The development and effect of genetic background on expressivity and penetrance of the Brachyury mutation in the mouse: a study in developmental genetics

Tint maps to mouse chromosome 6 and may interact with a notochordal enhancer of Brachyury

At the proximal part of mouse chromosome 17 there are three well-defined genes affecting the axis of the embryo and consequently tail length: Brachyury, Brachyury the second, and the t-complex tail interaction (T1, T2, and tct). The existence of T1 and tct in fact defines the classical "t-complex" that occupies approximately 40 cM of mouse chromosome 17. Their relationship to each other and various unlinked interacting genes has been enigmatic. The tint gene was the first of the latter to be identified. We report here its genetic mapping using a microsatellite scan together with outcrosses to Mus spretus and M. castaneous followed by a subsequent testcross to T, T1, and T2 mutants. Surprisingly, tint interacts with T2 but not with T1. The implications of our data suggest that T2 may be part of the T1 regulatory region through direct or indirect participation of tint.

Hop is an unusual homeobox gene that modulates cardiac development

Hop is a small, divergent homeodomain protein that lacks certain conserved residues required for DNA binding. Hop gene expression initiates early in cardiogenesis and continues in cardiomyocytes throughout embryonic and postnatal development. Genetic and biochemical data indicate that Hop functions directly downstream of Nkx2-5. Inactivation of Hop in mice by homologous recombination results in a partially penetrant embryonic lethal phenotype with severe developmental cardiac defects involving the myocardium. Inhibition of Hop activity in zebrafish embryos likewise disrupts cardiac development and results in severely impaired cardiac function. Hop physically interacts with serum response factor (SRF) and inhibits activation of SRF-dependent transcription by inhibiting SRF binding to DNA. Hop encodes an unusual homeodomain protein that modulates SRF-dependent cardiac-specific gene expression and cardiac development.

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.

Multiple gene control of the tRNA aminoacylating system in Drosophila melanogaster

The stimulation ofin vitrotRNA aminoacylation by post-microsomal supernatant enzymes isolated fromAbnormal Abdomen(A53g) adult flies is a function of the residual genome of these flies rather than of the major mutant gene,A53g. Genes controlling this stimulation are located on theXchromosome as well as on the autosomes. These observations are discussed in terms of the phenotypic effect of the major mutant gene in response to changes in protein synthesis which are under the control of modifier genes responsible for aberrant genetic translation mechanisms.