Germline Contributions to Clonal Hematopoiesis in Solid Cancer Patients
Authors: Franch-Expósito Sebastià , Srinivasan Preethi , Ptashkin Ryan , Bandlamudi Chaitanya , Bolton Kelly L. , Gedvilaite Erika , Menghrajani Kamal , Mandelker Diana , Comen Elizabeth , Norton Larry , Benayed Ryma , Gao Teng , Papaemmanuil Elli , Taylor Barry S , Levine Ross L. , Berger Michael F. , Zehir Ahmet 
Published: November 2020

Clonal hematopoiesis (CH), defined by expansion of clones in the hematopoietic system, has been linked to internal (aging) and external (smoking and oncologic therapy) factors in patients with advanced solid tumors. The effects of inherited variants, however, have yielded inconclusive results in few studies so far. While comparison of CH prevalence between monozygotic and dizygotic pairs failed to detect genetic predisposition to CH, others have shown associations with germline variants near TERT and TET2, and a potential link between pathogenic CHEK2 germline variants and CH has been proposed. Here, utilizing prospective tumor-blood paired sequencing data from a large institutional clinical cohort, we link inherited pathogenic variants in cancer predisposition genes to CH and show interactions between germline and somatic mutations in DNA damage repair (DDR) genes.
We analyzed sequencing data from 32,748 cancer patients who have undergone MSK-IMPACT testing using paired tumor and blood samples to study the associations between CH and rare pathogenic and likely-pathogenic (P/LP) germline variants. P/LP germline mutations were identified in the blood using an in-house developed random forest algorithm, trained using known P/LP mutations. Following joint anonymization of P/LP germline mutations and CH variants, linear regression models by gene were used to test for associations, accounting for known CH confounders.
In our cohort, 9,697 patients (29.6%) harbored at least one CH mutation, while P/LP germline variants were detected in 1,793 of these CH patients. P/LP germline variants in CHEK2 (OR=1.29, p=4.49x10-2) were found to be significantly associated with CH in a pan-cancer analysis. Interestingly, germline P/LP and CH co-mutations in CHEK2 were very rare; we identified only one patient with CH and germline mutations in CHEK2 out of 161 patients with germline CHEK2 mutations, compared to 304 patients with CH CHEK2 mutation when CHEK2 is wild-type in the germline. In contrast, ATM and TP53 showed increased number of patients with co-mutations in P/LP germline and CH mutations in the same gene, with 14 out of 84 patients with germline mutations in ATM compared to 254 when not mutated (OR=7.05, p=2.08x10-7) , and 3 out of 16 compared to 387 for TP53 (OR=10.94, p=5.44x10-3). Further, we found mutual exclusivity between CHEK2 P/LP germline variants and CH mutations in PPM1D (OR=0.18, p=1.79x10-2). This relationship was replicated by comparing CHEK2 germline mutations and somatic PPM1D mutations in the solid tumor samples within the same cohort and same pattern of mutual exclusivity also persisted in comparisons of other DDR pathway genes (i.e. ATM vs PPM1D and TP53 vs PPM1D).
When studying specific cancer types, a significant association between CHEK2 P/LP germline variants and CH in breast cancer patients was also identified (OR=2.33, p=2.64x10-3). Furthermore, trends between germline mutations in ATM and CH rates in breast cancer (OR=1.84, p=9.82x10-2) and lung adenocarcinoma (OR=2.22, p=8.91x10-2) patients were observed.
Our results support the already reported link between rare pathogenic germline mutations in CHEK2 with CH in cancer patients, with stronger association among breast cancer patients (Comen E. et al, 2019). Moreover, this study suggests a close relationship between inherited variants and CH mutations within the DDR genes in solid tumor patients and points out to interesting mutual exclusivity patterns between these same genes. Intriguingly, hematopoietic stem cells have been largely related to proficient DDR systems in order to regulate HSC maintenance and tissue homeostasis in the hematopoietic system. All in all, associations identified in this study might translate into enhanced clinical surveillance for CH and associated comorbidities cancer patients harboring these germline mutations.

Disclosures
Bolton: GRAIL: Research Funding. Papaemmanuil:Novartis: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Illumina: Consultancy, Honoraria; Prime Oncology: Consultancy, Honoraria; Isabl: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Kyowa Hakko Kirin: Consultancy, Honoraria; MSKCC: Patents & Royalties. Levine:Prelude Therapeutics: Research Funding; Qiagen: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy; Astellas: Consultancy; Morphosys: Consultancy; Novartis: Consultancy; Amgen: Honoraria; Gilead: Honoraria; Roche: Consultancy, Honoraria, Research Funding; Lilly: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; C4 Therapeutics: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Loxo: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Isoplexis: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Imago: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees. Berger:Illumina: Research Funding; Grail: Research Funding; Roche: Consultancy. Zehir:Illumina: Honoraria; Memorial Sloan Kettering Cancer Center: Current Employment.