Guest post by Basel Al-Barghouthi (ASHG talk: Wed Oct 17, 9:15am – 9:30am in Room 6D)
The sequencing of the human genome and other scientific advances have led to approaches that have revolutionized our ability to understand genetic contributions to disease. One example, genome-wide association studies (GWASs), have successfully identified thousands of associations for hundreds of complex diseases. In the case of osteoporosis, a complex disease characterized by reduced bone strength and increased incidence of fracture, human GWASs have focused on dissecting the genetic basis of bone mineral density (BMD). BMD, however, does not explain all of the phenotypic variance in bone strength and there are many other aspects of bone that influence its strength, such as bone size. Unfortunately, bone strength cannot be measured directly in humans. Therefore, we are performing GWAS for over 60 traits related to bone strength in mice, in order to bridge existing gaps in our knowledge.
In this project, we used the Diversity Outbred (DO), an outbred mouse population derived from eight genetically diverse mouse strains to perform a GWAS for size measurements of mouse femurs. The DO is particularly well-suited for high-resolution GWAS. In the DO, we measured bone size and identified a strong association influencing the width of femurs on Chromosome (Chr.) 1. To identify the gene responsible, we queried the Chr. 1 locus for associated genetic variants that potentially impacted protein activity; however, none were found, suggesting the locus was due to a genetic variant influencing gene expression. We then scanned the locus for variants affecting gene expression. Of all genes within the locus, Quiescin Sulfhydryl Oxidase 1 (Qsox1) was the only one whose expression in bone was regulated by the same variants associated with femoral width in a manner consistent with it being causal. The genetic data suggested that decreased Qsox1 levels would lead to wider bones.
Using CRISPR/Cas9 genome-editing, we tested this prediction by generating Qsox1 mutant mice that completely lacked active QSOX1 protein. Consistent with the genetic data, we observed significantly wider femurs in the absence of QSOX1. We also determined that the bones were wider specifically due to increased formation of bone along the left and right sides of the femur.
These data identify Qsox1 as a genetic determinant of bone size and highlight the power of the DO for the genetic analysis of complex traits, which can be particularly useful for traits that are difficult to measure in humans.