Susan E. Lewis

A null mutation of cytoplasmic malic enzyme in mice

In the course of conducting a biochemical screening program for mutant enzymes in mice, individuals with an apparent nonfunctional allele at the locus (Mod-1) responsible for cytoplasmic malic enzyme were observed. The variant, later attributed to a germinal mutation, was identified by starch gel electrophoresis and by enzyme activity measurements. A series of matings were made, and mice homozygous for the nonfunctional, null, allele (Mod-1″) were produced. In liver, kidney, and testis homogenates, the homozygous mutant exhibited less than 10% of the enzyme activity of the control mice. By an enzyme immuno-inactivation study, the residual enzyme activity was shown to be mitochondrial malic enzyme in all of the tissues examined. By double immuno-diffusion experiments, the kidney homogenate of the mutant formed no precipitin lines with the antiserum to cytoplasmic malic enzyme. Thus, the null mutants express no proteins that crossreact with the antiserum to cytoplasmic malic enzyme (CRM negative). Tissue enzyme assays revealed no significant differences between the normal and the mutant mice in activities of other enzymes in the related metabolic pathways. Because malic acid and malic enzyme together are reported to serve as a pump for NADPH generation in cytoplasm, total cellular NADP+ and NADPH concentrations in liver were determined for the control and the mutant mice. In liver from two individual mutant mice, lower NADPH/NADP+ ratio was detected in comparison to the level in liver from control mice. In spite of the lower levels of NADPH in the mutant mice, their body weight and lipid content were not significantly altered. Mice without cytoplasmic malic enzyme exhibited no striking deficiencies in metabolism or viability.

Dominant and Recessive Effects of Electrophoretically Detected Specific Locus Mutations

The heterozygous and homozygous effects of mutations recovered from a biochemical screen of mouse samples are described. Of the total of 28 electrophoretically detected mutations only one, a Pgm-2 mobility mutation appears to have caused a change to a previously known allelic form. All but one electrophoretically detected allele have been shown to be homozygous viable. Two mutations identified by other means are homozygous lethal; the first is a mutant detected because of its reduced PK-3 activity in heterozygotes, and the second is a morphologically detected mutation at the d locus. Three dominant visible mutations are also described briefly.

A null mutation at the mouse Phosphoglucomutase-1 locus and a new locus, Pgm-3

A null mutation at the phosphoglucomutase locus (Pgm-1) was discovered by electrophoretic analysis of the inbred mouse strain C57 BL/6J. The null allele (Pgm-1n) was shown to segregate as a Mendelian unit alternative to the Pgm-1a and Pgm-1b alleles. Mice expressing the Pgm-1n allele, either in the heterozygous or homozygous state, are viable, healthy, and fertile. The occurrence of the Pgm-1n mutant revealed a previously unreported genetic locus (Pgm-3) that controls the expression of a third phosphoglucomutase. Two electrophoretically expressed alleles of Pgm-3 (inherited without dominance) are found in the inbred mouse strains C57 BL/6J and DBA/2J. Linkage observed between the Pgm-3 locus, the dilute locus (d) and the cytoplasmic malic enzyme locus (Mod-1) has allowed assignment of the Pgm-3 locus to chromosome 9. A striking tissue specific expression of Pgm-1 and Pgm-3 was observed. Products of the Pgm-3 locus were detected in kidney, testes, brain, and heart. In contrast, Pgm-1 controlled isozymes were present in kidney, spleen, ovaries, and erythrocytes.

Linkage of Pep-2 and Apk on Mouse Chromosome 10:

An ethylnitrosourea (ENU)-induced electrophoretic variant of mouse PEP-2, a tripeptidase, has been used to determine the location of the structural gene on chromosome 10. Gene order and recombination frequencies were estimated as Apk-21.8±3.9%-Pep-2-3.7±2.1%-Sl. Methods for rapid determination of Apk and Pep-2 alleles by cellulose acetate electorphoresis and a valuable linkage testing stock carrying the unique Apkm and Pep-2b alleles are described.

THE HUMAN GENETIC RISK OF AIRBORNE GENOTOXICS: AN APPROACH BASED ON ELECTROPHORETIC TECHNIQUES APPLIED TO MICE

Among the most serious long-term adverse health effects potentially resulting from human exposure to airborne genotoxic agents is heritable damage. Environmentally-caused mutations transmitted through the gametes might not only persist for multiple human generations but, depending on the circumstances and the nature of the genetic alteration, may accumulate over time to be expressed as a gradual degradation in quality of life.