The small mutation causes hyperinsulinemia with only mild insulin resistance, a paucity of body fat, deficiencies in bone development and homeostasis, and hearing loss. Homozygotes can be distinguished by 2 weeks of age by their smaller body size and thin short tails, and adult homozygotes are visibly smaller with a tail that is 20% shorter than in wild-type controls. Hearing loss was shown through increased ABR threshold values when assessed between 8 and 12 weeks of age. The skeletal defects include shortened femurs with smaller growth plates, decreased cortical and trabecular thickness and abnormal trabecular number, with an increase in the number of osteoclasts and osteoblasts per bone perimeter but impaired osteoblast proliferation and differentiation in culture. This spontaneous point mutation is semidominant in some aspects of phenotype, unlike targeted mutations of this gene. Heterozygotes are statistically smaller than wild-type controls, but the severity of dwarfing is not readily apparent as it is with homozygotes. Heterozygotes also display an intermediate phenotype for serum insulin, femur length, reduction in femur growth plate size, bone mineral density, leanness, osteoblast function, cortical thickness, periosteal circumference, and mineral apposition rate. There is a variation by gender on the impact on bone structure such that female homozygotes have a decrease in trabecular number while males have an increase but those trabeculae are 34% thinner than normal, and the bone volume is more severely reduced in females than in males. An increase in osteoblasts and osteoclasts in the bone perimeter was found to reach statistical significance in female homozygotes but was less pronounced in male homozygotes. Additionally, in heterozygous females but not males fasting serum insulin like growth factor 1 is reduced to an intermediary level.
