Case Study 1• Dilated cardiomyopathy in RAG-1-/- mice• ? Viral ? Genetic• Sporadic occurrence in up to 20% of mice• Increased incidence after extensive backcrossing to

B10.BR - almost universal on histopathology, and clinically apparent particularly in female breeding stock

• Unable to segregate RAG-1-/- and cardiomyopathy, no way to genotype for susceptibility to cardiomyopathy

• Fortunately, RAG-1-/- subline sent to ARC was free of cardiomyopathy. Entire backcross to B10.BR was repeated starting with breeding nucleus from ARC

Case Study 2• Colitis in RAG-1-/- colony• Originally suspected due to Citrobacter but microbiological

tests negative• Genetic analysis of every case in past 12 months revealed that

colitis occurred only in mice with a particular genotype - T cell receptor transgenic with T cells produced by the thymus

• Solution 1: breed T cell receptor transgenic colony on a background that does not produce T cells in the thymus, and cross onto T-producing background only when necessary to produce experimental mice. Use mice in experiments before onset of colitis

• Solution 2: Infection with Helicobacter identified as a necessary co-factor for development of colitis. C-section colony onto Helicobacter-negative foster mothers from ARC

Case Study 3• Early death in homozygous CTLA-4-/- mice due to abnormal immune

activation• Occurrence of pathology and need for technically difficult genetic

screening in breeding colonies led to problems identifying sick animals quickly enough, maintaining lines and generating sufficient homozygous mice for experiments

• Solution: Breed two homozygous colonies of CTLA-4-/- mice on RAG-1-/- and RAG-2-/- backgrounds, respectively. The lack of immune system prevents immune pathology due to abnormal immune activation. When mice needed for experiments, intercross the lines on RAG-1-/- and RAG-2-/- backgrounds. This generates RAG-1+/- and RAG-2+/- mice with an immune system. All mice will be homozygous CTLA-4-/-, and experiments can be planned knowing how many mice are available.

• Note - total number of mice bred while establishing CTLA-4-/- lines on RAG-1-/- and RAG-2-/- backgrounds was significantly larger than required to maintain the previous strategy