Embryonic lethality and defective mammary gland development of activator‐function impaired conditional knock‐in Erbb3V943R mice

ERBB3 is a pseudokinase domain‐containing member of the ERBB family of receptor tyrosine kinases (RTKs). Following ligand binding, ERBB receptors homo‐ or hetero‐dimerize, leading to a head‐to‐tail arrangement of the intracellular kinase domains, where the “receiver” kinase domain of one ERBB is activated by the “activator” domain of the other ERBB in the dimer. In ERBB3, a conserved valine at codon 943 (V943) in the kinase C‐terminal domain has been shown to be important for its function as an “activator” kinase in vitro. Here we report a knock‐in mouse model where we have modified the endogenous Erbb3 allele to allow for tissue‐specific conditional expression of Erbb3V943R (Erbb3CKI‐V943R). Additionally, we generated an Erbb3D850N (Erbb3CKI‐D850N) conditional knock‐in mouse model where the conserved aspartate in the DFG motif of the pseudokinase domain was mutated to abolish any potential residual kinase activity. While Erbb3D850N/D850N animals developed normally, homozygous Erbb3V943R/V943R expression during development resulted in embryonic lethality. Further, tissue specific expression of Erbb3V943R/V943R in the mammary gland epithelium following its activation using MMTV‐Cre resulted in delayed elongation of the ductal network during puberty. Single‐cell RNA‐seq analysis of Erbb3V943R/V943R mammary glands showed a reduction in a specific subset of fibrinogen‐producing luminal epithelial cells. ERBB3 receptor tyrosine kinase, a member of the ERBB family, has an important role in development and disease. It is the only member of the ERBB family with a pseudokinase domain and it preferentially heterodimerizes with ERBB2 to activate ERBB2 through allosteric kinase domain interactions. While targeted replacement of a conserved amino acid that abolishes its residual protein kinase activity was consistent with life and normal development, replacement of a residue essential for activator‐receptor interactions showed embryonic lethality and delayed ductal elongation in the developing mutant mammary gland. Single cell analysis of the targeted mammary glands showed reduction in a specific subset of fibrinogen‐producing luminal epithelial cells, suggesting that these cells are dependent on ERBB3‐mediated allosteric signaling.