Kun, E. | bioRxiv Science

bioRxiv2024

The genetic architecture and evolutionary consequences of the human pelvic form

Human pelvic shape has undergone significant evolutionary change since the divergence from the chimpanzee lineage. This transformation, involving the reduction of the pelvic canal size to support bipedal locomotion, is thought to give rise to the obstetrical dilemma, a hypothesis highlighting the mismatch between the large brain size of infants and the narrowed birth canal in females. Empirical evidence for this classic hypothesis has been equivocal, largely due to a lack of sample size and appropriate types of data. To elucidate the genetic underpinnings of pelvic morphology, we applied a deep learning model to 31,115 dual-energy X-ray absorptiometry (DXA) from the UK Biobank, extracting a set of seven pelvic proportion (PP) phenotypes, including measures of the birth canal. All PPs were found to be highly heritable ([~]25-40%) and a genome-wide association study of these traits identified 179 independent loci. Unlike other skeletal proportions including long bone lengths, the subpubic angle associated with the birth canal exhibits a genetic correlation between sexes significantly less than 1, in line with sex-specific reproductive function. PPs were also left-right asymmetric but not heritable and instead associated with handedness. We conducted phenotypic and genetic association analyses to link PPs to 3 facets of the dilemma: locomotion, pelvic floor function and childbirth. Larger birth canal phenotypes were associated with reduced walking pace, decreased risk of back pain, and increased risk of hip osteoarthritis - phenotypes linked to locomotor efficiency. We also observed that a narrower birth canal width was associated with a reduced risk of pelvic floor disorders. When examining childbirth-related outcomes, narrower birth canal phenotypes were associated with increased risk of emergency cesarean sections and obstructed labor due to insufficient dilation, but not obstructed labor due to positioning of the fetus. Finally, we examined whether the dilemma might have been alleviated through evolution. We found no association between any PPs and gestational duration, contrary to the initial prediction by Washburn in 1960. However, we found that the birth weight of the child, a proxy for skull and brain size, was genetically correlated with birth canal width but not with other PPs. Collectively, our study offers fresh insight on a 60-year-old debate in human evolutionary studies. Our results support the idea that the obstetrical dilemma has played a central role in the co-evolution of the human brain and pelvis, while also highlighting the potential role of associated factors such as pelvic floor health.

genetics↗

bioRxiv2023

The genetic architecture of the human skeletal form

The human skeletal form underlies our ability to walk on two legs, but unlike standing height, the genetic basis of limb lengths and skeletal proportions is less well understood. Here we applied a deep learning model to 31,221 whole body dual-energy X-ray absorptiometry (DXA) images from the UK Biobank (UKB) to extract 23 different image-derived phenotypes (IDPs) that include all long bone lengths as well as hip and shoulder width, which we analyzed while controlling for height. All skeletal proportions are highly heritable ([~]40-50%), and genome-wide association studies (GWAS) of these traits identified 179 independent loci, of which 102 loci were not associated with height. These loci are enriched in genes regulating skeletal development as well as associated with rare human skeletal diseases and abnormal mouse skeletal phenotypes. Genetic correlation and genomic structural equation modeling indicated that limb proportions exhibited strong genetic sharing but were genetically independent of width and torso proportions. Phenotypic and polygenic risk score analyses identified specific associations between osteoarthritis (OA) of the hip and knee, the leading causes of adult disability in the United States, and skeletal proportions of the corresponding regions. We also found genomic evidence of evolutionary change in arm-to-leg and hip-width proportions in humans consistent with striking anatomical changes in these skeletal proportions in the hominin fossil record. In contrast to cardiovascular, auto-immune, metabolic, and other categories of traits, loci associated with these skeletal proportions are significantly enriched in human accelerated regions (HARs), and regulatory elements of genes differentially expressed through development between humans and the great apes. Taken together, our work validates the use of deep learning models on DXA images to identify novel and specific genetic variants affecting the human skeletal form and ties a major evolutionary facet of human anatomical change to pathogenesis.

genomics↗