Adverse Prognostic Impact of GAS6 Expression in De Novo Cytogenetically Normal Acute Myeloid Leukemia (CN-AML) (CALGB 8461, 9665, 20202; Alliance)

Abstract 1293

Receptor tyrosine kinases (RTKs) constitutively activated by gene mutation, overexpression and/or autocrine activation via ligand expression have been shown to negatively impact on outcomes of AML patients (pts). AXL, a member of the TAM (TYRO3, AXL, MERTK) RTK gene family was reported to be overexpressed and associated with poor survival in AML (Rochlitz, et al, Leukemia, 1999: 13:1352–8). No AXL mutations have been described, suggesting its activation may occur via aberrant expression in leukemic blasts of a TAM RTK ligand, GAS6. GAS6 was shown to be overexpressed in AML (Dirks, et al Leuk. Res. 23:643–51); yet its prognostic relevance is unknown. We report clinical and molecular associations and prognostic impact of aberrant GAS6 expression, in the context of TAM RTKs and known prognostic markers in de novo CN-AML pts (n=270; aged 18–83 y) treated with cytarabine/anthracycline-based therapies. Sixty-nine (26%) pts expressed GAS6 (>background signal; derived from microarray gene expression profiles of AML samples). TYRO3 expression status [positive (+) vs negative (–)] was similar in GAS6+ and GAS6– pts (P=.74), while AXL+ (P<.001) and low expression MERTK (P=.02) were more frequent in GAS6+ pts. Compared to GAS6– pts, GAS6+ pts were older (P=.02), had more platelets (P=.03), lower % blood blasts (P=.01) and increased frequency of hepatomegaly (P=.006); were more often NPM1 (P<.001) and CEBPA (P=.02) wild-type, and RUNX1 (P<.001) and ASXL1 (P=.002) mutated and expressed higher MN1 levels (P=.05). In univariable analyses, none of the TAM RTKs associated with complete remission (CR) and only TYRO3+ associated with reduced disease-free (DFS; P=.005), overall (OS; P=.005) and event-free survival (EFS; P=.008). GAS6+ vs GAS6– pts had lower CR rates (P<.001), shorter DFS (P=.03), OS (P=.004) and EFS (P<.001). While no TAM RTK entered the CR multivariable (MVA) model, GAS6+ expression status remained an independent marker for lower CR rate after adjusting for NPM1 status, white blood count (WBC) and age group (Table). In the DFS, OS and EFS models (Table), there was an interaction between GAS6 and the combined dual receptor (TYRO3/AXL) variable. GAS6 independently associated with shorter survival in TYRO3–/AXL– pts but not TYRO3+/AXL+ pts after adjusting for other variables. We show for the 1^st time that GAS6 expression is an independent prognostic marker in CN-AML; negatively impacting on CR attainment, independent of TAM RTKs and on survival endpoints in pts lacking TYRO3 and AXL expression, regardless of MERTK expression. Our results suggest that GAS6 expressed by AML blasts plays a role in chemotherapy resistance. As GAS6 is expressed but not its RTKs in a subgroup of pts with poor outcome, this may lead to the hypothesis that the prognostic impact of GAS6 in those patients is mediated by the encoded ligand acting on cells other than AML blasts including, for example, natural killer cells, where activation of AXL RTK is reported to suppress innate immunity.

ORs > (<) 1.0 mean higher (lower) CR rate, and HRs > (<) 1.0 mean higher (lower) risk for relapse or death (DFS, EFS), respectively, for the higher values of the continuous variables and the first category listed for the categorical variables. Variables significant at α =.20 in univariable models were considered, although all considered variables are not shown.

*There are interactions between GAS6 and TYRO3/AXL dual receptor status for DFS (P=.16), OS (P=.06) and EFS (P=.12). The P-values, HRs and CIs are for comparisons of GAS6+ v GAS6– pts within the TYRO3–/AXL– subset.