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Paternal Skeletal Size Predicts Intrauterine Bone Mineral Accrual

Medical Research Council Epidemiology Resource Centre (N.C.H., M.K.J., J.R.P., S.M.R., H.M.I., K.M.G., C.C., E.M.D.), University of Southampton, and Medical Physics and Bioengineering (P.T.), Southampton General Hospital, Southampton. SO16 6YD, United Kingdom

Address all correspondence and requests for reprints to: Cyrus Cooper, M.A., D.M., F.R.C.P., F.Med.Sci., Professor of Rheumatology and Director, Medical Research Council Epidemiology Resource Centre, Southampton General Hospital, Southampton SO16 6YD, United Kingdom. E-mail: cc{at}mrc.soton.ac.uk.

	Abstract

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Background: We have previously demonstrated that maternal body build and lifestyle factors predict neonatal bone mineral accrual. However, the paternal determinants of neonatal bone mass are not known. In this study we explored the relationship between a father’s bone mass and that of his offspring.

Methods: A total of 278 pregnancies (142 male and 136 female neonates) were recruited from the Southampton Women’s Survey, a unique, well-established cohort of women, aged 20–34 yr, who had been assessed before and during pregnancy. The neonates and their fathers underwent whole body dual-x-ray absorptiometry (DXA) within 2 wk of birth using a Lunar DPX (General Electric Corp., Madison, WI) and Hologic Discovery instrument (Hologic Inc., Bedford, MA), respectively; correlation and regression methods were used to explore the parental determinants of neonatal bone mass.

Results: After adjusting the paternal DXA indices for father’s age and the neonatal for baby’s gestational age and age at DXA scan, there were highly significant positive associations between baby’s whole body bone area, bone mineral content, and bone mineral density and the corresponding indices in the father (P = 0.003, 0.0002, 0.046, respectively) among female infants. These relationships were independent of maternal height and fat stores. The associations for male infants with paternal DXA indices did not achieve statistical significance.

Conclusions: The father’s skeletal size predicts skeletal size more strongly in female than male offspring, independently of the mother’s body build. These data point toward the importance of considering paternal genotype in studies exploring the developmental origins of osteoporotic fracture and raise intriguing mechanistic questions about the gender specificity of influences on intrauterine bone mineral accrual.

	Introduction

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Recent work has shown that peak bone mass, achieved in early adulthood, is a major determinant of osteoporosis risk in later life (1). Bone size and density appear to track throughout childhood to peak, and we have previously demonstrated that growth in early life, both in utero and in childhood, predicts adult bone mineral content (BMC) (2, 3) and risk of hip fracture (4) in later life. Maternal smoking, decreased fat stores, and vigorous exercise in late pregnancy are all associated with decreased bone mineral accrual in the offspring at birth (5, 6), but thus far there are few data on the contribution of the father to neonatal bone size and density. Although some maternal influences may be in a large part environmental, those from the father are necessarily genetic or epigenetic. Furthermore, the few studies using dual-x-ray absorptiometry (DXA) in parents and children, which report a heritability of around 50% (7, 8), have usually not explored gender-specific patterns of inheritance. We believe that our study is the first to measure, by DXA at birth, the contribution of paternal skeletal size and density to bone mineral accrual in the offspring, and to explore gender differences in these associations.

	Subjects and Methods

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Details of the Southampton Women’s Survey have been published previously (9), but briefly, nonpregnant women aged 20–34 yr were recruited via their general practitioners. Assessments of lifestyle, diet (by validated food-frequency questionnaire) (10), and anthropometry were performed at study entry, and then in early (11 wk) and late (34 wk) gestation among those women who became pregnant. At the early pregnancy visit, the mothers also underwent assessment of heel quantitative ultrasound, using a Hologic Sahara instrument (Sahara; Hologic Inc., Bedford, MA). The intermalleolar distance (using standard calipers) and room temperature were also recorded. In a repeatability study of 40 healthy nonpregnant women, the coefficients of variation (CVs) were found to be 0.8% (speed of sound) and 3.0% (bone ultrasound attenuation) (11). Mothers registered with specific general practitioner practices were invited to participate in the bone component of the Southampton Women’s Survey. These practices were selected to avoid the mothers participating in more than one substudy and were representative of the population of Southampton as a whole.

