Joseph T. Stout

Quantitative Trait Loci Analysis of Structural and Material Skeletal Phenotypes in C57BL/6J and DBA/2 Second-Generation and Recombinant Inbred Mice

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.

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

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.

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

A sample of 693 mice was used to identify regions of the mouse genome associated with trabecular bone architecture as measured using μCT. QTLs for bone in the proximal tibial metaphysis were identified on several chromosomes indicating regions containing genes that regulate properties of trabecular bone.

Genetic determinants of weight of fast- and slow-twitch skeletal muscles in old mice.

The main goal of the study was to explore the genetic architecture underlying muscle weight in old mice. Weight of soleus, tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius muscles was measured in the C57BL/6J (B6) and DBA/2J (D2) strains and derivative generations: a panel of the BXD recombinant inbred (RI) strains and a B6D2 F2 intercross at the age of 800 days. The between-strain difference in muscle weight (B6 > D2) ranged between 16% and 38%. Linkage analysis identified suggestive quantitative trait loci (QTL) on Chromosomes (Chr) 2, 6, 7, 8, 19, and X that influenced muscle weight in the 800-day-old group. Comparison of weights at 200, 500, and 800 days revealed a variable effect of age among the four muscles. Linkage analysis in the B6D2 F2 population combined across the three different age groups identified muscle-, sex-, and age-specific QTL on Chr 1, 2, 3, 5, 6, 8, 9, 11, 13, 17, X, and Y. Genetic factors that influence the rate of weight change (within-strain weight difference at two ages) over the lifespan of BXD RIs were mapped to the markers D2Mit369 and D3Mit130 at the genome-wide p < 0.05 for TA muscle in males (between 200 and 800 days) and females (between 500 and 800 days), respectively. Analysis of all age groups supported previous findings that the genetic effects may be muscle-, age-, and sex-specific.

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 unre-ported 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.

QTL influencing baseline hematocrit in the C57BL/6J and DBA/2J lineage: age-related effects

Baseline serum hematocrit varies substantially in the population. While additive genetic factors account for a large part of this variability, little is known about the genetic architecture underlying the trait. Because hematocrit levels vary with age, it is plausible that quantitative trait loci (QTL) that influence the phenotype also show an age-specific profile. To investigate this possibility, hematocrit was measured in three different age cohorts of mice (150, 450, and 750 days) of the C57BL/6J (B6) and the DBA2/J (D2) lineage. QTL were searched in the B6D2F2 intercross and the BXD recombinant inbred (RI) strains. The effects of these QTL were explored across the different age groups. On the phenotypic level, baseline serum hematocrit declines with age in a sex-specific manner. In the B6D2F2 intercross, suggestive QTL that influence the phenotype were located on Chromosomes (Chr) 1, 2, 7, 11, 13, and 16. With the exception of the QTL on Chr 2, all of these QTL exerted their largest effect at 750 days. The QTL on Chr 1, 2, 7, 11 and 16 were confirmed in the BXD RIs in a sex- and age-specific manner. Linkage analysis in the BXD RIs revealed an additional significant QTL on Chr 19. Baseline serum hematocrit is influenced by several QTL that appear to vary with the age and sex of the animal. These QTL primarily overlap with QTL that have been shown to regulate hematopoietic stem cell phenotypes.

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.

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.

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.

Tail Tendon Break Time: A Biomarker of Aging?

Research has attempted to identify biomarkers of aging that are predictive of longevity and specific age-related changes during animal life span. Tail tendon break time (TTBT), one presumed biomarker, measures collagen cross-linking, known to increase with age. Significant differences in the rate of increase of TTBT with age have been reported between mouse strains and animal species. We measured both TTBT and longevity in C57BL/6J, DBA/2J, and 23 recombinant inbred (RI) strains (B×D RIs), with TTBT measured at 200, 500, and 800 days of age. Longevity demonstrated considerable variability among these strains (116-951 days). TTBT, also highly variable, increased significantly with age in both sexes and all genotypes. Neither TTBT nor its rate of change correlated significantly with life span. There were suggestive trends for rate of TTBT change to correlate with male longevity and strain longevity to correlate with female TTBT. We conclude that for the range of genetic variation found among these mouse genotypes, TTBT cannot be considered a robust biomarker of longevity.