Gerald E. McClearn: complexity in behavior genetics.

A festskrift in honor of the career contributions of Gerald E. McClearn was held in State College, Pennsylvania in May 2009. A selection of papers presented at that celebration is included in this issue of Behavior Genetics. These papers illustrate contemporary progress in research areas that have been chosen to reflect key aspects of Jerry's career accomplishments.

Genomic analysis of variation in hindlimb musculature of mice from the C57BL/6J and DBA/2J lineage.

The precise locations of attachment points of muscle to bone-the origin and insertion sites-are crucial anatomical and functional characteristics that influence locomotor performance. Mechanisms that control the development of these interactions between muscle, tendon, and bone are currently not well understood. In a subset of BXD recombinant inbred (RI) strains derived from the C57BL/6J and DBA/2J strains, we observed a soleus femoral attachment anomaly (SFAA) that was rare in both parental strains (Lionikas, Glover et al. 2006). The aim of the present study was to assess suitability of SFAA as a model to study the genetic mechanisms underlying variation in musculoskeletal anatomy. We scored the incidence of SFAA in 55 BXD strains (n = 9 to 136, median = 26, phenotyped animals per strain, for a total number of 2367). Seven strains (BXD1, 12, 38, 43, 48, 54, and 56) exhibited a high incidence of unilateral SFAA (47-89%), whereas 23 strains scored 0%. Exploration of the mechanisms underlying SFAA in 2 high incidence strains, BXD1 and BXD38, indicated that SFAA-relevant genes are to be found in both C57BL/6J and DBA/2J regions of the BXD1 genome. However, not all alleles relevant for the expression of the phenotype were shared between the 2 high-incidence BXD strains. In conclusion, the anatomical origin of the soleus muscle in mouse is controlled by a polygenic system. A panel of BXD RI strains is a useful tool in exploring the genetic mechanisms underlying SFAA and improving our understanding of musculoskeletal development.

Stability of genetic influences on pulmonary function in a longitudinal study of octogenarian twins.

Using data from the first four waves of the OCTO-Twin study (twins 80 + years), the present study investigated the stability and change of genetic and environmental contributions to pulmonary function. Using a genetic simplex model, variance in peak expiratory flow (PEF) at each wave was decomposed into additive genetic and nonshared (specific) environmental factors. Additionally, this analysis distinguished the source of these influences, either from previous waves (transmissions) or from novel influences at each wave (innovations). At each time point (except wave 1), the genetic variance was due to genetic transmissions from prior time points. Conversely, the specific environmental variance in PEF at each time point was mainly due to environmental innovations. These results imply that genetic factors contribute to the stability of pulmonary function over time whereas environmental factors contribute to its change.

Blood pressure and heart rate QTL in mice of the B6/D2 lineage: sex differences and environmental influences.

A quantitative trait locus (QTL) approach was used to define the genetic architecture underlying variation in systolic blood pressure (SBP) and heart rate (HR), measured indirectly on seven occasions by the tail cuff procedure. The tests were conducted in 395 F(2) adult mice (197 males, 198 females) derived from a cross of the C57BL/6J (B6) and DBA/2J (D2) strains and in 22 BXD recombinant-inbred (RI) strains. Interval mapping of F(2) data for the first 5 days of measurement nominated one statistically significant and one suggestive QTL for SBP on chromosomes (Chr) 4 and 14, respectively, and two statistically significant QTL for HR on Chr 1 (which was specific to female mice) and Chr 5. New suggestive QTL emerged for SBP on Chr 3 (female-specific) and 8 and for HR on Chr 11 for measurements recorded several weeks after mice had undergone stressful blood sampling procedures. The two statistically significant HR QTL were confirmed by analyses of BXD RI strain means. Male and female F(2) mice did not differ in SBP or HR but RI strain analyses showed pronounced strain-by-sex interactions and a negative genetic correlation between the two measures in both sexes. Evidence for a role for mitochondrial DNA was found for both HR and SBP. QTL for HR and SBP may differ in males and females and may be sensitive to different environmental contexts.

