Genetics of palate development

Identification of a murine locus conveying susceptibility to cadmium-induced forelimb malformations

The heavy metal cadmium (Cd), an environmentally ubiquitous contaminant, is a potent teratogen in mice. When administered parenterally, it induces an array of malformations that vary in scope and severity with the route, dose, time of administration, and the strain of the animal. When administered intraperitoneally on day 9.0 of gestation, 4 mg/kg cadmium chloride produces forelimb defects (predominantly ectrodactyly) in over 80% of fetuses of the C57BL/6 mouse strain, while no limb defects are observed in the identically treated SWV strain. Like other examples of strain-specific teratogenic activity, the underlying nature of the differential susceptibility remains unknown. The present study investigates the segregation of sensitivity to Cd-induced forelimb defects in crosses between C57BL/6 and SWV mice and provides evidence for the involvement of both maternal and fetal factors in the determination of defect expression. In addition, quantitative trait loci (QTL) analysis of the fetal genetic component was performed among 198 backcross progeny, utilizing a genomic linkage map of 149 informative microsatellite markers. One QTL demonstrating significant linkage to expression of the defect, designated Cadfar (cadmium-induced forelimb autopod reduction), was mapped to the distal end of chromosome 6 with a lod score of 3.1.

Review of the toxicology of beclomethasone dipropionate

This paper reviews the published toxicity of beclomethasone dipropionate (BDP). BDP is a synthetic glucocorticosteroid which has a powerful local anti-inflammatory effect but little systemic action. It has been developed for both dermatological and inhaled applications. LD50 values and other acute studies indicated low toxicity. Findings published for repeat dose and reproductive toxicity studies embraced the known range of metabolic and physiological effects of glucocorticoids. For repeat dose studies, these included reduction in body weight gains, cushingoid syndrome in dogs, reductions in the numbers of lymphocytes and the weights of the tissues connected with the immune system, and hepatic glycogen deposition and fatty liver changes. In reproductive studies, there was an increase in the prevalence of cleft palate in mice and rabbits and in the number of dead foetuses, and ossification was retarded. Despite the route of administration, there was a general similarity of effects within and between species. All observations were characteristic of synthetic glucocorticoids and related to the intrinsic effects of these drugs.

The influence of earthquake-induced stress on human facial clefting and its simulation in mice

A large earthquake (8-9 on the Richter scale) and a series of aftershocks took place on 2 March 1985 in Santiago, Chile. The characteristics of over 22,000 births registered in three public hospitals in the same year were reviewed. A significant increase in the rate of facial clefts was found; 2.01 per 1000 births in contrast to 1.6 per 1000 births in previous years. The increase was greater in those born in September: 3.8 per 1000 births. This increase in clefting could be related to the effects of stress in mothers induced by the earthquake, and to test this hypothesis 13.5-day-old embryos from two inbred mouse strains, A/Sn and C57BL/10, were subjected to a similar stress using a vibrator cage to imitate the main shock and the first five replicas of the earthquake. The same intensity and duration of shock as in the original earthquake were applied. The results were 19.8% cleft palates in stressed A/Sn mice and no clefting in C57BL/10. This was highly significant in A/Sn mice (chi 2 = 19.9; P < 0.001) but not in C57BL/10. No clefting was found in controls in both strains. A surprising finding was the proportion of resorbed embryos in the stressed groups, which increased from 8.3 to 49.3% in A/Sn and from 5.8 to 48.3% in C57BL/10. It is known that A/Sn mice are genetically sensitive to cleft palate induction by cortisone, while C57BL/10 are not. These findings in mice support the stress hypothesis for the increase in cleft palate observed in humans. The increase in resorbed embryos in both strains also suggests an effect on stress.