After her baby’s birth, the mother was asked to give written informed consent for her baby to undergo a DXA within 2 wk of birth. A Lunar DPX instrument with neonatal scan mode and specific pediatric software (General Electric Corp., Madison, WI) was used. The instrument underwent daily quality assessment and was calibrated against a water phantom weekly. At the visit to the scan room, the baby was pacified and fed if necessary, undressed completely, and then swaddled in a standard towel. It was placed on a waterproof sheet in a standard position on the scanner. Measurement of whole body bone area (BA), BMC, areal bone mineral density (aBMD) and body composition was performed, using specific software protocols, while the baby was kept in position using rice bags placed over the bottom end of the towel. The baby was weighed at the end of the visit on calibrated digital scales, and this weight and the previously recorded birth length were entered into the DXA record on the computer. The short-term and long-term CVs for whole body aBMD for the DXA instrument were 0.8 and 1.4%, respectively. The radiation exposure to the baby was estimated as a maximum of 8.9 µSv for whole body and 8.8 µSv for lumbar spine, and institutional review board approval was gained from the local research ethics committee (Southampton and Southwest Hampshire, UK).

In a subset of pregnancies, the fathers were invited, at the neonatal DXA scan, to attend the Osteoporosis Centre at Southampton General Hospital for a DXA scan. After informed written consent was obtained, their height and weight were measured, and whole body, lumbar spine and hip bone mineral, and whole body lean and fat mass were assessed using a Hologic Discovery DXA instrument (Hologic Inc.). The manufacturer’s CV for the instrument was 0.75% for whole body aBMD. Again, institutional review board approval was gained from the local research ethics committee (Southampton and Southwest Hampshire, UK).

Statistical analysis

All variables were checked for normality. Paternal weight, fat mass, and percentage fat mass were skewed and transformed logarithmically to normality. Neonatal fat and percentage fat mass were best transformed by square root and were subsequently standardized to standard deviate units. Volumetric bone mineral density (vBMD) was estimated by adjusting BMC for BA, length for weight and weight. Correlation and linear regression methods were used to explore the relationships between paternal and offspring whole body BA, BMC, aBMD, estimated vBMD, and body composition using Stata V8.2 (StataCorp LP, College Station, TX). The results were further explored with correction for multiple testing using the Bonferroni method (12). Based on a previous parent-offspring cohort, we estimated that to give 90% power to detect a correlation of 0.20 between father and offspring bone mass as being statistically different from zero at the 5% level, we would need 260 subjects.

	Results

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Characteristics of the father and neonates

After matching mothers, fathers, and neonates, there were 278 mother-father-neonate triplets with complete data. The mean (SD) age of the fathers was 33.9 yr (5.2). The mean (SD) height was 1.78 m (0.07), and median (interquartile) body mass index (BMI) was 26.0 kg/m² (23.6–28.7). The mean (SD) whole body BA, BMC, and aBMD were 2252 cm² (153), 2856 g (381), and 1.3 g/cm² (0.1), respectively. Data on the fathers’ height were cross-sectional, and, thus, secular changes by year of birth were possible. Therefore, we explored the relationship between paternal height and age. This revealed a significant tendency for older fathers to be shorter (height decreased by 0.2 cm with each year of increasing age; P = 0.002) There were similar associations between fathers’ height and BA, BMC, and aBMD at each site. Thus, these variables (height and DXA-derived measures of bone mass) were adjusted for paternal age. The mean (SD) height of the mothers was 1.63 m (0.06). All but six mothers and three fathers were white, and ethnicity differed between mother and father in seven cases.

The characteristics of the neonates are summarized by gender in Table 1. Birth weight and neonatal DXA measures increased linearly with gestational age, and in a quadratic relationship with age at DXA. The latter association was consistent with the normal pattern of weight loss immediately after birth, with subsequent gain over the next week, thus giving a nonlinear relationship between age and size in the first 2 wk of life. Thus, birth weight was adjusted for gestational age and gender, and DXA outcomes additionally for age at DXA.

When the neonates were analyzed together, there were statistically significant associations between the paternal and corresponding neonatal whole body DXA indices. However, when the associations were explored by offspring gender, it was apparent that the father-daughter relationships were strong (Fig. 1A), whereas the father-son relationships did not reach statistical significance (Fig. 1B). Limiting the analysis to the lowest 75% of birth weight (to investigate whether it was the girls’ lower mean birth weight that allowed a greater paternal influence) did not change the pattern of associations. However, the interaction terms for paternal BA, BMC, and aBMD and offspring gender failed to attain statistical significance (P = 0.114, P = 0.061, P = 0.709, respectively). Paternal whole body BMC was positively associated with female neonatal total lean (r = 0.25; P = 0.0037) and fat (r = 0.22; P = 0.009) mass, and negatively with percentage lean (r = –0.18; P = 0.034). Consistent with this pattern, there was a positive association with percentage fat (r = 0.17; P = 0.052). Thus, the relative proportions of fat and lean changed as overall neonatal size increased. Table 2 summarizes the relationships between paternal and neonatal bone mass/body composition. After adjusting for multiple comparisons with a Bonferroni correction, the relationships were attenuated but retained statistical significance for paternal with neonatal BA (P = 0.012) and BMC (P = 0.0032), although not aBMD (P = 0.736).

