Global ENU Mutagenesis Screen for Genetic Modifiers in Sickle Cell Disease Mice.

Genetic modifiers of sickle cell disease (SCD) will be identified in an animal model of SCD after mutagenesis with the chemical mutagen N-ethyl-N-nitrosourea (ENU). This phenotype driven approach utilizes a third generation knock-in (KI) mouse model of SCD that reproduces most if not all of the pathology of the disorder. This model was produced by targeted gene replacement of the murine α-globin genes with human α-globin and the murine β globin genes with a human γ- and β^s-globin gene cassette that mimics the fetal to adult hemoglobin switch that occurs in man. Sickle embryonic stem (ES) cells were derived from developing blastocysts isolated from female sickle mice that were mated with sickle males. Sickle ES cells were treated with ENU and a mutant library of independent subclones was established and archived. The optimal ENU dosage was empirically determined through a series of pilot experiments that measured the HPRT mutation frequency and the efficiency of producing sickle mice from the mutagenized cells by tetraploid embryo complementation. Animals harboring mutations that affect hematological indices, kidney function, or liver function are identified in the mutagenized sickle mice by comparison to cloned control sickle mice. Microsatellite linkage analyses of mutant offspring outcrossed to congenic SCD mice and direct sequence comparison to the murine genome will allow the positional cloning of modifier genes. Putative modifying factors will be positively confirmed by introducing the exact germline modification discovered during the ENU screen into the unmutagenized ES cells, followed by the direct examination of the phenotype in mice generated from the modified cells by cloning. These studies will define gene(s) responsible for the phenotypic variation in disease severity that is observed in the SCD population. By experimental design, the therapeutic benefit or detriment associated with each modifying gene(s) on the in vivo pathophysiology of sickle cell anemia will be tested directly in our animal model of this disorder.