Genotype-by-sex and environment-by-sex interactions influence variation in serum levels of bone-specific alkaline phosphatase in adult baboons (Papio hamadryas)

The role of sex in the genomics of human complex traits

Nearly all human complex traits and disease phenotypes exhibit some degree of sex differences, including differences in prevalence, age of onset, severity or disease progression. Until recently, the underlying genetic mechanisms of such sex differences have been largely unexplored. Advances in genomic technologies and analytical approaches are now enabling a deeper investigation into the effect of sex on human health traits. In this Review, we discuss recent insights into the genetic models and mechanisms that lead to sex differences in complex traits. This knowledge is critical for developing deeper insight into the fundamental biology of sex differences and disease processes, thus facilitating precision medicine.

Baboons as a model to study genetics and epigenetics of human disease

A major challenge for understanding susceptibility to common human diseases is determining genetic and environmental factors that influence mechanisms underlying variation in disease-related traits. The most common diseases afflicting the US population are complex diseases that develop as a result of defects in multiple genetically controlled systems in response to environmental challenges. Unraveling the etiology of these diseases is exceedingly difficult because of the many genetic and environmental factors involved. Studies of complex disease genetics in humans are challenging because it is not possible to control pedigree structure and often not practical to control environmental conditions over an extended period of time. Furthermore, access to tissues relevant to many diseases from healthy individuals is quite limited. The baboon is a well-established research model for the study of a wide array of common complex diseases, including dyslipidemia, hypertension, obesity, and osteoporosis. It is possible to acquire tissues from healthy, genetically characterized baboons that have been exposed to defined environmental stimuli. In this review, we describe the genetic and physiologic similarity of baboons with humans, the ability and usefulness of controlling environment and breeding, and current genetic and genomic resources. We discuss studies on genetics of heart disease, obesity, diabetes, metabolic syndrome, hypertension, osteoporosis, osteoarthritis, and intrauterine growth restriction using the baboon as a model for human disease. We also summarize new studies and resources under development, providing examples of potential translational studies for targeted interventions and therapies for human disease.

The variability is in the sex chromosomes

Sex differences in the mean trait expression are well documented, not only for traits that are directly associated with reproduction. Less is known about how the variability of traits differs between males and females. In species with sex chromosomes and dosage compensation, the heterogametic sex is expected to show larger trait variability ("sex-chromosome hypothesis"), yet this central prediction, based on fundamental genetic principles, has never been evaluated in detail. Here we show that in species with heterogametic males, male variability in body size is significantly larger than in females, whereas the opposite can be shown for species with heterogametic females. These results support the prediction of the sex-chromosome hypothesis that individuals of the heterogametic sex should be more variable. We argue that the pattern demonstrated here for sex-specific body size variability is likely to apply to any trait and needs to be considered when testing predictions about sex-specific variability and sexual selection.

Bone mineral density variation in men is influenced by sex-specific and non sex-specific quantitative trait loci

INTRODUCTION

A major predictor of age-related osteoporotic fracture is peak areal bone mineral density (aBMD) which is a highly heritable trait. However, few linkage and association studies have been performed in men to identify the genes contributing to normal variation in aBMD. The aim of this study was to perform a genome wide scan in healthy men to identify quantitative trait loci (QTL) that were significantly linked to aBMD and to test whether any of these might be sex-specific.

METHODS

aBMD at the spine and hip were measured in 515 pairs of brothers, aged 18-61 (405 white pairs, 110 black pairs). Linkage analysis in the brother sample was compared with results in a previously published sample of 774 sister pairs to identify sex-specific quantitative trait loci (QTL).

RESULTS

A genome wide scan identified significant QTL (LOD>3.6) for aBMD on chromosomes 4q21 (hip), 7q34 (spine), 14q32 (hip), 19p13 (hip), 21q21 (hip), and 22q13 (hip). Analysis suggested that the QTL on chromosomes 7q34, 14q32, and 21q21 were male-specific whereas the others were not sex-specific.

