| Human T cell Leukemia Virus type 1 (HTLV-1) was the first human retrovirus associated with malignancy. The transforming properties of HTLV-1 are attributed to a protein encoded at the 5' end of the viral genome termed tax. Tax residues 89 and 90 have been shown to be crucial for CBP/p300 and NF-kappabeta recruitment and viral transactivation. In vitro assays of Tax mutants V89A and L90A displayed a 75 % and 50 % reduction in HTLV LTR transactivation, respectively. Due to the in vitro attenuation of viral transactivation observed by V89A and L90A Tax mutants, it was hypothesized that development of neurofibromas would be delayed in transgenic mice carrying V89A and L90A tax sequences. Neurofibromas were expected at 26 and 52 weeks for V89A and L90A mice, respectively, as compared to 13 weeks for wild-type tax mice. Transgenic mouse models were utilized for investigation of the contributions of Tax residues 89 and 90 towards tumorigenesis. Four of 41 V89A mice and 2 of 15 L90A pups were identified as transgene-positive. Necropsy of Tax-positive V89A mouse 16 on day 96 revealed abnormal development of the left rear femur associated with a thin cortex and elevated number of osteoclasts. Additionally, abnormal lymph node structure was observed with poor follicle development and no germinal centers. Tax protein expression was identified in V89A mouse 16 femur, lymph node and muscle. Tax-positive L90A mouse 2 was sacrificed at day 99 for developmental deficits. However, histologic and biochemical analysis of L90A mouse 2 did not reveal an etiology for the observed deficit. Remaining V89A and L90A founders were euthanized at two years of age. No tumors were observed for necropsied or euthanized V89A or L90A mice. Attenuation of HTLV LTR activation by mutated Tax resulted in a complete loss of tumorigenicity for V89A and L90A mice. |
