Impact of Non-JAK2 Molecular Mutations and Cryptic SNP Lesions in Myelofibrosis Patients Treated with Ruxolitinib
Authors: Ai Jing , Visconte Valeria , Tabarroki Ali , Zarzour Ahmad , Gerace Christopher , Al-Issa Karam , Hsi Evan , Hirbawi Yasmine , Hamilton Betty K. , Hsi Eric D. , Rogers Heesun J. , Tiu Ramon V. 
Published: November 2019

Abstract
The identification of the JAK2V617F mutation in myeloproliferative neoplasms (MPN) paved the way for the pivotal studies that led to the FDA approval of a JAK1/2 inhibitor, ruxolitinib (rux) in patients (pts) with myelofibrosis (MF). Improvement in splenomegaly and debilitating disease-related symptoms were the primary clinical responses observed with rux. Although JAK2 mutational status did not impact response/survival in MF pts, cytogenetics had an impact on prognosis. In a related myeloid neoplasm specifically myelodysplastic syndromes, molecular mutations (TET2/DNMT3A) predict for better therapeutic response to DNA methyltransferase inhibitors. We hypothesized that somatic mutations and single nucleotide polymorphism array (SNP-A) lesions are frequent in MF pts treated with rux and may affect their clinical outcomes. To further investigate the predictive and prognostic impact of SNP-A lesions and somatic mutations in MF pts in the rux era, we studied 54 MF pts who received at least 12 weeks of rux therapy (tx) using a modified dose escalation approach (Tabarroki A et al. 55th ASH; Abstract 1586). Clinical (total symptom score [TSS], spleen size), cytogenetic (metaphase cytogenetics [MC], SNP-A), hematologic and survival data were collected before and 12-weeks post rux tx. Categorical data were analyzed using X2 test. A p-value of <.05 was considered statistically significant. Sanger sequencing for genes relevant to myeloid neoplasm pathophysiology like TET2, CBL, LNK, DNMT3A, TP53, SF3B1, U2AF1, SRSF2, ASXL1, EZH2, JAK2, CALR, and IDH1/2 was performed. The median age of the cohort was 66 yrs (41-89); male/female: 28/26. The median follow-up time after initiation of tx was 17 months. The median overall follow-up of the cohort from the time of diagnosis was 35 months. Using DIPSS-plus, pts were stratified as high (24, 44%), int-2 (22, 41%) and int-1 (8, 15%) risk groups. Baseline median WBC=9.4k/μL, Hgb=10.2g/dL, PLT=212k/μl, TSS=20, and median palpable spleen size=13cm. Post-tx median WBC=9.9k/μL, Hgb=10.1g/dL, PLT=150k/μL, TSS=4, and spleen size=6cm. MC identified cytogenetic abnormalities in 24/54 (44.4%) pts. The most frequent chromosomal defects included del(20), +8, and +9. Serial MC was available for 20 pts and no cytogenetic evolution was identified. SNP-A data were available for 29 pts, of which 28 pts had SNP-A lesions. The most commonly involved chromosomes were 9 (15.1%), 20 (14.1%), and 14 (8.5%). Compared to MC analysis, additional SNP-A lesions were found in 66% of pts. Of note 39% of the pts had normal karyotypes but with pathologic SNP-A lesions; another 27% had pathologic SNP-A lesions besides the abnormal MC. Serial SNP-A analysis was available in 10 pts who while on rux tx did not develop any additional/new SNP-A lesion. There was no difference in spleen response rates or TSS between those who carried SNP-A lesions versus those who did not. Molecular analysis was possible for 34 pts. The most frequent somatic mutations observed involved JAK2 (70.6%), ASXL1 (24%), CALR (24%), SRSF2 (15%), and U2AF1 (9%). Pts with int-2 and high DIPSS plus scores were more likely to carry at least 1 mutation in any gene compared to pts with int-1 scores (int-2 vs int-1, p=.05; high vs int-1, p=.06). After a median follow-up of 35 months from diagnosis, 95% of the pts were still alive. 3 pts died from disease progression: 1 had a sole SRSF2 mutation, 1 had an SRSF2 plus CALR mutation, and 1 had a TET2 plus TP53 mutation. SRSF2 mutant pts had more severe thrombocytopenia pre-rux tx (91 vs. 203k/μL; p=.04). ASXL1 mutant pts had increased spleen sizes pre-rux (21 vs. 15cm, p=.06), but had similar response post-rux (10 vs. 8cm, p=0.6) compared to the wild-type. SRSR2 mutant pts had higher DIPSS-plus score (4.4 vs. 3; p=.05). Our study showed that MF pts treated with a rux modified dose escalation approach resulted in meaningful clinical and splenic responses regardless of molecular mutation status. Frequently found cryptic SNP-A lesions in MF pts may explain their poorer outcomes compared to pts with other MPNs. The fact that pts did not acquire additional/new SNP-A lesions during rux tx may be one of the mechanisms of improved survival in these pts. ASXL1, CALR, SRSF2 and U2AF1 were the most frequent non-JAK molecular mutations in MF pts treated with rux and were more frequent in high risk pts. Further studies are necessary to elucidate the clinical/ biological effects of these mutations in MF pts treated with rux.

Disclosures
Tiu: Incyte: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Speakers Bureau; Gilead: Membership on an entity's Board of Directors or advisory committees.