Analysis of candidate genes and hypertension in African American adults.

OBJECTIVE: We investigated the associations between hypertension status and the genotypes of four single nucleotide polymorphism (SNP) sites in four hypertension-related genes (Angiotensinogen [AGT], Angiotensin I Converting Enzyme [ACE], Angiotensinogen II receptor, subtype 1 [AGTR1], and Alpha 1-Antichymotrypsin [ACT or SERPINA3]), in an African American sample. METHODS: DNA from 628 participants of the Carolina African American Twin Study of Aging project, a population-based study of African American adult twins, was genotyped using SNPs shown to be associated with hypertension in other studies. RESULTS: The ACE SNP (ACE4 or A-240T) was associated with hypertension (P = .047 in a generalized estimating equations alternating logistics regression model that included age, body mass index, sex, and education. The analysis indicated a protective effect of the TT genotype (odds ratio [OR] 1.59, 95% confidence interval [CI] 1.03-2.48, P = .04) and of the AT genotype (OR 1.91, 95% CI 1.01-3.62, P = .047) compared with the AA genotype. DISCUSSION: These results extend previous findings of associations of various polymorphisms of ACE to hypertension and support the association of hypertension to the A allele of ACE4. The potential for this polymorphism to alter expression by its position in the gene's promoter region suggests that future studies of altered ACE protein activity are warranted.

QTL analysis of trabecular bone in BXD F2 and RI mice.

UNLABELLED: A sample of 693 mice was used to identify regions of the mouse genome associated with trabecular bone architecture as measured using microCT. QTLs for bone in the proximal tibial metaphysis were identified on several chromosomes indicating regions containing genes that regulate properties of trabecular bone. INTRODUCTION: Age-related osteoporosis is a condition of major concern because of the morbidity and mortality associated with osteoporotic fractures in humans. Osteoporosis is characterized by reduced bone density, strength, and altered trabecular architecture, all of which are quantitative traits resulting from the actions of many genes working in concert with each other and the environment over the lifespan. microCT gives accurate measures of trabecular bone architecture providing phenotypic data related to bone volume and trabecular morphology. The primary objective of this research was to identify chromosomal regions called quantitative trait loci (QTLs) that contain genes influencing trabecular architecture as measured by microCT. MATERIALS AND METHODS: The study used crosses between C57BL/6J (B6) and DBA/2J (D2) as progenitor strains of a second filial (F2) generation (n = 141 males and 148 females) and 23 BXD recombinant inbred (RI) strains (n approximately 9 of each sex per strain). The proximal tibial metaphyses of the 200-day-old mice were analyzed by microCT to assess phenotypic traits characterizing trabecular bone, including bone volume fraction, trabecular connectivity, and quantitative measures of trabecular orientation and anisotropy. Heritabilities were calculated and QTLs were identified using composite interval mapping. RESULTS: A number of phenotypes were found to be highly heritable. Heritability values for measured phenotypes using RI strains ranged from 0.15 for degree of anisotropy in females to 0.51 for connectivity density in females and total volume in males. Significant and confirmed QTLs, with LOD scores 4.3 in the F2 cohort and 1.5 in the corresponding RI cohort were found on chromosomes 1 (43 cM), 5 (44 cM), 6 (20 cM), and 8 (49 cM). Other QTLs with LOD scores ranging from 2.8 to 6.9 in the F2 analyses were found on chromosomes 1, 5, 6, 8, 9, and 12. QTLs were identified using data sets comprised of both male and female quantitative traits, suggesting similar genetic action in both sexes, whereas others seemed to be associated exclusively with one sex or the other, suggesting the possibility of sex-dependent effects. CONCLUSIONS: Identification of the genes underlying these QTLs may lead to improvements in recognizing individuals most at risk for developing osteoporosis and in the design of new therapeutic interventions.

Anomaly of anatomical origin of soleus muscle: a mouse model.

