A Human SIL-SCL Fusion Gene Displays a Development Block at the CD4− CD8− to CD4+CD8+ Transition in Transgenic Mice.

Chromosomal rearrangements leading to dysregulation of the SCL gene, located at chromosome 1p32, is a common event in the development of human T-cell acute lymphocytic leukemia (T-ALL). In the most common form of SCL gene rearrangement, an interstitial deletion of approximately 90 kb brings SCL under the control of regulatory elements that normally govern expression of the ubiquitously expressed SIL (SCL interrupting locus) gene, which is located directly upstream of SCL. To investigate the consequences of this event, we reproduced this gene alteration by using cre-mediated recombination. A BAC clone containing both human SIL and SCL genes was isolated and loxP sites were cloned into the intron 1 region of both the SIL and SCL genes, corresponding to the sites at which recombination leading to an interstitial deletion occurs in human T-ALL patients. This BAC clone was used to generate transgenic SILloxloxSCL mice. The SILloxloxSCL mice were bred to mice that express the cre recombinase in the thymus (Lck-cre mice). The BAC transgene was recombined between the two loxP sites in over 50% of the thymocytes from double transgenic Lck-cre/SILloxloxSCL mice, faithfully recapitulating the event seen in human T-ALL patients, and bringing the SCL gene under the direct control of SIL regulatory elements. Aberrant SCL gene expression was verified by RT-PCR. Using FACS analysis, we found that mice carrying both the SILloxloxSCL transgene and the Lck-cre transgene have decreased CD4+/CD8+, CD4+/CD8−, CD4−/CD8+ and increased CD4−/CD8− thymocytes compared to transgene-negative mice or mice that carried the SILloxloxSCL transgene but not the Lck-cre transgene. These findings were detected in mice from 6 to 15 months of age. Interestingly, increased numbers of CD44+ thymocytes can also been identified in SILloxloxSCL/Lck-cre mice. In the spleen, the SILloxloxSCL/Lck-cre mice show a marked reduction in the number of mature CD4+ or CD8+ cells. These results indicate an accumulation of immature T-cells in mice transgenic for both the SILloxloxSCL transgene and the Lck-cre transgene, and demonstrate that conditional activation of SCL under control of SIL regulatory elements can impair normal T-cell development.