Novel Synthetic Lethal Targets for Myeloid Neoplasms with Loss of Chromosome 7
Authors: Mori Minako , Adema Vera , Gurnari Carmelo , Pagliuca Simona , Terkawi Laila , Bahaj Waled , Kewan Tariq , Barnard John , Visconte Valeria , Haferlach Torsten , Maciejewski Jaroslaw P. 
Published: November 2021

Abstract
Loss of chr7 (-7) and partial deletions of its long arm (del7q) are observed in 10% of de novo myeloid neoplasms (MNs), 50% of therapy related MDS, up to 60% of post aplastic anemia MNs and occur frequently as evolution of congenital bone marrow failure syndromes (e.g., GATA2 and SAMD9L deficiency, FA). LOH of one or more chr7 genes has been considered the culprit in the pathogenesis of -7/del7q MNs. In addition to the loss of protective alleles, deletion resulting in haploinsufficiency (HI) of tumor suppressor genes (TSGs) in CDRs might be also a cause of leukemogenic drive behind -7/del7q. To date, albeit many candidate genes have been associated with -7/del7q, the search for genes responsible for clinical phenotype has failed to identify causative -7/del7q TSGs and their putative loss would be difficult to target. Irrespective of the important goal of clarification of leukemogenic effects of -7/del7q, loss of genes in CDRs may create a vulnerability phenotype, which could be exploited with synthetic lethal approaches. Such strategies would rely on the higher resistance of diploid vs -7/del7q cells, thus allowing for a therapeutic window.
Here, we studied the molecular profile of 8160 MN patients (del7q: 1.7%; -7: 6%). EZH2 mutations were enriched in -7/del7q compared to chr7 diploid cases (3.8 vs 1.2%, P&lt;.0001) also by absolute numbers heterozygous mutations were more numerous. We also detected somatic CUX1 mutations (1.7 vs 0.9%), SAMD9/SAMD9L (0.3 vs 0.1%), and LUC7L2 (0.3 vs 0.1%) in -7/del7q vs diploid. In -7/del7 cases somatic alterations were detected in BRAF (n=7), POT1 (n=3), PCLO (n=5) and PSMC2 (n=1) while no mutations in CUL1 and KMT2C were found. We then investigated the presence of driver mutations located on other chromosomes in -7/del7q. Del7q/-7 cases showed a lower frequency of TET2 and SF3B1 mutations vs diploid cases. In isolated/+1 del7q/-7 cases, TP53 mutations were significantly less frequent, but were increased in -7/del7q with complex karyotype (P&lt;.0001). Higher frequencies of RAS genes, RUNX1 and ETV6 hits were also found. Del7q and TP53 mutations were founder lesions (dominant) in 38% and 54% of -7/del7q, while -7 was dominant in 63% of -7 cases. TP53 was the only mutation significantly associated with further worsening the already poor prognosis of -7/del7q cases (HR=1.629 P&lt; .01). Germline alterations were more common in -7/del7q as compared to diploid cases (13 vs 5% P&lt; .0001) of which most were FA or DNA repair gene variants also in other genes including (e.g., SAMD9L, 7%, DDX41, 3.7%).
Having defined the genotype of -7/del7q, we set to identify genes which could be possible targets for the therapy chiefly synthetic lethality. Criteria for selection included: consistent HI in most of the patients, genes not affected by hemizygous LOF mutations and embryonic lethality in knockout (KO) configuration. Expression data of -7/del7q (n=86), diploid cases (n=1066) and healthy controls (n=84; MLL and BEAT AML to increase precision) were analyzed. Our algorithm included selection of genes with mRNA expression inversely correlating with copy number (deletion copy number). Out of 694 genes on chr7, 147 genes were deleted in all patients and 101 genes had more inconsistent HI levels. In total 35 genes showed significant negative correlation with -7/del7q ploidyincluding ACTR3B,AGK,ATP06V0E2,CUL1,FASTK,GALNT11,GSTK1, IMPDH1, PLXNA4, SLC37A3, ZNF277, KMT2C, NUP205, TMEM209, ZC3HC1 and GIMAP1/2/4/6, a cluster ofnucleotide binding proteins. Following adjustment to ploidy, HI was found for EZH2 (76% cases), CUX1 (76%), KMT2C (70%), LUC7L2 (60%), and SAMD9/9L (32%/50%) but also even more consistently in SSBP1 (88%), PSMC2 (86%), CUL1, ZNF398, and RHEB (all 84%) and TNPO3 (82%). Among those genes homozygous KO of Ezh2 and Cul1 lead to embryonic lethality, Gimap family deletion reduces normal hematopoiesis, Samd9l +/-and Samd9l-/- mice develop MDS and die after 1.5yrs and Cux1 knockdown causes an MDS like phenotype. Existing inhibitors are available for CUL1 (MLN4924), CUX1 (BER modulating agents) and EZH2 (EPZ6438, GSK343), but the presence of homozygous mutations (UPD7q) argues that EZH2 inhibition is unlikely to be successful.
In conclusion, we showed a comprehensive molecular topography of -7/del7q and identified novel HI genes which could be targeted by novel or repurposed drugs. Ongoing drug screens for identified targets performed in cells with -7/del7q will be presented at the meeting.

Disclosures
Haferlach: MLL Munich Leukemia Laboratory: Other: Part ownership. Maciejewski: Bristol Myers Squibb/Celgene: Consultancy; Novartis: Consultancy; Regeneron: Consultancy; Alexion: Consultancy.