In the laboratory mouse, the soleus muscle arises at the head of the fibula and inserts via the Achilles tendon on the tuber calcanei together with the gastrocnemius muscle. During routine dissection of mice from the BXD recombinant inbred (RI) strains, we found that the soleus often originated from the lateral epicondyle of the femur instead of the head of the fibula. This soleus femoral attachment anomaly (SFAA) changes the soleus from being a single-joint to a two-joint muscle. The incidence of SFAA was 45% in the BXD38 RI strain. Bilateral inspection indicated that SFAA may be present unilaterally or bilaterally within an individual mouse. We explored the effect of SFAA on muscle weight in mice with unilateral expression. The weight of SFAA soleii was significantly less (P < 0.01) than that of the soleii with normal attachment by 6% (females) and 14% (males). Similar anatomical anomalies of the soleus muscle have been noted in humans. The mouse model will provide the means to explore the physiological consequences and genetic basis for such anomalies.

Quantitative trait loci analysis of structural and material skeletal phenotypes in C57BL/6J and DBA/2 second-generation and recombinant inbred mice.

UNLABELLED: QTL analyses identified several chromosomal regions influencing skeletal phenotypes of the femur and tibia in BXD F2 and BXD RI populations of mice. QTLs for skeletal traits co-located with each other and with correlated traits such as body weight and length, adipose mass, and serum alkaline phosphatase. INTRODUCTION: Past research has shown substantial genetic influence on bone quality, and the impact of reduced bone mass on our aging population has heightened the interest in skeletal genetic research. MATERIALS AND METHODS: Quantitative trait loci (QTL) analyses were performed on morphologic measures and structural and material properties of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) F2 (second filial generation; n = 400) and BXD recombinant inbred (RI; n = 23 strains) populations of mice. Body weight, body length, adipose mass, and serum alkaline phosphatase were correlated phenotypes included in the analyses. RESULTS: Skeletal QTLs for morphologic bone measures such as length, width, cortical thickness, and cross-sectional area mapped to nearly every chromosome. QTLs for both structural properties (ultimate load, yield load, or stiffness) and material properties (stress and straincharacteristics and elastic modulus) mapped to chromosomes 4, 6, 9, 12, 13, 15, and 18. QTLs that were specific to structural properties were identified on chromosomes 1, 2, 3, 7, 8, and 17, and QTLs that were specific to skeletal material properties were identified on chromosomes 5, 11, 16, and 19. QTLs for body size (body weight, body length, and adipose mass) often mapped to the same chromosomal regions as those identified for skeletal traits, suggesting that several QTLs identified as influencing bone could be mediated through body size. CONCLUSION: New QTLs, not previously reported in the literature, were identified for structural and material properties and morphological measures of the mouse femur and tibia. Body weight and length, adipose mass, and serum alkaline phosphatase were correlated phenotypes that mapped in close proximity of skeletal chromosomal loci. The more specific measures of bone quality included in this investigation enhance our understanding of the functional significance of previously identified QTLs.

Adjusting data to body size: a comparison of methods as applied to quantitative trait loci analysis of musculoskeletal phenotypes.