Genetic analysis of the maternal factors controlling the survival of trisomy 16 mouse fetuses

The B x H recombinant inbred strains of mice were used to undertake a genetic analysis of the maternal factors controlling the survival of trisomy 16 fetuses. The data presented indicate that the prevalence of trisomic fetuses on day 15 of gestation varies significantly with the genetic background of the mother. The strain difference in the frequency of trisomy appears to be the result of selective elimination of trisomic fetuses. Various statistical methods employed to elucidate the genetic architecture of the trait from the recombinant inbred strains data indicate that the number of loci involved in the selection process ranges from one to five. Linkage association with two loci has been found; however, with a low probability level (P = 0.292).

Deficiency of the corpus callosum: incomplete penetrance and substrain differentiation in BALB/c mice

In a foundation population of BALB/c mice used to establish a colony at Waterloo in 1977, about 11% of adults showed either total absence or gross deficiency of the corpus callosum. Comparisons between parents and offspring, between progenies of different males, and between 13 separate lines established by full-sib inbreeding showed that variation in the adult corpus callosum reflected a genuine incomplete penetrance in a genetically uniform population. However, after seven generations of inbreeding, a spontaneous change occurred in one line (BALB/cWah 1), resulting in more than 50% of adults with deficient corpus callosum, among which about 20% have complete absence of callosal axons traversing the hemispheres.

Genetic differences in the frequency of acetazolamide-induced ectrodactyly in the mouse exhibit directional dominance of relative embryonic resistance

Eleven of the common inbred strains of the mouse were surveyed for their teratogenic response to acetazolamide that was administered three times per os at 1,000 mg/kg (9 A.M. and 4 P.M. on day 9 and 9 A.M. on day 10). The products of conception were examined for gross malformations on day 15. One strain, SJL/J, exhibited maternal toxicity to the dosage regime and was excluded from the survey. Five strains exhibited significantly increased resorption rates after treatment. All strains responded with the expected malformation of postaxial forelimb ectrodactyly with a right-sided predominance. Nine of the strains could be assigned to one of four mutually exclusive classes of frequency of ectrodactyly and the tenth strain (BALB/cByJ) showed overlap between the two intermediate classes. The data suggest major genes determine the difference in sensitivity to ectrodactyly rather than a polygenic mode of inheritance. Induced cleft lip was found in four strains and one of these strains, SWR/J, exhibited a significantly higher frequency. The strain differences in sensitivity to induced resorption, forelimb ectrodactyly, and cleft lip were genetically independent. A reciprocal cross study was conducted with five of the strains from the four classes of frequency of ectrodactyly response in order to determine gene action. A significant maternal effect on the ectrodactyly response was found only with one of the strain pairs in the ten sets of reciprocal crosses with the five strains. When there was a significant difference between two strains, the F1 embryos exhibited dominance of relative resistance to ectrodactyly. The directional dominance of relative resistance to acetazolamide-induced ectrodactyly suggests that regulatory genes control the embryonic differences in frequency of ectrodactyly response to acetazolamide. By analogy with other metric traits of development that exhibit directional dominance, the genetic variation in ectrodactyly response that has been observed so far in the mouse embryo may not be involved with the primary target of acetazolamide teratogenesis.

Heterochrony and human malformation

The role of altered developmental timing or heterochrony in morphologic evolution has intrigued classical and modern biologists. Analogous manifestations of developmental asynchrony occur in human dysmorphogenesis where they illustrate the residue and repertoire of phylogenetic change. Certain single malformations such as holoprosencephaly immediately suggest heterochrony by their resemblance to antecedent phylogenetic or embryologic structures. Multiple malformation syndromes of genetic, chromosomal, or teratogenic etiology may have altered developmental timing as an underlying theme. The persisting alpha-fetoprotein synthesis in ataxia-telangiectasia, the morphologic atavisms in Down or trisomy 13 syndromes, and the delayed growth or fetal to adult hemoglobin switch in diabetic embryopathy all exemplify developmental asynchrony. The perspective of heterochrony stresses the molecular history and hierarchy which is recapitulated with each pregnancy, and reconciles apparent discrepancies between the rates of molecular and morphologic evolution. Recognition of heterochrony places isolated anomalies in the context of pattern and suggests monitoring of teratogenesis through altered expression of ontogenetically regulated, phylogenetically relevant molecules.