Predisposition to Myeloproliferative Neoplasms

Familial forms of myeloproliferative neoplasms (MPN) and genetic contribution to sporadic cases of MPN have long been recognized. In the majority of cases, familial MPN is inherited as an autosomal dominant trait. The penetrance varies from around 20% to up to 100% in some pedigrees. We can distinguish two types of familial MPN. Type 1 has high penetrance, polyclonal hematopoiesis and hyperproliferation of a single hematopoietic lineage, caused by mutations in a single gene and usually manifesting at birth or early childhood. Examples are mutations in the genes for the erythropoietin receptor, thrombopoietin or its receptor, MPL. The type 2 familial MPNs are characterized by clonal hematopoiesis, low penetrance and manifestation beginning in most cases later in adult life. These type 2 familial MPNs are classical examples of inherited predisposition to a clonal malignant disease, in which acquired somatic mutations in hematopoietic cells are required for disease manifestation. Affected family members typically display the same acquired driver mutations in the genes for JAK2 (JAK2-V617F or JAK2-exon12), MPL (MPL-W515), or calreticulin (CALR) as patients with sporadic MPN. The mutated genes and the mechanism of how these inherited germline mutations predispose to MPN have not yet been elucidated. The search for these germline mutations has been hampered by the low penetrance of MPN manifestation and the rare occurrence of pedigrees that are large enough for genetic studies. Furthermore, the few candidate gene mutations that have been identified to date do not map to one gene locus and the function of the candidate genes does not fall into a common category. Two models of how the germline predisposition interacts with acquired driver mutations can be considered. First, the germline mutation may increase the mutation rate for gene mutations in JAK2, MPL, and CALR. Second, the germline mutation functionally synergizes with mutations in JAK2, MPL, and CALR and promotes disease initiation. The current state of our studies and studies in other laboratories will be discussed.