Cryptic XPO1-MLLT10 translocation is associated with HOXA locus deregulation in T-ALL

To the editor:

Biological subclasses of T-cell acute lymphoblastic leukemia (T-ALL) can be defined by recurrent gene expression patterns, which typically segregate with specific chromosomal anomalies. The HOXA⁺ subgroup is characterized by deregulated homeobox A (HOXA) gene expression and is associated with translocations involving the mixed lineage leukemia (MLL) and/or MLLT10 loci, SET-NUP214, or TCRB-HOXA.^1,2  Nevertheless, the genetic basis for many HOXA⁺ cases remains unexplained.

Diagnostic assessment of a 33-year-old man with T-ALL revealed high leukemic blast expression of HOXA9 at levels comparable to those in known HOXA⁺ cases (Figure 1A). Tests for PICALM-MLLT10, SET-NUP214, MLL-AF6, and TCRB-HOXA were negative. Leukemic cells exhibited a complex karyotype (46,XY,add(2)(p14),-10,-17,+2mars,inc[11]), which led us to speculate that HOXA positivity might be caused by a structural genetic abnormality. We therefore performed poly(A)-enriched sequencing (RNA-sequencing) of diagnostic RNA, analysis of which revealed fusion of exon 24 of XPO1 to exon 6 of MLLT10 (Figure 1B, upper panel). Expression of an in-frame XPO1-MLLT10 fusion transcript was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) and direct sequencing (Figure 1B, lower panel).

We hypothesized that the common involvement of MLLT10 would result in similar deregulation of HOXA locus expression in XPO1-MLLT10⁺ and PICALM-MLLT10⁺ T-ALL. We tested the expression of a range of HOX genes by quantitative RT-PCR. As predicted, the pattern of HOXA gene transcription in the XPO1-MLLT10⁺ case was very similar to that in the PICALM-MLLT10⁺ cases (Figure 1C). A targeted RT-PCR screen of 84 HOXA⁺ T-ALL samples that lacked known explicatory genetic anomalies identified no further XPO1-MLLT10⁺ cases (Figure 1D), suggesting rarity and/or breakpoint heterogeneity.

Each of the genes involved in this fusion has been previously implicated in leukemia. Notably, MLLT10 (which encodes the AF10 protein) is involved in the recurrent PICALM-MLLT10³  and MLL-MLLT10⁴  translocations in both T-ALL and acute myeloblastic leukemia. Recently reported results of RNA-sequencing have identified HNRNPH1 and DDX3X as MLLT10 fusion partners in HOXA⁺ T-ALL.⁵  Our data provide further evidence of shared fusion partner–independent mechanisms of AF10-mediated transcriptional dysregulation, and this case adds to the repertoire of MLL and/or AF10-rearranged T-ALL that might be candidates for targeted DOT1L-directed therapy.⁶ MLLT10 breakpoints are heterogeneous, and increasing truncation of the transcript was reported to correlate with an earlier maturation block in T-ALL, although this was not confirmed in a later series.^3,7  In this case, detailed characterization of T-cell receptor (TR) gene configuration revealed monoallelic TRG and TRD and incomplete TRB diversity-joining rearrangements (data not shown), consistent with an immature pre-β-selection immunogenotype.⁸ 

XPO1 (also CRM1) encodes exportin 1, a transport protein that mediates nuclear export of multiple tumor suppressor and growth regulatory molecules (eg, P53 and RB1). Pharmacologic XPO1 inhibition has shown promising antileukemic activity in preclinical models via a mechanism that is believed to involve either nuclear retention of XPO1 cargo upon which the leukemic cells depend for survival,⁹  and/or reactivation of nuclear protein phosphatase 2A.¹⁰  It is tempting to speculate that HOXA-independent activity of the XPO1-AF10 fusion protein could also contribute to leukemogenesis in this case, for example through aberrant transport of proteins that mediate proliferation and survival and/or by dominant negative inhibition of wild-type XPO1.