UNLABELLED: The aim of this study was to compare three methods of adjusting skeletal data for body size and examine their use in QTL analyses. It was found that dividing skeletal phenotypes by body mass index induced erroneous QTL results. The preferred method of body size adjustment was multiple regression. INTRODUCTION: Many skeletal studies have reported strong correlations between phenotypes for muscle, bone, and body size, and these correlations add to the difficulty in identifying genetic influence on skeletal traits that are not mediated through overall body size. Quantitative trait loci (QTL) identified for skeletal phenotypes often map to the same chromosome regions as QTLs for body size. The actions of a QTL identified as influencing BMD could therefore be mediated through the generalized actions of growth on body size or muscle mass. MATERIALS AND METHODS: Three methods of adjusting skeletal phenotypes to body size were performed on morphologic, structural, and compositional measurements of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) second generation (F(2)) mice (n = 400). A common method of removing the size effect has been through the use of ratios. This technique and two alternative techniques using simple and multiple regression were performed on muscle and skeletal data before QTL analyses, and the differences in QTL results were examined. RESULTS AND CONCLUSIONS: The use of ratios to remove the size effect was shown to increase the size effect by inducing spurious correlations, thereby leading to inaccurate QTL results. Adjustments for body size using multiple regression eliminated these problems. Multiple regression should be used to remove the variance of co-factors related to skeletal phenotypes to allow for the study of genetic influence independent of correlated phenotypes. However, to better understand the genetic influence, adjusted and unadjusted skeletal QTL results should be compared. Additional insight can be gained by observing the difference in LOD score between the adjusted and nonadjusted phenotypes. Identifying QTLs that exert their effects on skeletal phenotypes through body size-related pathways as well as those having a more direct and independent influence on bone are equally important in deciphering the complex physiologic pathways responsible for the maintenance of bone health.

Serum alkaline phosphatase activity is regulated by a chromosomal region containing the alkaline phosphatase 2 gene (Akp2) in C57BL/6J and DBA/2J mice.

Quantitative trait locus (QTL) analyses were conducted to identify chromosomal regions that contribute to variability in serum alkaline phosphatase (AP) enzyme activity in mice derived from the C57BL/6J (B6) and DBA/2J (D2) inbred strains. Serum AP was measured in 400 B6D2 F2 mice at 5 mo and 400 B6D2 F2 mice at 15 mo of age that were genotyped at 96 microsatellite markers, and in 19 BXD recombinant inbred (RI) strains at 5 mo of age. A QTL on the distal end of chromosome 4 was present in all sex- and age-specific analyses with a peak logarithm of odds (LOD) score of 20.36 at 58.51 cM. The Akp2 gene, which encodes the major serum AP isozyme, falls within this QTL region at 70.2 cM where the LOD score reached 13.2 (LOD significance level set at 4.3). Serum AP activity was directly related to the number of D2 alleles of a single nucleotide polymorphism in the 5'-flanking region of the Akp2 gene, although no strain-related differences in hepatic expression of Akp2 RNA were found. A variety of sequence polymorphisms in this chromosomal region could be responsible for the differences in serum AP activity; the Akp2 gene, however, with several known amino acid substitutions between protein sequences of the B6 and D2 strains, is a leading candidate.

An examination of early smoking experiences and smoking status in a national cross-sectional sample.

AIMS: To extend previous work showing lightheadedness from and liking for smoking to be associated with continued smoking while controlling for demographics and social influences that can also contribute to progression to established smoking. DESIGN AND SETTING: Random digit dialing telephone survey conduced on 3383 never smokers, non-smokers, former smokers and current smokers in the continental United States. MEASUREMENTS: Demographic information (sex, race, age, education level), smoking history, reactions to early experiences with smoking (lightheadedness, liking), whether parents, siblings or friends smoked when respondent was a teenager. FINDINGS: Lightheadedness and liking interacted-those who liked smoking (regardless of lightheadedness) were very likely to progress to established smoking, while non-likers who experienced lightheadedness were more likely than non-likers who did not experience lightheadedness to progress. These results held even after adjusting for demographic (sex, age, race, education) and social influences (parents, siblings and friends smoking). CONCLUSIONS: Lightheadedness from early smoking appears to be associated with having smoked 100 cigarettes only among those who report not liking early smoking. Overall, this study supports the literature suggesting that early experiences, particularly liking smoking, are associated with becoming a regular smoker.

Evidence for a QTL on chromosome 19 influencing LDL cholesterol levels in the general population.