CONCLUSIONS

This study demonstrates that six QTL were significantly linked with aBMD in men. One was linked to the spine and five were linked to the hip. When compared to published data in women from the same geographical region, the QTL on chromosomes 7, 14 and 21 were male-specific. The occurrence of sex-specific genes in humans for aBMD has important implications for the pathogenesis and treatment of osteoporosis.

Genetic variation in baboon craniofacial sexual dimorphism

Sexual dimorphism is a widespread phenomenon and contributes greatly to intraspecies variation. Despite a long history of active research, the genetic basis of dimorphism for complex traits remains unknown. Understanding the sex-specific differences in genetic architecture for cranial traits in a highly dimorphic species could identify possible mechanisms through which selection acts to produce dimorphism. Using distances calculated from three-dimensional landmark data from CT scans of 402 baboon skulls from a known genealogy, we estimated genetic variance parameters in both sexes to determine the presence of gene-by-sex (G x S) interactions and X-linked heritability. We hypothesize that traits exhibiting the greatest degree of sexual dimorphism (facial traits in baboons) will demonstrate either stronger G x S interactions or X-linked effects. We found G x S interactions and X-linked effects for a few measures that span the areas connecting the face to the neurocranium but for no traits restricted to the face. This finding suggests that facial traits will have a limited response to selection for further evolution of dimorphism in this population. We discuss the implications of our results with respect to the origins of cranial sexual dimorphism in this baboon sample, and how the genetic architecture of these traits affects their potential for future evolution.

Osteopenia and osteoporosis in adult baboons (Papio hamadryas)

BACKGROUND

Little is known about the degree to which baboons, an important animal model in skeletal research, spontaneously experience age-related osteopenia and osteoporosis.

METHODS

We measured bone mineral density (BMD) in 667 baboons, assigned T-scores to older animals based on sex-specific young adult reference groups, and compared reproductive history in older females with low BMD to those with normal BMD.

RESULTS

Approximately 25% of older baboon females were osteopenic. No females or males were osteoporotic. Neither parity nor interbirth interval spine clearly distinguished low vs. normal BMD groups. Intersite correspondence in low BMD was highest between sites in the same region rather than sites of the same bone type.

CONCLUSION

As with humans, osteopenia is common among older females. The absence of osteoporotic animals may be due to colony maintenance resulting in truncation of the aged population and selection for healthier animals in the oldest ranges.

Quantitative genetics of cortical bone mass in healthy 10-year-old children from the Fels Longitudinal Study

The genetic influences on bone mass likely change throughout the life span, but most genetic studies of bone mass regulation have focused on adults. There is, however, a growing awareness of the importance of genes influencing the acquisition of bone mass during childhood on lifelong bone health. The present investigation examines genetic influences on childhood bone mass by estimating the residual heritabilities of different measures of second metacarpal bone mass in a sample of 600 10-year-old participants from 144 families in the Fels Longitudinal Study. Bivariate quantitative genetic analyses were conducted to estimate genetic correlations between cortical bone mass measures, and measures of bone growth and development. Using a maximum likelihood-based variance components method for pedigree data, we found a residual heritability estimate of 0.71 for second metacarpal cortical index. Residual heritability estimates for individual measures of cortical bone (e.g., lateral cortical thickness, medial cortical thickness) ranged from 0.47 to 0.58, at this pre-pubertal childhood age. Low genetic correlations were found between cortical bone measures and both bone length and skeletal age. However, after Bonferonni adjustment for multiple testing, rho(G) was not significantly different from 0 for any of these pairs of traits. Results of this investigation provide evidence of significant genetic control over bone mass largely independent of maturation while bones are actively growing and before rapid accrual of bone that typically occurs during puberty.

