GCGR Loss-of-Function Mutations Promote Amino Acid-Dependent Pancreatic Neuroendocrine Tumor Initiation from a Subpopulation of Alpha Cells

The cellular origin of sporadic pancreatic neuroendocrine tumors (PNETs) is obscure. Dysregulated hormone production and the absence of obvious driver mutations mask tumor origins although they presumably arise from endocrine cell lineages such as glucagon-producing alpha cells or insulin-expressing beta cells. A reciprocal feedback circuit involving pancreatic glucagon and hepatic amino acids regulates alpha cell mass such that loss of hepatic glucagon receptor (GCGR) signaling drives alpha cell hyperproliferation and tumor formation. This genetically-defined system is an ideal model for parsing the mechanisms of PNET origin and progression. Utilizing a combination of genetically engineered Gcgr knockout mice and GCGR-inhibiting antibodies, we found that elevated plasma amino acids drive reversion to a proliferative SLC38A5+ embryonic progenitor-like status. Chronic activation of this mTOR-dependent state drives tumor initiation and single cell profiling reveals tumors originate from a subpopulation of alpha cells. Profiling RNA, tumor signaling, and immune content of biopsies from rare human PNETs found in individuals with germ-line GCGR loss-of-function variants showed active mTOR and prominent tumor cell-associated expression of amino acid transporter SLC7A8. This work, for the first time, identifies metabolic control of an adult-to-progenitor alpha cell identity switch and shows that chronic maintenance of progenitor identity in the adult organ drives proliferation and tumor initiation.