Froggy; a new mutation on Chromosome 13 affecting skull shape and body size
Coleen Marden, Michelle M. Curtain, Julie Hurd, Leah Rae Donahue
Source of Support: NIH-NEI grant EY015073 to Leah Rae Donahue
Mutation (allele) symbol: frg
Mutation name: froggy
Strain of origin: A/J
Current strain name: A/J-frg/J
Stock #3485 (view JAX® Mice Data Sheet for additional information including Price and Supply Information)
Abstract
We report a spontaneous mutation with a shortened face and smaller body that has been mapped to chromosome 13.
Origin and Description
The recessive froggy mutation arose in Ed Birkenmeier’s research colony at The Jackson Laboratory and was discovered by C. Kelly and A. Williams in 1992. We confirmed a recessive mode of inheritance by mating a mouse homozygous for the froggy mutation to a C57BL/6J inbred mouse and there were no affected progeny observed in the F1 offspring of this mating. Froggy has been on an inbred A/J background for 46 generations and is maintained by mating a homozygous female or male to a heterozygous littermate. Heterozygous by heterozygous mice are also mated since homozygous mice are not always dependable breeders. The average litter size is 6 to 7 pups for a +/frg by +/frg mating. The litter size is much less 3 to 4 pups, when a homozygote is mated to a heterozygote with about half of these matings being nonproductive.
The froggy mutation affects body size and skull shape. (See Photo) The mutation has varied penetrance ranging from an obvious shortened face to less severe were the length of the face is less affected. Despite this varied phenotype, mutants can always be determined due to their wide set eyes. Both males and females are affected and have normal fertility and life span.
Genetic Analysis
Using our standard mapping protocol, a linkage cross was set up by mating a female C57BL/6J to a homozygous male frg/frg. The heterozygous offspring of this mating were then backcrossed to a frg/frg and produced 50 affected N2 mice that were used for linkage analysis. A genome scan confirmed a map location on Chromosome 13. The mutation maps between D13Mit88 (NCBI 36 position 38.8 Mb) and D13Mit248 (NCBI 36 position 53 Mb) and is non-recombinant with D13Mit63 (NCBI 36 position 42.7 Mb), D13 Mit179 (NCBI 36 position 44.8 Mb), D13Mit91 (NCBI 36 position 46.9 Mb) and D13Mit139 (NCBI 36 position 51.8 Mb).
Biological Characterization
Craniofacial morphology of sixteen-week-old mice: Skulls were prepared by incomplete maceration in potassium hydroxide, stained with alizarin red, and stored in undiluted glycerin (Green, 1952). Morphological measurements of the skull (Table 1-See protocol on this website.) were made using digital calipers (Stoelting, Wood Dale, Ill) with previously established landmarks (Richtsmeier, 2000, see protocol this website). Male and female froggy mutants had significantly shorter skull, nose and upper jaw lengths compared to same sex controls. Mutants had significantly wider inner canthal distance than controls. Male mutants also had significantly less skull height than male controls along with significantly less skull height/width ratio. Skull length to nose length and skull height to skull length ratios were greater in the mutants. Jaw Length ratio and skull length to skull width was significantly less in mutants. (graph 1, graph 2 and graph 3)
DEXA analysis of whole body aBMD and body composition of sixteen-week-old mice: Whole body, areal bone mineral density (aBMD), bone mineral content (BMC) and body composition (lean, fat and % fat mass) was assessed by PIXImus densitometry (GE LUNAR, Madison, WI) (Table 2-See protocol on this website). Male and female mutants were both significantly different for skull BMD with female mutants having significantly more than female controls while male mutants had significantly less than male control and female mutants. All other significant differences within sexes were with males; male mutants had less whole body fat, total mass, skull BMC, and ear pinna length. (graph 4 & graph 5)
Male and female mutants are nearly equal with their short and wide face phenotype. Froggy male body composition is more affected than females.
Pathology
A routine pathological screen was done on seven mice. Two were one- year-old and five were 12-weeks-old; all seven were homozygotes. All these mice showed mild dystrophic muscle due to the A/J background (Oxford Journal: Disruption of muscle membrane and phenotype divergence in two novel mouse models of dysferlin deficiency). Froggy bones were normal with no shortening of leg bones or vertebral bodies. A clinical eye check was done on both affected and controls at 18, 22, and 26 weeks and all were normal. (Mouse models of ocular disease) Hearing was assessed by auditory brainstem response (ABR) by testing two mutants and one control at three months old. Both genotypes showed severe hearing loss due to the A/J background. (Science Direct: Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses)
Summary
The froggy skeletal mutation affects both sexes, however the body composition of males is more affected than females. Hearing is impaired and there is a muscle mutation, but this is due to A/J background.
Acknowledgements
The authors thank Norm Hawes for eye examination, Ronald Hurd for ERG and Heping Yu for ABR testing.