View larger version (19K): [in this window] [in a new window]   FIG. 1. Paternal and neonatal whole body bone mineral. A, Female neonates. B, Male neonates.

Maternal-neonatal relationships

Maternal and paternal height predicted neonatal birth weight similarly (maternal height: β = 10.5 g/cm, P = 0.010; paternal height: β =11.0 g/cm, P = 0.008). Increasing parity was associated with increasing birth weight, BA, and BMC (P < 0.01). Thus, the mean (SD) birth weight of first-order children was 3450 g (436), and whole body BMC was 61.3 g (12.7), and the corresponding figures for higher birth-order children were 3643 g (407) for birth weight and 66.6 g (13.8) for BMC. Children of mothers with greater fat stores (measured by triceps skinfold thickness in late pregnancy) were also of greater birth weight and bone mass (birth weight: β = 72.8 g/SD, P = 0.005; whole body BMC: β = 2.6 g/SD, P = 0.001). These associations were independent of the paternal-neonatal relationships.

Maternal heel speed of sound and bone ultrasound attenuation, and milk intake in pregnancy were not statistically significantly associated with neonatal bone size or density. However, the maternal serum 25(OH)-vitamin D level, measured in late pregnancy, did show a positive association with bone size in the female offspring in the larger birth cohort. Thus, the mean whole body BA of the female offspring of mothers with levels less than 33 nmol/liter was 110 vs. 119 cm² in the offspring of mothers more than 33 nmol/liter (P = 0.04). The results for whole body BMC were 58 vs. 63 g (P = 0.04), respectively. These associations were again independent of the paternal relationships.

	Discussion

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Paternal skeletal size and areal density were positively associated with intrauterine bone mineral accrual more strongly among female than male offspring. These associations were independent of maternal body build. As far as we are aware, this is the first study using DXA to relate the bone mass of fathers to that of their offspring at birth.

The strongest associations detected were between corresponding paternal and female neonatal DXA indices, and the pattern suggests possible discordance between the inheritance of bone size and bone density. Paternal BA, BMC, aBMD, and vBMD predicted neonatal BA and BMC more strongly than aBMD and vBMD. This finding is consistent with previous reports showing that skeletal size, rather than density, is influenced by early life factors (2, 13). Density tends to be more dependent on environmental influences later in the life course, such as loading and nutrition. The lack of association with paternal height, and the strong relationship between neonatal BMC and paternal vBMD, suggest that both paternal skeletal size and volumetric density are predictors of neonatal BMC. However, to investigate fully the influences on volumetric density, a three-dimensional technique such as peripheral quantitative computed tomography would be needed.

The influence of the father on skeletal development in the offspring is necessarily genetic, in contrast to the combined genetic and environmental influences of the mother. Birth weight has been shown to be only modestly inherited (14), with maternal constraint, i.e. the ability of the mother to supply the fetus with required nutrition, a more important determinant. Thus, in a Shetland pony/Shire horse cross, the foal is smaller at birth when the mother is the pony rather than the horse (15). Factors such as maternal smoking, nutritional status, and physical activity, which have influenced intrauterine bone mineral accrual (5), are likely to act via modulation of nutrient flow to the fetus; the effect of parental height, similar for mother and father in our study, most likely reflects genetic inheritance.

Maternal and paternal genetic influences may be altered as a result of epigenetic modification of the genome early in development (16). This may result in a phenomenon known as imprinting, in which only the maternal or paternal allele is expressed in the fetus, allowing one parent, rather than both, to determine a particular trait (17, 18). The extent to which such epigenetic phenomena may explain the difference in magnitude we observed between associations in male and female offspring remains uncertain.

There are few previous data relating to offspring-parent inheritance of bone mass. Daughters of women with osteoporosis have had reduced bone density (7, 19); another study demonstrated mother-daughter and father-son heritability of aBMD (20). However, greater heritability of femoral neck aBMD for father-daughter than father-son associations has been found (21). In addition, a study of Indian families, in which DXA in both mother and father was obtained, showed associations of similar magnitude between offspring (age 6 yr) BMC and either parent (whole body BMC: mother to offspring r = 0.36, father to offspring r = 0.38; both P < 0.001) (22). However, the relationships were not examined separately in male and female offspring. The correlation coefficient for father-offspring whole body BMC in this study was somewhat stronger than our figure of 0.19 for both genders combined and 0.32 for females only. This might reflect a paternal genetic influence becoming more marked as the effect of intrauterine constraint declines and would be consistent with documented patterns of infant vs. intrauterine growth (23).

