The Jackson Laboratory

Overview

Our lab seeks to identify genes that regulate bone density, size and shape. All of these skeletal phenotypes have been shown to be genetically regulated and to contribute to fracture susceptibility. To better understand the interaction between genes and environment in determining skeletal integrity, we use inbred strains of mice with skeletal differences as well as mice with spontaneous mutations that result in skeletal abnormalities. Our lab also studies the structure of the skull and face, and develops new spontaneous mutations in mice with craniofacial dysmorphologies. Our goal is to identify and characterize reliable genetic and physiological mouse models for human craniofacial disorders and to share these and other models for human skeletal diseases with the scientific public.

Scientific report

Mouse Models of Skeletal and Craniofacial Development and Morphology

Quantitative trait loci analyses of bone density and body composition in mice with growth hormone deficiency

Quantitative Trait Loci (QTL) analyses are powerful tool for the discovery of regulatory genes for complex traits such as body composition and skeletal phenotypes. Crosses between inbred strains of mice have provided data that are contributing to new genetic insights regarding bone and body morphology. Further, QTL analyses can be used to partition phenotypes into their regulatory pathways by using mice with altered genomes. We use the little (Ghrhr^lit) mutation, a model of growth hormone and IGF-1 deficiency, which arose spontaneously on the C57BL/6J (B6) low-bone-density inbred strain background, coupled with a new congenic strain we made by introgressing the little mutation onto the high-bone-density C3H/HeJ (C3H) inbred strain background. Homozygous mice on both backgrounds, (B6-lit/lit and C3.B6-lit/lit), have undetectable circulating growth hormone (GH), low serum insulin-like growth factor 1 (IGF1), and reduced bone mineral density (BMD), femur length, and body mass compared to normal littermates. However, although C3.B6-lit/lit mice are of the same body weight and femur length as B6-lit/lit mice, C3.B6-lit/lit mice have higher BMD and female C3.B6-lit/lit mice have lower percent body fat than B6-lit/lit females, while males of the two strains do not differ in body fat mass. To explore the genetic regulation of these differences, we crossed homozygous mutant mice of the two little strains, B6-lit/lit and C3.B6-lit/lit, to produce 1,008 male and 1,062 female F2 mice, all GH/IGF-1 deficient, but segregating alleles from B6 and C3H. QTL analyses of genome-wide data from 120 polymorphic markers (nearly 211,761 genotypes) revealed 14 regulatory loci for total body BMD, 14 for femoral BMD, 15 for periosteal circumference, 14 for femur length, 8 for L2-L4 lumbar vertebral BMD, 14 for body weight, 12 for percent body fat, and 11 for serum IGF-1 with highly significant LOD scores. All nineteen chromosomes had QTL for multiple phenotypes.

Further, we have identified interactive pairs of QTL in which a particular QTL is not detected until it is paired with another QTL. For example, the QTL on Chromosome (Chr) 2 at 26cM for whole body BMD was not detected in the main effect analysis; however the interactive pair 2:18 (Chr 18 at 42cM) has a significant LOD of 5.09. Again for the whole body BMD phenotype, the QTL on Chr 15 must be paired with either the QTL on Chr 5 or 7 to have a significant effect. In contrast, the contributing QTL to the interactive pairs Chr 9:11 and 12:16 each has a significant individual effect, as well as a significant interactive effect. Similar examples can be found for other phenotypes. The importance of these interacting QTL is threefold: the proteins encoded by the genes within the QTL may interact physically, e.g., ligand and receptor; they may represent a biological pathway; or they may be part of a redundant physiological system. These possibilities can only be evaluated once the genes within the QTL are identified; this work is ongoing in my laboratory.