The genetic basis of cardiovascular disease (CVD) with its complex etiology is still largely elusive. Plasma levels of lipids and apolipoproteins are among the major quantitative risk factors for CVD and are well-established intermediate traits that may be more accessible to genetic dissection than clinical CVD end points. Chromosome 19 harbors multiple genes that have been suggested to play a role in lipid metabolism and previous studies indicated the presence of a quantitative trait locus (QTL) for cholesterol levels in genetic isolates. To establish the relevance of genetic variation at chromosome 19 for plasma levels of lipids and apolipoproteins in the general, out-bred Caucasian population, we performed a linkage study in four independent samples, including adolescent Dutch twins and adult Dutch, Swedish and Australian twins totaling 493 dizygotic twin pairs. The average spacing of short-tandem-repeat markers was 6-8 cM. In the three adult twin samples, we found consistent evidence for linkage of chromosome 19 with LDL cholesterol levels (maximum LOD scores of 4.5, 1.7 and 2.1 in the Dutch, Swedish and Australian sample, respectively); no indication for linkage was observed in the adolescent Dutch twin sample. The QTL effects in the three adult samples were not significantly different and a simultaneous analysis of the samples increased the maximum LOD score to 5.7 at 60 cM pter. Bivariate analyses indicated that the putative LDL-C QTL also contributed to the variance in ApoB levels, consistent with the high genetic correlation between these phenotypes. Our study provides strong evidence for the presence of a QTL on chromosome 19 with a major effect on LDL-C plasma levels in outbred Caucasian populations.

Simple tests to detect errors in high-throughput genotype data in the molecular laboratory.

With the advent of high-density DNA marker data sets for the mouse and other model systems, 100 or more genotype are routinely generated from large groups of mice. Issues of the accuracy and reliability of the genotyping are extremely important but often not addressed until genetic analysis is conducted. Simple tests that rely on the robust predictions arising from Mendelian genetics can be made quickly in the molecular laboratory as the data are generated, and require only a spreadsheet program. In this report, genotype data from 392 mice tested at 96 marker sites were analyzed for errors that are typical when handling large volumes of data generated in a repetitive process. The testing consisted of: (1) repeating the genotyping of approximately 1% of the samples; (2) examining the deviation from the expected segregation ratio ( 1:2:1 ) on a marker-by-marker basis; and (3) testing the correlation of the genotype at one marker with that at neighboring genetic markers on a chromosome. These three steps allowed analysis at the level of the microtiter plate, where errors are most likely to occur. A set of 96 dinucleotide repeat markers that are polymorphic between the C57BL/6J and DBA/2J mouse strains and can be multiplexed is reported for use in other genotyping projects.

Anogenital distance measured at weaning is correlated with measures of blood chemistry and behaviors in 450-day-old female mice.

In female mice, anogenital distance (AGD), measured at weaning, provides an estimate of uterine exposure to testosterone from flanking male mouse littermates. A variant of the anogenital distance index (AGDI) that uses the residual value of AGD after accounting for the effect of weight by regression (AGDWTRES) was measured at weaning in F(2) female mice from a C57BL/6J x DBA2/J cross. AGDWTRES was used to examine the relationship between intrauterine environment and blood chemistry variables and activity-related behaviors when the females were 450 days old. Longer AGDWTRES values correlated with lower levels of calcium, cholesterol, phosphorus, iron, and protein, which is opposite to the expected direction, based on underlying sex differences for blood chemistry. A positive correlation was found between AGDWTRES and two activity-related measures (the number of rears in a test of exploration, and the number of sectors of a rod that are entered by the mouse). These findings suggest that in utero proximity to males, as indexed by AGDWTRES, may have effects on fundamental aspects of blood chemistry and behavior that extend well into mouse middle age, and could play an important role in health.

Effects of periadolescent ethanol exposure on alcohol preference in two BALB substrains.

