Interactions between cycloheximide and T-locus alleles during mouse embryogenesis

Female CD-1 mice were mated with CD-1 X T/ + F1 males that were heterozygous for the brachyury (T') semidominant lethal gene or were +/+. Fetuses from CD-1 X +/+ matings were normal when observed on gestation Day 17 (plug day = Day 0). Those from the CD-1 X T/+ cross exhibited the expected 1:1 ratio of short:normal tail lengths, but 10% of these fetuses were tailless, apparently due to factors in the CD-1 genotype that increased the expressivity of the T-gene with regard to reduction of tail length. Additional CD-1 females were mated with CD-1 X tw18/+ F1 males. Fetuses from the CD-1 X tw18/+ matings were normal. CD-1 females carrying CD-1 X +/+, CD-1 X T/+, or CD-1 X tw18/+ litters were injected ip on gestation Day 9 with 30 mg/kg cycloheximide or were untreated. Cycloheximide was teratogenic for litters from all three crosses. Polydactyly, oligodactyly, and a variety of skeletal abnormalities were observed. Gross malformations and total skeletal malformations were increased in treated CD-1 X T/ + or tw18/+ litters in comparison with CD-1 X +/+ litters, as were nonvertebral skeletal defects in CD-1 X tw18/+ litters. Prenatal mortality was also greater in treated mutant-containing litters than in +/+ litters, and fetal weights were similarly decreased in treated CD-1 X tw18/+ litters. The incidence of taillessness was also higher in treated (26%) than in control (10%) CD-1 X T/+ litters. Thus both the T and tw18 alleles appear to have enhanced the teratogenicity of cycloheximide, and the inhibitor may have increased the expressivity of T.

Differences in the toxicity and teratogenicity of cytochalasins D and E in various mouse strains

Adult toxicity, embryo lethality, and teratogenicity of cytochalasins D and E (CD and CE) were determined for A/J, C57BL/6JH, aeae, and OELN mouse strains. The adult LD50 for CD (1.9-2.6 mg/kg) and CE (3.1-3.3) were similar for the strains. Although adult LD50s were similar, the embryo lethality and teratogenicity of CD and CE vary from strain to strain when all were treated with intraperitoneal injections on days 7-9 of gestation. CD was more teratogenic than CE and frequently produced exencephaly while CE did not. CD was more teratogenic in OELN and aeae (ED50 +/- 0.7 mg/kg) than in A/J or C57BL/6J (ED50 greater than 1.2 mg/kg). OELN fetuses were more resistant to lethal effects of CD than the other strains which had fetal LD50s +/- 1.2 mg/kg maternal weight. The F1 fetuses of a cross of sensitive aeae and resistant C57BL/6J showed intermediate sensitivity to exencephaly. The results suggest a genetic contribution to the response and differences in action of CD and CE as teratogens.

The susceptibility to cortisone-induced cleft palate of recombinant inbred strains of mice: lack of association with the H-2 haplotype

Recombinant-inbred (RI) strains of mice derived from the cross of strains C57BL/6J and DBA/2J were used to study the inheritance of susceptibility to cortisone-induced cleft palate. Most of the RI strains could be classified as either resistant, like C57BL/6J, or susceptible, like DBA/2J, suggesting the segregation of a major locus. An association with the phosphoglucomutase-1 locus (Pgm-1) on Chromosome 5 was observed. There was no association with the H-2 locus on Chromosome 17 as had been reported in previous studies utilizing different strains. We conclude that susceptibility to cortisone-induced cleft palate may be determined by different loci depending on the strains studied.

Genetics society of Canada award of excellence lecture. The genetics of common familial disorders--major genes or multifactorial?

The common familial disorders were, until recently, neglected by geneticists because their familial distributions did not neatly fit the Mendelian mold, and no specific genes could be identified. The multifactorial-threshold model made the familial characteristics of these disorders more intelligible. Although it originally postulated a polygenic genetic component the model can also accommodate one or more major genes with low penetrance. The resulting upsurge of interest has led to (1) the development of increasingly sophisticated mathematical models from which to calculate recurrence risks for specific family situations and (2) the identification of specific predisposing genes in a number of such disorders. One of the corollaries of the model is that any pharmacological agent at therapeutic doses is likely to be teratogenic to at least some embryos, so that regulation should be in terms of "acceptably low" levels of teratogenicity rather than "safety".

Dissimilar effects of thalidomide in dizygotic twins

Among 93 cases of thalidomide embryopathy ascertained in Brazil, one pair of female dizygotic twins was found. One of them presented complete paraxial hemimelia in the right arm while the other showed a triphalangeal thumb at the left hand and had duodenal stenosis. Of the eight dizygotic twin pairs exposed to thalidomide in utero described thus far, only four were concordant for the type of malformation.