Mouse models of craniofacial disorders

In addition to the axial and appendicular skeleton, my laboratory studies craniofacial morphology. Our goal is to identify and characterize reliable genetic and physiological mouse models for human craniofacial dysmorphologies and to share these models with the scientific public. To date, we have identified more than one hundred deviants with heritable craniofacial abnormalities including: abnormal ear pinnae shape, size, and placement; shortened snouts; shortened, domed, and asymmetrical skulls; abnormal dentition; abnormal eye morphology; and deafness. We perform comprehensive genetic studies and preliminary characterization studies that include clinical examination, basic histology, and evaluation of bone-density and skeletal anomalies, dentition, vision, and hearing. Recently, we have identified the gene responsible for three new mutants: one with extra teeth, one with glaucoma, and one with a short lower jaw, small ears, and a sparse coat. Manuscripts describing these new mouse models and their importance to human health are in preparation. In addition, the chromosomal locations of several other new mutants have been identified. We provide both uncharacterized and completed models to the scientific community via our web site www.jax.org/cranio/ and distribute live mice upon request. We have a dedicated email address, faces@jax.org, for direct communication with interested investigators.

Mouse models of fracture susceptibility

The skeleton serves ambulatory, as well as metabolic and protective, functions. Because bone is important for mobility and muscle attachment, the ultimate test of skeletal integrity is its resistance to fracture. Although fracture often is associated with osteoporotic patients, stress fracture from constant overuse and acute fracture from trauma can occur in any population. In the human clinic, the gold standard used to predict fracture risk is bone mineral density (BMD) as measured by dual energy X-ray absorptiometry. Yet other characteristics of bone architecture including size, shape, and quality are important contributors to bone strength and should be considered when predicting fracture risk. In addition, it has been shown that these morphological phenotypes are often associated with differences in muscle mass. Coupled with this expanded interest in phenotyping of bone and muscle is an intense search for genetic predisposition for fracture based on these newly identified indices of bone strength. The interpretation of these studies has been complicated by the heterogeneity of the human population and by environmental differences. Taken together, however, the studies demonstrate that genetic regulation of bone strength is complex and that animal models are critical to progress in this field.

To determine if BMD, muscle mass, or bone morphological phenotypes are good predictors of skeletal strength in mice, we collected data on skeletal geometry, muscle mass, BMD, and bone strength from mice of nine genetically diverse inbred strains. We used three measures of bone strength: peak load (the maximum amount of stress that the femur can withstand before breaking); stiffness (the amount of stress the bone can withstand and still return to its original shape); and energy to break (a measure of the amount of energy needed to cause a fracture, often termed the toughness of bone).

We found that BMD at the mid-shaft of the femur was a significant predictor of peak load and stiffness, but was a poor predictor of energy to break. In contrast, measures of femoral geometry-femur length and dimension-were significant predictors of peak load, stiffness, and energy to break. Similarly, both the area and weight of the quadricep muscle were good predictors of all three measures of bone strength in both male and female mice, although the weight of the isolated quadricep muscle was a stronger predictor of energy to break in females than in males, and a better predictor of peak load in males than in females. These data support the hypothesis that skeletal geometry and muscle mass are important determinants of bone strength and fracture risk. In addition, we have shown that there are sex differences for some predictors of bone strength. Finally, our data demonstrate that inbred strains of mice can be used as models to discover the genetic regulation of bone geometry and strength.

Mouse models of heritable skeletal diseases

With the Mouse Mutant Resource, we continue to develop mouse models for skeletal diseases that result from spontaneous mutations in single genes. We are currently characterizing and mapping several new mutations that result in appendicular and axial skeletal malformations, craniofacial abnormalities, and both loss and abnormal accretion of bone mineral. These mutants will become important models for studying bone physiology and morphology.

Lab staff

Principal Investigator: Leah Rae Donahue, Ph.D.
Co-Principal Investigators: Wesley Beamer, Ph.D., Muriel T. Davisson, Ph.D., Stephen A. Murray, Ph.D., Clifford J. Rosen, M.D. 
Research Assistant II: Michelle M. Curtain
Biomedical Technologist II: Harold Coombs III, Coleen Kane
Executive Assistant: Aimée Picard

Publication listings

Selected Publications