Susceptibility to cleft palate and the major histocompatibility complex (H-2) in the mouse

Effects of the Ay gene on susceptibility to hydrocortisone fetotoxicity and teratogenicity in mice

Effects of the Ay gene, a coat color gene, on susceptibility to hydrocortisone fetotoxicity and teratogenicity were investigated by using the congenic strain of C57BL/6-Ay (Ay/a) which had been maintained by repeated back-crosses of the Ay gene to the C57BL/6 (a/a) background. Matings were conducted as follows (female x male): group I, a/a; group II, a/a x Ay/a; and group III, Ay/a x a/a. Pregnant females were subcutaneously given daily doses of 0, 12.5, 25, or 50 mg/kg of hydrocortisone on days 10-13 of pregnancy. On day 18 of pregnancy, fetuses were sexed, weighed, and examined for external abnormalities. In group I, the mean fetal weight was significantly decreased at a dose of 25 mg/kg or more. The incidences of cleft palate were 3.2 and 22.7% at 25 and 50 mg/kg, respectively. In group II, in which half of the fetuses were expected to carry the Ay gene, the mean fetal weight was decreased significantly at 12.5 mg/kg or more. The incidence of cleft palate in group II at 50 mg/kg was 44.2%, which was significantly higher than that in group I. In group III, in which maternal mice as well as half of their fetuses carried the Ay gene, a decrease in the mean fetal weight was greater than in group II. In addition, the mean percentage of fetal resorptions was significantly increased at 50 mg/kg. The incidence of cleft palate in group III was significantly increased at 25 mg/kg (10.5%) when compared with those in groups I and II. These results indicate that the Ay gene may be associated with susceptibility to hydrocortisone fetotoxicity and teratogenicity in mice.

Increased susceptibility to 6-aminonicotinamide-induced cleft lip of heterozygote Dancer mice

Dancer heterozygotes (Dc/+) very rarely have cleft lip and show a dancing behaviour due to inner ear defects while homozygotes (Dc/Dc) have cleft lip. Males of the two genotypes Dc/+ and +/+ were mated to C3H strain and R stock females and Dc/+ males to Dc/+ females. On day 10/8 of gestation females were treated with 6-aminonicotinamide (6AN) at either 19 mg/kg or 28.5 mg/kg followed 3 h later by a protective dose of nicotinamide. Controls were untreated. Both 6AN treatments caused a significant increase in cleft lip to between 25% and 29% for crosses of Dc/+ males to C3H and R females whereas crosses with +/+ males gave 0% cleft lip. In the controls the cleft lip frequency was: for Dc/+ X Dc/+ 14%, Dc/+ X C3H 1.4%, and for the other three crosses 0%. The four crosses given the high dose of 6AN and the +/+ X C3H and Dc/+ X R cross at the low dose showed significantly increased resorption rates to between 23% and 47% over the control rates of from 5% to 11%. The presence of the Dc gene increased the susceptibility to cleft lip caused by 6AN.

Genetics of palate development

We are beginning to see a tantalizing picture by looking into the "genetic window" on the teratogenesis of cortisone-induced cleft palate and on normal palate development. We must continue to open this window. So far, all strains of mice that have been treated with cortisone do react with induced cleft palate and, from their dose-response behaviors, they appear to react in the same way. It is tempting to conclude that dosage tolerance is the only genetic variation in the reaction. We do not attach any significance to this because we must emphasize the fact that these different strains represent an insignificant sample of the hundreds of genetically different, recognized strains of the mouse. We have emphasized previously that systematic strain surveys must become an active part of teratological genetics in order to obtain an approximation of how much genetic variation exists in a trait in the species (e.g., Biddle, 1981). Now, we need to go beyond strain surveys. Crosses between strains must become part of this activity. Dominance appears to be a property of the homeostatic model of palate development as far as the cortisone probe is concerned, but it may not be a universal property. The significance of dominance and the architecture of gene action can only be interpreted by exploring more strain pairs in the mouse. A study of two or three or four strains is simply inadequate. From the crosses between a limited number of strains, major genetic factors in cortisone reactivity have been identified. These major genetic factors may regulate the homeostatic model of palate development, but we do not know this for a fact. The identification of major genetic factors has moved us to a new horizon; we can move now from a purely descriptive biology to a study of the functional biology of the process of palate development. This move will require not only the molecular probes with which to walk along the specific chromosomes to the major genetic factors but also a library of fully characterized strains of the mouse in which to use these probes. The library of strains that have been genetically characterized for the cleft palate reaction is missing.