RPS15 mutations and Treatment Outcome in Chronic Lymphocytic Leukemia - Results from the CLL11 Phase III Trial (Chlorambucil (Clb) with or without the Addition of Rituximab (R) or Obinutuzumab (GA101, G)) of the German CLL Study Group

BACKGROUND

Genetic factors such as IGHV mutation status, deletion 17p and mutations of e.g. TP53, NOTCH1, ATM, SF3B1, BIRC3, XPO1 and FBXW7 carry predictive and/or prognostic impact in CLL. Recently, recurrent mutations have been found in RPS15, a gene encoding an element of the 40S ribosomal subunit, resulting in damaged regulation of endogenous p53 and being associated with adverse outcome (Ljungström et al., Blood 2016; Landau et al., Nature 2015). Here, we studied the incidence and clinical impact of RPS15 mutations (mut) in the CLL11 trial evaluating front line treatment with Clb vs. R-Clb vs. G-Clb in physically unfit patients.

METHODS

We performed Sanger sequencing of the C-terminal region of RPS15 (exon 4) in a representative subset of 700 (89.6%) patient samples from the CLL11 trial. Sensitivity was determined as somatic variant detection of >10%.

RESULTS

The incidence of RPS15 mutation was 4.7% (33/700 pts) and most mutations were somatic missense variants. We found significant correlations with genetic baseline characteristics: RPS15mut was associated with IGHVunmut (30/31 pts, p<0.001) and TP53mut (7/32 pts, p=0.028), but not with del17p (4/33 pts, NS). Regarding clinical parameters, RPS15mut was associated with high baseline white blood cell count (≥ 50 G/l, 29/33 pts, p=0.006) and a higher CLL-IPI (27/33 pts, p=0.014).

Clinical response to treatment was not significantly different when comparing patients with and without RPS15mut, but there was a lower CR rate (9.7% vs. 18.7%, NS) when analyzing all treatment arms combined, and a trend also when considering the treatment arms R-Clb (6.7% vs. 10.9%, NS) and G-Clb (15.4% vs. 32.4%, NS). MRD positivity (≥10^-4) at the end of treatment was significantly more frequent in patients carrying RPS15mut (100% vs. 82.9%, p=0.021). When considering treatment arms separately, this also applied to G-Clb (100% vs. 62.7%, p=0.015), but not to R-Clb (100% vs. 96.2%, NS).

At a median observation time of 41.6 months there were 533 (76.1%) events for progression free survival (PFS) and 195 (27.9%) for overall survival (OS) in the cohort studied.

Considering all treatment arms together, the presence of RPS15mut was associated with inferior PFS (HR 1.502, CI 1.038-2.173, p=0.031). When looking at the treatment arms separately, PFS was mainly affected in the R-Clb group (HR 1.888, CI 1.133-3.148, P=0.015), but less so in the Clb or G-Clb groups (HR 0.720 , CI 0.227-2.289, NS; HR 1.525, CI 0.829-2.805, NS, respectively).

Regarding OS, no significant differences could be found depending on RPS15 mutational status (HR 1.182, CI 0.643-2.172, NS) and also when considering treatment arms G-Clb and R-Clb separately (HR 1.698, CI 0.682-4.227, NS, and HR 0.977, CI 0.396-2.413, NS). Neither addition of R nor G had an effect on outcome OS as compared to Clb monotherapy in the presence of RPS15mut.

We performed multivariable Cox regression analysis including clinical and genetic prognostic parameters that were significant in univariate analyses to evaluate their independent prognostic value. For PFS we identified G-Clb vs. Clb (HR 0.211, CI 0.161-0.277, p<0.001), R-Clb vs. Clb (HR 0.492, CI 0.380-0.636, p<0.001), del17p (HR 1.848, CI 1.206-2.832, p=0.005), TP53mut (HR 1.923, CI 1.323-2.796, p=0.001), IGHVunmut (HR1.974, CI 1.610-2.422, p<0.001), del11q (HR 1.366, CI 1.083-1.723, p=0.009), Binet Stage A (HR 0.795, CI 0.636-0.993, p=0.043) and serum thymidine kinase >10 U/l (HR 1.209, CI 1.006-1.452, p=0.043) as independent prognostic factors. Multivariate analysis was not performed for OS since RPS15mut was not significantly associated with OS.

CONCLUSION

RPS15 as a putative CLL driver gene affecting the p53 pathway was mutated in 4.7% patients in need of front line treatment enrolled within the CLL11 trial. Patients with RPS15mut remained MRD positive at the end of treatment and had inferior PFS in univariate analysis. However, RPS15mut was not identified as an independent prognostic marker, possibly due to its close colinearity with other high risk markers such as IGHVunmut and TP53mut.