Therefore, there is little existing evidence to support a differential association between father and offspring bone mass in male and female offspring, respectively. The weakly positive (although not statistically significant) offspring gender-bone mass interaction terms for BA and BMC in our work would be consistent with such a notion, but this interaction term demonstrated no relationship for aBMD. Thus, any gender association is more likely to be reflected in skeletal size than in volumetric density. This conclusion is supported by the lack of association between paternal and neonatal aBMD after correction for multiple testing. We considered that the girls, having a lower mean birth weight, might be less susceptible to maternal constraint and, thus, more susceptible to paternal influence. However, in analyses limiting the subjects to those in the lowest 75% of the birth weight distribution, the gender disparity was still apparent (data not shown). If our observations reflect a true difference, we speculate that there might be a gender/imprinting interaction, such that the paternal allele of a gene influencing skeletal growth tends to be expressed in girls but not boys. An alternative explanation could be that other sex-dependent factors, such as estrogen/androgen balance, might modify the genetic relationship.

This was a prospective study, with detailed characterization of the mothers, babies, and fathers, and a gold standard measurement of bone mineral. However, there were also several limitations. First, we did not have objective evidence of paternity. True paternity has been estimated at 85–90% (24), but this would tend to make finding significant associations less likely. Second, although the use of DXA is well established in adults, there are limitations to its use in babies. Specific pediatric software was used to minimize the loss of edge detection, and movement artifact was modest and uniform across the cohort. The few babies with excessive movement were excluded from our analysis. Third, for ethical reasons we did not have prenatal maternal DXA data to explore the relative magnitude of parental influences. Maternal BMC and aBMD have been documented to change over pregnancy and 1 yr or more after delivery, particularly with breast-feeding (25, 26), and, thus, it is difficult to define a post-pregnancy time point when a reliable measure could be made by DXA. Instead, we used heel ultrasound measurements in early pregnancy, a technique that carries no risk to the baby. Finally, our study population was a randomly selected subset of a larger cohort; nevertheless, we had ample statistical power to detect parent-offspring relationships.

In conclusion, we have demonstrated that paternal skeletal size and density predict intrauterine bone mineral accrual more strongly in female than male offspring, and that these associations are independent of maternal factors. These data suggest that the genotype of the father should be considered in future studies of the early life determinants of adult osteoporotic fracture, and raise intriguing mechanistic questions regarding the gender specificity of influences on intrauterine bone mineral accrual.

	Acknowledgments

 
* Members of the Southampton Women’s Survey Study Group were David J. P. Barker, Catherine M. Law, Vanessa Cox, Patricia Coakley, and Julia Hammond.

We thank the mothers who gave us their time, I. Cameron and T. Wheeler for allowing us to include their patients, and a team of dedicated research nurses and ancillary staff for their assistance. We also thank Mrs. G. Strange for helping prepare the manuscript.

	Footnotes

 
This work was supported by the Medical Research Council, Arthritis Research Campaign, and National Osteoporosis Society, International Osteoporosis Foundation. Participants were drawn from a cohort study funded by the Medical Research Council. M.K.J. was in receipt of a Claus Christiansen Fellowship from the International Osteoporosis Foundation to undertake this study.

Disclosure Statement: N.C.H., M.K.J., E.M.D., and C.C. have all undertaken lecturing commitments for Procter & Gamble Pharmaceuticals, Glaxo Smith Kline/Roche Ltd., Merck Sharp and Dohme, Eli Lilly, and Servier Ltd. C.C. has also been a consultant with the aforementioned companies.

First Published Online February 19, 2008

¹ For a list of members of the Southampton Women’s Survey Study Group, see Acknowledgments.

Abbreviations: aBMD, Areal bone mineral density; BA, bone area; BMC, bone mineral content; BMI, body mass index; CV, coefficient of variation; DXA, dual-x-ray absorptiometry; vBMD, volumetric bone mineral density.

Received February 6, 2007.

Accepted February 7, 2008.

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This Article Abstract Full Text (PDF) Submit a related Letter to the Editor Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Harvey, N. C. Search for Related Content PubMed PubMed Citation Articles by Harvey, N. C. Related Collections Pediatric Endocrinology Calcium and Bone Metabolism