Genetic and environmental determinants of variation of soluble adhesion molecules

In our research we examined the contribution of putative genetic sources on interindividual variation and cross-sectional correlations of several adhesion molecules, including intracellular (ICAM-1) and vascular cell adhesion molecules (VCAM-1) and E-selectin, in a population-based sample of ethnically homogeneous families of European origin. The plasma levels of these molecules were measured in 947 apparently healthy individuals from 217 nuclear families. Quantitative statistical-genetic analysis implementing the model fitting technique revealed significant parent/offspring and sibling correlations (p < 0.01) for all three molecules. The putative genetic effects explained 55.2 +/- 7.2% (VCAM-1), 63.3 +/- 7.5% (ICAM) and 63.8 +/- 8.1% (E-selectin) of the variation. Common family environmental factors also significantly influenced the variation of E-selectin (13%) and VCAM-1 (28.6%). The main results of our bivariate analysis showed that the observed phenotypic correlations between ICAM-1 and VCAM-1, and between ICAM-1 and E-selectin, were mostly attributable to shared environmental factors (r(E)= 0.896 and 0.737, respectively; p < 0.01). However, the correlation between VCAM-1 and E-selectin was likely caused by common genetic effects (r(G)= 0.334, p < 0.05). Our results show that familial clustering of adhesion molecules is likely due to strong genetic effects, supplemented with shared environmental factors.

QTL with pleiotropic effects on serum levels of bone-specific alkaline phosphatase and osteocalcin maps to the baboon ortholog of human chromosome 6p23-21.3

Bone ALP and OC are under partial genetic control. This study of 591 pedigreed baboons shows a QTL corresponding to human 6p23-21.3 that accounts for 25% (bone ALP) and 20% (OC) of the genetic variance. A gene affecting osteoblast activity, number, or recruitment likely resides in this area.

INTRODUCTION

Serum levels of bone alkaline phosphatase (ALP) and osteocalcin (OC) reflect osteoblast activity. Both of these measures are under partial genetic control. Genetic effects on bone ALP have not been previously localized to chromosomal regions in primates, nor has the degree to which genetic effects are shared (pleiotropic) between bone ALP and OC been studied.

MATERIALS AND METHODS

We applied variance components methods to a sample of 591 adult pedigreed baboons to detect and quantify effects of genes that influence bone ALP and that have pleiotropic effects on bone ALP and OC. A univariate linkage analysis was conducted for bone ALP. Bivariate linkage analyses were conducted in areas for which the bone ALP results presented here and a previous univariate OC linkage analysis showed evidence for linkage on the same chromosome for both bone ALP and OC.

RESULTS

A quantitative trait locus (QTL) for serum levels of bone ALP is evident on the baboon ortholog of human chromosomal region 6p (LOD 2.93). Thirty-seven percent (genetic correlation [rho(G)] = 0.61) of the genetic variance in bone ALP and OC is caused by pleiotropic effects of the same gene(s). Bivariate linkage analysis revealed a QTL in the region corresponding to human chromosome 6p23-21.3, with the strongest evidence for bivariate linkage near D6S422 (LOD = 2.97 at 22 cM from our pter-most marker). D6S422 maps to 20.4 Mb in the human genome. The QTL-specific heritability (h2) is 0.25 and 0.20 for bone ALP and OC, respectively.

CONCLUSIONS

This first formal test for shared genetic effects on two serum markers of osteoblast activity indicates that a significant pleiotropic effect on bone ALP and OC levels, and thus on bone formation, is detectible. The fact that this region corresponds to one on mouse chromosome 13 that has repeatedly yielded QTLs for BMD should encourage more intensive study of the effect of genes in this region on bone maintenance and turnover.

Bone ALP and OC reference standards in adult baboons (Papio hamadryas) by sex and age

BACKGROUND

Serum bone-specific alkaline phosphatase (bone ALP) and intact osteocalcin (OC) are markers of bone formation of interest because of easy measurability and potential utility as identifiers of those at risk for fractures associated with bone metabolism disorders. The baboon (Papio hamadryas) exhibits extensive biological similarities to humans making it particularly well suited to studies of bone maintenance and turnover.

METHODS

We measured serum bone ALP and OC in 591 baboons.

RESULTS

We report significant sex and age effects and present reference ranges and percentile distributions for these markers.

CONCLUSIONS

This study is the first to characterize normal variation in bone ALP and OC levels in baboons and to assess the age and sex effects on this variation. The results provide much-needed reference standards to allow researchers to evaluate the status of their animals in cross-sectional studies and assess the meaning of changes in bone ALP and OC levels in longitudinal studies.