Gene-teratogen interaction in insulin-induced mouse exencephaly

Heterozygotes for the mutant genes crooked (Cd) or rib fusion (Rf), crossed to either strain A/J or SWV produce mutant F1 offspring with minor skeletal defects. If the mothers are also treated with teratogenic doses of insulin, the mutant F1 heterozygotes develop exencephaly significantly more often than nonmutants. That the heterozygous mutant gene increased the embryo's sensitivity to insulin-induced exencephaly was also demonstrated by dose-response curves. For A/J and SWV crosses, probit regression lines for the mutant crosses were fitted to common slopes that were significantly different from those of probit regression lines for the nonmutant crosses. The nonparallelism of the mutant versus the nonmutant groups of lines indicated the presence of a gene-teratogen interaction. Early exencephalic embryos showed delayed turning in one-third of treated mutant crosses and less than a quarter of treated nonmutant crosses. Another one-third of exencephalics, in treated mutant crosses only, had abnormally twisted hindbodies and appeared to be phenocopies of untreated homozygotes. Insulin treatment significantly slowed the rates of embryo turning and neural tube closure but did not affect somite number. Histologically, insulin did not appear to cause cell death, and mitotic index of mesoderm was unaffected. Only exencephalic embryos showed a significant reduction in neurectoderm mitotic index. Insulin caused a reduction in neurectoderm cell proliferation, delay in pattern of turning and delay in neural tube closure to the greatest extent in exencephalic embryos. It was postulated that extensive delays in turning could provide a mechanical basis for the failure of the neural tube to close.

The murine Ah locus: in utero toxicity and teratogenesis associated with genetic differences in benzo[a]pyrene metabolism

Benzo[a]pyrene, at dose between 50 and 300 mg per kg body weight given at Day 7 or 10 of gestation, causes in utero toxicity and teratogenicity more so in genetically "responsive" C57BL/6 than in "nonresponsive" AKR inbred mice. With the use of AKR X (C57BL/6) (AKR)F1 and (C57BL/6) (AKR)F1 X AKR backcrosses, it was shown that allelic differences at the Ah locus in the fetus can be correlated with dysmorphogenesis. If the mother is nonresponsive (Ahd/Ahd), the Ahb/Ahd genotype in the fetus is associated with more stillborns and resorptions, decreased fetal weight, increased congenital anomalies, and enhanced P1-450-mediated covalent binding of BP metabolites to fetal protein and DNA, when compared with the Ahd/Ahd genotype in the fetus from the same uterus. If the mother is responsive (Ahb/Ahd), however, none of these parameters can be distinguished between Ahb/Ahd and Ahd/Ahd individuals in the same uterus, presumably because enhanced BP metabolism in maternal tissues and placenta cancels out these differences between individual fetuses. Of particular interest in our study is the fact that the mother and the father both must be of a particular genotype before differences in teratogenesis among fetuses (due to their genotype) will be expressed. These data might provide an example in attempting to explain clinically why only one child is affected with an apparent "drug-induced syndrome" although the mother has taken the same dose of the particular drug during each of numerous pregnancies.

Genetic independence of the embryonic reactivity difference to cortisone- and 6-aminonicotinamide-induced cleft palate in the mouse

The A/J strain of mice is more reactive than C57BL/6J to both cortisone- and 6-aminonicotinamide-induced cleft palate. A breeding study was set up to determine the genetic control of the differences in embryonic reactivity between the two strains to the two teratogens. In this paper the test for possible association between the two response traits is presented. In the second-backcross generation of embryos where segregation of the two traits could be studied no association was found. Therefore, any embryonic genes making major contributions to differences in reactivity between the two strains are not the same for the two teratogens.

Cadmium induced limb defects in mice: strain associated differences in sensitivity

Cadmium (CdSO4) was given ip on day 9 at 12 or 24 mumol/kg to pregnant CD-1 (non-inbred) mice. Fetuses showed malformations of the limbs, face, trunk, and tail. There was a statistically significant relationship between the dose of cadmium and the malformation rate. Cadmium (12 mumol/kg ip on day 9) was then given to mice of six inbred strains three of which (A/J, BALB/cJ, and C57BL6J) carry a gene cdm for resistance to cadmium-induced testicular damage, and three strains (AKR/J, CBA/J, and DBA/2J) which do not. Paradoxically, the three strains resistant to cadmium induced testicular damage were significantly more sensitive to its teratogenic effects than were the other three strains. In all inbred strains most malformations involved the limbs. All forelimb defects found in inbred or non-inbred cadmium treated mice were postaxial and indistinguishable from those produced by acetazolamide in mice. The remarkable similarity of the cadmium- and acetazolamide-induced forelimb malformations may be a reflection of the limited number of ways that a rodent forelimb can react to a teratogenic insult. The hindlimb defects were all preaxial.

