher levels compared to 26 non-APL AML (p<0.001), with the levels in M3v being similar to those found in T-ALL. Amongst control AMLs, pTαlevels were lower (p<0.001) in FAB M4/5 AML compared to FAB M0/1/2 AML, which were not different from pTαlevels in classical APL (p=0.31). Unlike T-ALL, in which pTαand RAG1 expression cosegregate, M3v and classical APL did not express RAG1 transcripts. However, TEA was expressed at significantly higher levels in M3v APL compared to classical APL (p<0.001) and non-APL AML (p=0.002). TCR γ and δ rearrangements tended to be more common in M3v as compared with classical APL (p=0.11); moreover, expression of cytoplasmic CD3 (2/12) and TdT (3/13) were also restricted to these cases (0/3 and 0/5 in classical APL). Only M3v morphology and CD34 expression correlated with higher levels of pTαexpression amongst APL. Notably, CD2 expression, known to correlate with M3v APL, including in this series, did not. These data demonstrate that M3v APL blasts exhibit several features suggestive of early T-lymphoid orientation which may reflect the progenitor subject to leukemic transformation. They suggest that the PML-RARA fusion gene may arise in more primitive hematopoietic stem cells in M3v than previously thought to be the case and that the biological differences between classical and hypogranular variant forms of the disease may in part reflect differing cellular origins. Moreover these data are consistent with the existence of bipotent myeloid/T-lymphoid progenitors recently proposed by Katsura and colleagues and which could be the progenitor population subject to leukemic transformation in the hypogranular variant form of APL.