HSCI Retreat 2020 Abstract 4

ABSTRACT

The fundamental importance of the insulin-like growth factor-1 (IGF-1)-IGF1R (IGF-I receptor) system in skeletal acquisition and bone mineral density (BMD) accretion during developmental growth and aging is supported by mouse studies and limited human case reports. We now report two siblings, born IUGR to consanguineous parents, had severe post-natal growth retardation with height SDS of -6.2 (P1, female, 9.9 years) and -4.5 (P2, male, 7.1 years). Co-morbidities included microcephaly, dysmorphic features, and a markedly reduced BMD of -6.9 SD (P1) and -3.7 SD (P2). Whole exome sequencing identified a novel homozygous IGF1Rc.2132_2143del, which resulted in an IGF1R variant that was appropriately expressed and localized, but IGF-I responsiveness was significantly blunted. To assess the impact of a human homozygous IGF1R p.Ala711_Glu714del in-frame mutation on skeletal accretion in a knock-in mouse model. A viable knock-in mouse model of the human mutation (IGF-1RKI) provided in vivo evidence that the private IGF1R mutation was the cause for the loss of bone integrity and impaired growth. Igf1r c.2134_2145del (p,Ala712_Glu715del) in C57BL/6J background was generated by gene-editing with CRISPR/Cas9 methodology and mice bred for heterozygous and homozygous knock-in genotypes. Bone morphology and BMD were analyzed by high-resolution micro-computed tomography (mCT), mechanical properties were studied by 3-point bending and micro-indentation assays. Male and female IGF-1RKI homozygous mice, born IUGR, displayed significant post-natal growth retardation. Reduced ossification of the axial and appendicular skeleton detected in one-day-old heterozygous mice was more severe in the homozygous mice. Both sexes showed 45% reductions in femur mid-diaphysial total cross-sectional area and bone area, and a 30% decrease in cortical bone thickness that resulted in reductions in polar moment of inertia and mechanical strength at 8 and 16 weeks of age. Despite the severely compromised morphology of the femur in the homozygous mice, BMD of the cortical bone compartment was surprisingly similar to control mice. In contrast, the trabecular bone at the femur distal metaphysis of homozygous males showed 40% and 30% reductions in bone volume/total volume (BV/TV) and BMD, respectively, while homozygous females showed minor reductions in both BV/TV and BMD at 8 and 16 weeks.