Ethanol exposure during adolescence is a rite of passage in many societies, but only a subset of individuals exposed to ethanol becomes dependent on alcohol. To explore individual differences in response to ethanol exposure, we compared the effects of periadolescent ethanol exposure on alcohol drinking in an animal model. Male and female mice of two BALB substrains were exposed to ethanol in one of three forms--choice [water vs. 10% (volume/volume) ethanol], forced (10% ethanol in a single bottle), or gradual (single bottle exposure, starting with 0.5% ethanol and increasing at 2-day intervals to 10% ethanol)--from the 6th through the 12th week of age and administered two-bottle alcohol preference tests (10% ethanol vs. water) for 15 days immediately thereafter. All three forms of ethanol exposure increased alcohol preference in male and female BALB/cByJ mice, relative to findings for ethanol-naive control animals. Only gradual ethanol exposure produced an increase in alcohol preference in BALB/cJ mice. During extended alcohol preference testing (for a total of 39 days) of mice in the gradual ethanol exposure group, the higher alcohol preference of the gradual ethanol-exposed BALB/cByJ male mice persisted, but alcohol preference of control group female mice in this strain--formerly ethanol naive, but at this point having received 10% ethanol in the two-bottle paradigm for 15 days--rose to the level of alcohol preference of female mice in the gradual ethanol exposure group. This finding demonstrated that both adolescent and adult ethanol exposure stimulated alcohol preference in female mice of this strain. Across days of testing in adulthood, alcohol preference of the gradual ethanol-exposed BALB/cJ mice decreased, resulting in a lack of effect of gradual exposure to ethanol on alcohol preference in both male and female mice of this strain during the period of extended testing. These strain differences support a genetic basis for the effects of ethanol exposure on alcohol preference and fit within a body of literature, showing substantial individual differences in the effects of ethanol exposure among genetically undefined rats and differences in response to ethanol exposure among inbred rat strains. Exploration of the mechanisms underlying this gene by environment interaction in a mouse model may help elucidate individual differences in the effects of ethanol exposure in human beings and contribute to the understanding of the causes of alcoholism.

Genetic and environmental influences on blood pressure and pulse pressure among adult African Americans.

OBJECTIVE: The purpose of the present study was to identify sources of variability for systolic blood pressure (SBP), diastolic blood pressure (DBP), and pulse pressure (PP) in a sample of adult African-American twins. DESIGN: The classic twin design was employed to examine genetic and environmental sources of variance in the outcome measures of interest. PARTICIPANTS: Participants were 143 (71 MZ and 72 DZ) same-sex, intact twin pairs (mean age = 49.87 years; SD 13.62), who took part in the Carolina African-American Twin Study of Aging (CAATSA). MAIN OUTCOME MEASURES: Outcome measures of interest included SBP and DBP, and PP. RESULTS: For older twins, heritabilities were .52 for SBP, .36 for DBP, and .14 for PP. However, for younger twins, heritabilities were .44 for SBP, .27 for DBP, but no genetic influence on PP was observed. CONCLUSION: The results indicate that genetic factors are a significant source of variance in hemodynamic indices, and also suggest that, with advancing age, genetic factors play an increasing role in determining blood pressure and PP in this population.

Quantitative trait loci analysis of tail tendon break time in mice of C57BL/6J and DBA/2J lineage.

Tail tendon break time (TTBT), a measure of collagen cross-linking, shown to increase with age differs significantly among inbred strains of mice, indicating underlying genetic influences. This study was aimed to identify quantitative trait loci (QTLs) associated with tail tendon break time at three ages (200, 500, and 800 days of age) for 23 BxD recombinant inbred strains of mice and B6D2F(2) mice derived from C57BL/6J and DBA/2J strains. Heritability estimates were calculated, and QTL analyses were conducted using interval-mapping methods. Mean tail tendon break time values were higher in males and increased nonlinearly with age. Eight total QTLs were nominated in the B6D2F(2) mice at the three measured ages, with the QTL at 800 days confirmed in the recombinant inbred strains. Allelic effect modeling for the identified QTLs suggests differences in gene action between sexes. Candidate genes in the QTL regions include collagen genes and an advanced glycation end-product receptor. The QTLs identified demonstrate influence at some but not all ages.

Quantitative trait loci (QTL) analysis of longevity in C57BL/6J by DBA/2J (BXD) recombinant inbred mice.

