Histiocytic sarcoma (HS) is a rare but aggressive cancer in humans and dogs. The spontaneous canine model, with the clinical, epidemiological and histological similarities with human HS and specific breed predispositions, is a unique model/opportunity to unravel the genetic bases of this cancer. In this study, we aimed to identify germline risk factors associated with the development of HS in canine predisposed breeds. We used a methodology that combined several genome-wide association studies in a multi-breed and multi-cancer approach, as well as targeted next generation sequencing, and imputation combining several breeds (Bernese mountain dog, Rottweiler, flat coated retriever and golden retriever) and three haematopoietic cancers (HS, lymphoma and mast cell tumor). Results showed that we not only refined the previously identified HS risk CDKN2A locus but we identified new loci on canine chromosomes 2, 5, 12, 14, 20, 26 and X. Capture and targeted sequencing of specific loci pointed towards the existence of regulatory variants in non coding regions and/or methylation mechanisms linked to risk haplotypes, leading to strong cancer predispositions in specific dog breeds. Our results showed that these canine cancer predisposing loci appear to be due to the additive effect of several risk haplotype involved also in other haematopoietic cancers such lymphoma or mast cell tumor, illustrating the pleiotropic nature of these canine cancer loci as observed in human oncology, thus reinforcing the interest of predisposed dog breeds to study cancer initiation and progression.