Lysosomal Degradation Targets Mutant Calreticulin and the Thrombopoietin Receptor in Myeloproliferative Neoplasms

• Lysosomal degradation regulates cellular CRTDel52 and MPL levels.

• mTOR inhibitors reduce CRTDel52 and MPL levels and reduce CRTDel52-induced cell proliferation.

Somatic mutants of calreticulin (CRT) drive myeloproliferative neoplasms (MPNs) via binding to the thrombopoietin receptor (MPL) and aberrant activation of the JAK/STAT pathway. Compared with healthy donors, platelets from MPN patients with CRT mutations display low cell surface MPL. Additionally, co-expression of MPL with an MPN-linked CRT mutant (CRTDel52) reduces cell surface MPL, suggesting that CRTDel52 may induce MPL degradation. We show that lysosomal degradation is relevant to the turnover of CRTDel52 and MPL. Furthermore, CRTDel52 increases the lysosomal localization and degradation of MPL. Mammalian target of rapamycin (mTOR) inhibitors reduce cellular CRTDel52, MPL and secreted CRTDel52 levels, and impair CRTDel52-mediated cell proliferation. mTOR inhibition also reduces colony formation and differentiation of CD34+ cells from MPN patients but not healthy donors. Together, these findings indicate low surface MPL as a biomarker of mutant CRT-mediated MPN and induced degradation of CRTDel52 and MPL as an avenue for therapeutic intervention.