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Targeting cell surface molecules using radioligand and antibody-based therapies has yielded considerable success across cancers. However, it remains unclear how the expression of putative lineage markers, particularly cell surface molecules, varies in the process of lineage plasticity, wherein tumor cells alter their identity and acquire new oncogenic properties. A notable example of lineage plasticity is the transformation of prostate adenocarcinoma (PRAD) to neuroendocrine prostate cancer (NEPC)-a growing resistance mechanism that results in the loss of responsiveness to androgen blockade and portends dismal patient survival. To understand how lineage markers vary across the evolution of lineage plasticity in prostate cancer, we applied single-cell analyses to 21 human prostate tumor biopsies and two genetically engineered mouse models, together with tissue microarray analysis on 131 tumor samples. Not only did we observe a higher degree of phenotypic heterogeneity in castrate-resistant PRAD and NEPC than previously anticipated but also found that the expression of molecules targeted therapeutically, namely PSMA , STEAP1 , STEAP2 , TROP2, CEACAM5 , and DLL3 , varied within a subset of gene-regulatory networks (GRNs). We also noted that NEPC and small cell lung cancer subtypes shared a set of GRNs, indicative of conserved biologic pathways that may be exploited therapeutically across tumor types. While this extreme level of transcriptional heterogeneity, particularly in cell surface marker expression, may mitigate the durability of clinical responses to current and future antigen-directed therapies, its delineation may yield signatures for patient selection in clinical trials, potentially across distinct cancer types., Competing Interests: Competing interests statement:P.S.N. has received consulting fees from Janssen, Merck and Bristol Myers Squibb and research support from Janssen for work unrelated to the present studies. S.Z. has received consulting fees from Guidepoint and GLG consulting. J.L.Z. is a current employee of AstraZeneca. M.C.H served as a paid consultant/received honoraria from Pfizer and has received research funding from Merck, Novartis, Genentech, Promicell and Bristol Myers Squibb. C.L.S is on the board of directors of Novartis, is a co-founder of ORIC Pharmaceuticals, and is a co-inventor of the prostate cancer drugs enzalutamide and apalutamide, covered by U.S. patents 7,709,517, 8,183,274, 9,126,941, 8,445,507, 8,802,689, and 9,388,159 filed by the University of California. C.L.S. is on the scientific advisory boards of the following biotechnology companies: Beigene, Blueprint, Cellcarta, Column Group, Foghorn, Housey Pharma, Nextech, PMV.

Leucine-rich repeat-containing G-protein coupled receptor 5-positive (Lgr5(+)) stem cells reside at crypt bottoms of the small and large intestine. Small intestinal Paneth cells supply Wnt3, EGF, and Notch signals to neighboring Lgr5(+) stem cells. Whereas the colon lacks Paneth cells, deep crypt secretory (DCS) cells are intermingled with Lgr5(+) stem cells at crypt bottoms. Here, we report regenerating islet-derived family member 4 (Reg4) as a marker of DCS cells. To investigate a niche function, we eliminated DCS cells by using the diphtheria-toxin receptor gene knocked into the murine Reg4 locus. Ablation of DCS cells results in loss of stem cells from colonic crypts and disrupts gut homeostasis and colon organoid growth. In agreement, sorted Reg4(+) DCS cells promote organoid formation of single Lgr5(+) colon stem cells. DCS cells can be massively produced from Lgr5(+) colon stem cells in vitro by combined Notch inhibition and Wnt activation. We conclude that Reg4(+) DCS cells serve as Paneth cell equivalents in the colon crypt niche., Competing Interests: The authors declare no conflict of interest.