BACKGROUND AND AIMS: Genes associated with longevity have been identified using both single gene and genome-wide approaches in a variety of species. The aim of this study was to identify quantitative trait loci (QTLs) that influence longevity in male and female mice from twenty-three C57BL/6J by DBA/2J (BXD) recombinant inbred (RI) strains. METHODS: Approximately 12 animals of each sex for each RI strain were maintained under standard conditions until natural death or moribundity criteria were met. RESULTS: A number of life span-relevant loci previously reported on chromosomes (Chrs) 7, 8, 10 and 11 were confirmed. In addition, 5 previously unreported QTLs for mouse life span on Chrs 1, 2, 6, 11, and X were identified as significant and 3 QTLs on Chrs 5, 8, and 16 were suggestive. CONCLUSIONS: Several QTLs were coincident in males and females although the modest correlation between male and female median lifespans and the identification of sex specific QTLs provide evidence that the genetic architecture underlying longevity in the sexes may differ substantially. The identification of multiple QTLs for longevity will provide valuable resources for both reductionist and integrationist research into mechanisms of life span determination.

Bone, muscle, and physical activity: structural equation modeling of relationships and genetic influence with age.

Correlations among bone strength, muscle mass, and physical activity suggest that these traits may be modulated by each other and/or by common genetic and/or environmental mechanisms. This study used structural equation modeling (SEM) to explore the extent to which select genetic loci manifest their pleiotropic effects through functional adaptations commonly referred to as Wolff's law. Quantitative trait locus (QTL) analysis was used to identify regions of chromosomes that simultaneously influenced skeletal mechanics, muscle mass, and/or activity-related behaviors in young and aged B6xD2 second-generation (F(2)) mice of both sexes. SEM was used to further study relationships among select QTLs, bone mechanics, muscle mass, and measures of activity. The SEM approach provided the means to numerically decouple the musculoskeletal effects of mechanical loading from the effects of other physiological processes involved in locomotion and physical activity. It was found that muscle mass was a better predictor of bone mechanics in young females, whereas mechanical loading was a better predictor of bone mechanics in older females. An activity-induced loading factor positively predicted the mechanical behavior of hindlimb bones in older males; contrarily, load-free locomotion (i.e., the remaining effects after removing the effects of loading) negatively predicted bone performance. QTLs on chromosomes 4, 7, and 9 seem to exert some of their influence on bone through actions consistent with Wolff's Law. Further exploration of these and other mechanisms through which genes function will aid in development of individualized interventions able to exploit the numerous complex pathways contributing to skeletal health.

Dopamine receptor genes (DRD2, DRD3 and DRD4) and gene-gene interactions associated with smoking-related behaviors.

Cigarette smoking, like many addictive behaviors, has a genetic component, and the dopamine D2-like receptor genes (DRD2, DRD3 and DRD4) are candidates for contributing to these behaviors. Phenotypic information concerning smoking-related behaviors from a nationally representative sample of research volunteers was analyzed for association with polymorphisms in these genes. Genotype status at the DRD2 intron 2 simple tandem repeat was related to cigarettes per day (P = 0.035) and heaviness of smoking index (P = 0.049). The presence of the glycine allele at the S9G polymorphism of the DRD3 gene was associated with frequency/quantity measures of smoking [log-transformed time to first cigarette (P = 0.031) and heaviness of smoking index (P = 0.035)]. There was a trend for DRD4 long alleles of the variable number of tandem repeats polymorphism to be associated with reduced severity of three withdrawal symptoms [desire/craving (P = 0.054); anger/irritability (P = 0.10); and trouble sleeping (P = 0.068)]. Interactions between genotypes at all three genes were associated with nervousness (P = 0.020) and trouble sleeping (P = 0.015). An interaction between DRD2 and DRD3 was found for trouble concentrating (P = 0.020). These relationships present possible dopamine-related responses to nicotine that warrant further study.