Mouse Models of Tumor Development in Neurofibromatosis Type 1

1. Mouse Models of Tumor Development in Neurofibromatosis Type 1 Karen Cichowski, T. Shane Shih, Earlene Schmitt, Sabrina Santiago, Karlyne Reilly, Margaret McLaughlin, Roderick Bronson, and Tyler Jacks. 1999. Science 286 : 2171-6 Presented by Brooke Pinney

2. Neurofibromatosis Type 1 • Familial cancer syndrome • Autosomal dominant disorder caused by mutations in the NF1 tumor suppressor gene Affects 1 in 4000 births worldwide

3. Inherited • Spontaneous Mutation (sperm or egg) Does not skip generations or have any racial, geographic, or ethnic distribution

4. Characteristics of NF • Café-au-lait spots • Neurofibromas plexiform neurofibromas – diffuse tumors under the skin dermal neurofibromas – nodule-like skin tumors benign and malignant nerve sheath tumors (MPNST)

5. Skeletal abnormalities scoliosis – lateral curvature of the spine • Learning disabilities – reading, writing, and arithmetic in normally intelligent individuals

6. Pigmented bumps on the iris of the eye • Optic nerve tumors (gliomas) bulging eye impaired vision surgical correction necessary

7. What causes NF? • NF1 gene codes for a protein neurofibromin member of the GTPase – activating protein (GAP) Stimulates the GTPase activity of Ras pathway in cells

8. Deregulation of Ras-mediated signaling is likely to contribute to the pathology of NF1 • Look at Ras pathway

11. Purpose • To test the possibility that a mutation in the wild-type NF1 allele is required and rate-limiting in the formation of neurofibromas in NF1+/- mice • To look at a cooperative and causal role for p53 mutations in MPNST development

12. NF1 heterozygosity causes development of benign neurofibromas • Loss of heterozygosity (NF1 function) leads to progression of malignant peripheral nerve sheath tumors (MPNSTs)

13. Testing mice • Chimeric mice composed of NF1 -/- mice and NF1 +/+ mice Greatest chimerism died within 1 month Lowest degree lived a typical life with no unusual pathology

14. Moderate degree of chimerism - shortened life span - progressive neuromotor defects • Neurofibromas in every animal -usually multiple tumors

15. Schwann cells were most common type of cells • Upon histological analysis lesions from mice similar to humans

16. Look at cooperation of p53 for tumor growth • Mice with germ line mutations in NF1 and p53 were generated Generated mice with NF1 and p53 on opposite chromosomes (NP trans) Crossed NP trans mice with wild type so both mutations on chromosome 11 (NP cis)

17. NP trans mice • Loss of heterozygosity at either locus • Tumor phenotype similar to parental NF+/- or p53+/- benign tumors

18. NP cis mice • Loss of heterozygosity at both loci • Deficient for both NF1 and p53 malignant tumor growth

19. Percentage of soft tissue sarcomas • NF1+/- 5% • P53 57% • NP trans 36% • NP cis 81%

20. Results for NP trans • NP trans animals had more tumors than mice heterozygous for a single mutation • Survived 10 months • Loss of heterozygosity at either location -one of the tumor suppressors still functioning

21. Results for NP cis • NP cis mice had increased soft tissue sarcomas compared to other mice • Survived 5 months • Loss of both wild type allele – loss of both genes cooperates in formation of lesions • Histology similar to MPNSTs

23. Conclusions • NF1-/-:NF1+/+ chimeric mice prove that loss of NF1 is obligate step in neurofibroma development • Neurofibromin-deficient cells in the developing nerve is rate-limiting step in tumor formation • Loss of both wild type alleles supports a causal role for p53 mutations in development of malignant tumors in NF1 patients

24. Loss of p53 and NF1 function allow for the outgrowth of cells that would have undergone growth arrest or apoptosis if NF1 mutated alone