6-Aminonicotinamide-induced cleft palate in the mouse: the nature of the difference between the A/J and C57Bl/6J strains in frequency of response and its genetic basis

Cleft palate induction by 6-aminonicotinamide (6-AN) was examined in the A/J and C57BL/6J strains of mice to determine the nature of the strain difference in frequency of cleft-palate response. Probit analysis of the cleft-palate response to dose of different genotypes revealed a family of linear and parallel dose-response curves. The genotypes differ only in dosage tolerance (log ED50) to 6-AN that is required for the cleft-palate response. No evidence for a maternal cytoplasmic effect on 6-AN-induced cleft palate was found under the conditions of the present study. When the difference is dosage tolerance to 6-AN between A/J and C57BL/6J was examined with a single dose and measured by differences in frequency of induced cleft palate on a probit scale, there was some departure from genetic additivity. There was an indication off dominance deviation of the F1 embryos in the direction of C57BL/6J.A3-locus, epistatic model is proposed to account for the difference in embryonic tolerance ot 6-AN-induced cleft palate. There was a suggestion of association with the brown (b) locus.

Chemical mutagenesis: an emerging issue for public health

Chemical mutagens are recognized as prevalent in the environment and a potential threat to the health of future generations. This paper presents an overview of chemical mutagenesis as an issue for public health. Several problems in the determination of risk to human populations are discussed, including difficulties of extrapolating scientific data to humans, the latency period between exposure and recognizable genetic damage, and the large number of chemicals which must be tested. Test systems are described. Possibilities of control through federal regulation are discussed.

Genetically mediated induction of drug-metabolizing enzymes associated with congenital defects in the mouse

Various polycyclic aromatic compounds induce certain monooxygenase activities, including aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.2), and cytochrome P1-450 in the liver and many nonhepatic tissues of the mouse. This induction process is controlled by the Ah locus. Genetic differences that have been shown in the past to be associated with the Ah locus include an increased susceptibility to chemical carcinogenesis, mutagenicity in vitro, and drug toxicity--manifested as hepatic necrosis, aplastic anemia, or shortened survival time. Pregnant mice received a single injection of 3-methylcholanthrene or 7,12-dimethylbenz[a] anthracene between day 5 and day 13 of gestation, and the uterine contents were examined on day 18. Striking increases were observed in the incidence of MC-1 and DMBA-induced resorptions and congenital malformations in the aromatic hydrocarbon "responsive" C57BL/6N inbred strain, and of DMBA-induced resorptions in the "responsive" C3H/HeN and BALB/cAnN strains--when compared with the similarly treated genetically "nonresponsive" AKR/N strain. These data suggest but do not prove that an association exists between the Ah locus and developmental toxicity, i.e., teratogenesis. Although numerous teratogenic differences among inbred mouse strains have previously reported, this study is unique in that the genetic differences in teratogenicity observed were predicted in advance on the basis of known differences among these strains in polycyclic hydrocarbon metabolism regulated by the Ah locus.

Cortisone-induced cleft palate in the mouse. A search for the genetic control of the embryonic response trait

The cause of the difference in the mean tolerance (ED50) to cortisone-induced cleft palate between the embryos of the A/J and C57BL/6J strains appears to be due to a small number of genes. A single major gene effect and a polygenic model, in the sense of many equal and additive genes, have been ruled out. The embryonic tolerance of C57BL/6J is greater than and dominant to that of A/J; two or three loci, possibly with independent effects, appear to explain the variability. A component of the variation in embryonic response may be associated with or linked to the major histocompatibility locus (H-2). No evidence was found to support the hypothesis of X-chromosome linked susceptibility to cortisone-induced cleft palate.

The multifactorial/threshold concept -- uses and misuses

The common congenital malformations have familial distributions that cannot be accounted for by simple Mendelian models, but can be explained in terms of a continuous variable, "liability," with a threshold value beyond which individuals will be affected. Both genetic and environmental factors determine liability, making the system multifactorial. Cleft palate is a useful experimental model, illustrating a number of factors that contribute to palate closure, the nature of a developmental threshold, and how genes and teratogens can alter the components of liability to increase the probability of cleft palate. The nature of the genetic component to liability in human malformations in not clear, and various possibilities, ranging from polygenic in the strict sense to a major gene with reduced penetrance are compatible with the data -- but the important feature is the threshold. Much of the confusion over the concept results from inconsistent use of terminology. The term "multifactorial" should be used for "determined by a combination of genetic and environmental factors," without reference to the nature of the genetic factor(s). "Polygenic" should be reserved for "a large number of genes, each with a small effect, acting additively." When several genes, with more major effects are involved, "multilocal" can be used. When it is not clear which of these is applicable the term "plurilocal" is suggested, in the sense of "genetic variation more complex than a simple Mendelian difference." Since teratological data often represent threshold characters the concept also has important implications for the interpretation of data on dose-response curves, synergisms, and strain differences in response to teratogens.

Teratogenic effect of anticonvulsant drugs

Multiple malformations in all four children of a mother taking anticonvulsant drugs are reported, along with the extended family pedigree.

Congenital malformations in four siblings of a mother taking anticonvulsant drugs

Four siblings have various congenital malformations attributable to the teratogenic effect of anticonvulsant drugs. Their mother has 23-year history of continuous medication for seizures. Since the malformations noted in her four offspring are more extensive and severe in each subsequent child, the question arises as to the possible cumulative effect of antiepileptic drugs in producing congenital malformations. The observations in this family strongly support the need to carefully evaluate all offspring of mothers receiving anticonvulsant drugs.