Maternal transmission of an Igf2r domain 11: IGF2 binding mutant allele (Igf2r ^I1565A) results in partial lethality, overgrowth and intestinal adenoma progression

The cation-independent mannose 6-phosphate/insulin-like growth factor-2 receptor (M6P/IGF2R or IGF2R) traffics IGF2 and M6P ligands between pre-lysosomal and extra-cellular compartments. Specific IGF2 and M6P high-affinity binding occurs via domain-11 and domains-3-5-9, respectively. Mammalian maternal Igf2r allele expression exceeds the paternal allele due to imprinting (silencing). Igf2r null-allele maternal transmission results in placenta and heart over-growth and perinatal lethality (>90%) due to raised extra-cellular IGF2 secondary to impaired ligand clearance. It remains unknown if the phenotype is due to either ligand alone, or to both ligands. Here, we evaluate Igf2r specific loss-of-function of the domain-11 IGF2 binding site by replacing isoleucine with alanine in the CD loop (exon 34, I1565A), a mutation also detected in cancers. Igf2r^I1565A/+p maternal transmission (heterozygote), resulted in placental and embryonic over-growth with reduced neonatal lethality (<60%), and long-term survival. The perinatal mortality (>80%) observed in homozygotes (Igf2r^{I1565A/I1565A}) suggested that wild-type paternal allele expression attenuates the heterozygote phenotype. To evaluate Igf2r tumour suppressor function, we utilised intestinal adenoma models known to be Igf2 dependent. Bi-allelic Igf2r expression suppressed intestinal adenoma (Apc^Min). Igf2r^I1565A/+p in a conditional model (Lgr5-Cre, Apc^loxp/loxp) resulted in worse survival and increased adenoma proliferation. Growth, survival and intestinal adenoma appear dependent on IGF2R-domain-11 IGF2 binding.