Prognostic and oncogenic relevance of TLX1/HOX11 expression level in T-ALLs

TLX1 is a homeodomain transcription factor generally associated with a favorable outcome in T-cell acute lymphoblastic leukemia (T-ALL). However, the molecular mechanisms of TLX1 deregulation remain unclear and various transcript levels in the absence of 10q24 abnormalities have been reported. A reproducible and accurate delineation of TLX1(+) T-ALL will be necessary for proper therapeutic stratification. We have studied 264 unselected T-ALLs (171 adults and 93 children) and show that T-ALLs expressing high levels of TLX1 (n = 35,13%), defined as a real- time quantitative polymerase chain reaction (RQ-PCR) level of TLX1 greater than 1.00 ABL, form a homogeneous oncogenic group, based on their uniform stage of maturation arrest and oncogenetic and transcriptional profiles. Furthermore, TLX1 -high T-ALLs harbor molecular TLX1 locus abnormalities in the majority (31/ 33), a proportion largely underestimated by standard karyotypic screening. T-ALLs expressing TLX1 at lower levels (n = 57, 22%) do not share these characteristics. Prognostic analysis within the adult LALA94 and GRAALL03 prospective protocols demonstrate a better event free survival (P =.035) and a marked trend for longer overall survival (P =.059) for TLX1-high T-ALLs, while the expression of lower levels of TLX1 does not impact on prognosis. We propose that TLX1+ T-ALLs be defined as cases expressing TLX1/ABL ratios greater than 1 and/or demonstrating TLX1 rearrangement.