Spontaneous fracture 2 Jackson (sfx2J), a second mutation of the gulonolactone oxidase gene (Gulo). 

Belinda S. Harris, Patricia F. Ward Bailey, Leah Rae Donahue, Kenneth R. Johnson, Roderick T. Bronson, and Muriel T. Davisson

Source of Support: This research was supported by NIH/NCRR grant RR01183 to the Mouse Mutant Resource (M.T.Davisson, PI) and Cancer Center Core Grant CA34196.

Mutation (allele) symbol: sfx2J

Mutation (allele) name: spontaneous fracture 2 Jackson

Gene symbol: Gulosfx-2J

Strain of origin: RB156BNR/Ei-rul/J

Current strain name: RB156BNR/Ei-rul/J-Gulosfx-2J

Stock #: 005354 (view JAX® Mice Data Sheet for additional information including Price and Supply Information)

Phenotype categories: skeletal

Abstract

We have identified a remutation to spontaneous fracture (Gulosfx) by a direct test for allelism. The phenotype of this remutation is identifiable at 4-5 weeks of age when the mutants appear smaller than their control littermates and begin to hobble about their cage. Between 5 and 8 weeks of age the mutants develop rear limb paralysis and many die by 8 weeks of age. The phenotypic characteristics of this mutation are similar to the original mutation spontaneous fracture (Gulosfx) except for the eye phenotype described below that is inherent in the background strain on which the sfx2J mutation arose.

Origin and Description

This recessive mutation arose in 2002 in the strain RB156BNR/Ei-rul/J (ruffled) which is maintained in the Mouse Mutant Resource at the Jackson Laboratory.The ruffled mutation in the background strain is still segregating in the  RB156BNR/Ei-rulJ-Gulosfx2Jstrain and when homozygous causes the sfx2J mice to have a ruffled looking coat.

Genetic Analysis 

A direct test for allelism was set up by mating a female heterozygote carrying this new mutation to a male sfx heterozygote. This cross produced three affected mutants out of 10 born, proving that the new mutation is allelic with sfx.

Pathology

X-Ray analyses of seven mutants between 4 and 8 weeks of age show thin cortical bone, absence of trabeculae, and complete fractures of the femur with mineralized callus in all seven mice. The forelimbs and spine were less affected than the rear legs, but all bone displayed thin cortices and very little, if any trabeculae.  Bone mineral density was measured by PIXImus in 3 mutants and very low amounts of mineral were detected in the overall mutant skeleton compared to age matched littermates.

Three mutants were perfused for our standard pathological screen and found to have severe osteolysis with fibrous osteodystrophy, and thin cortical bone, particularly in long bones. These 3 mutants all had complete fractures in the femur or at the knee near the epiphysis and beneath the growth plate. Fracture sites have extensive callus formation (See Figure 1). The severity of the fractures in sxf 2J  appears to be worse than in the original spontaneous fracture mutant. It is possible that a modifying gene is present in the RB156BNR/Ei background that exacerbates the fracture phenotype. Mutants   also had cataracts and rosettes and wavy outer nuclear layer of retinas. The eyes of a female mutant were checked using an opthalmascope and it was found to have cataracts on both eyes (as the strain background has characteristically) It has not yet been determined if the rosettes and wavy outer nuclear layer of the retinas is also characteristic of the background strain.

Hearing as assessed by Auditory brain stem testing (ABR) of three mutants and three controls tested at one month of age was normal.

Discussion

A remutation to sfx has been confirmed by a direct test for allelism.  The phenotype of this remutation is more severe than the original sfx mutation, suggesting a modifying gene may be present in the RB156BNR/Ei-rul/J background strain.

Acknowledgements

The authors would like to thank the following for their excellent technical expertise: Norm Hawes for eye examination, Heping Yu for hearing assessement, and Coleen Marden for perfusions.

References

MGD 2005, Mouse Genome Database, Mouse Genome Informatics Project, The Jackson Laboratory, Bar Harbor, ME. (URL: http://www.informatics.jax.org).