Lethal Recessive Mutations Research Articles

Establishing Control Frequency of Embryonic and Fetal Loss in a Mutant Mouse Colony.

In the analysis of prenatal lethal recessive mutations, one must account for embryonic losses that are not related to the mutant phenotype. This protocol details the way to determine what the background level of unrelated embryonic loss is by a simple backcrossing strategy in the particular mouse strain that carries the lethal recessive mutation.

Detection of homozygous genotypes for a putatively lethal recessive mutation in the porcine argininosuccinate synthase 1 (ASS1) gene.

The sequencing of the pig genome revealed the existence of homozygous individuals for a nonsense mutation in the argininosuccinate synthase 1 (ASS1) gene (rs81212146, c.944T>A, L315X). Paradoxically, an AA homozygous genotype for this polymorphism is expected to abolish the function of the ASS1 enzyme that participates in the urea cycle, leading to citrullinemia, hyperammonemia, coma and death. Sequencing of five Duroc boars that sired a population of 350 Duroc barrows revealed the segregation of the c.944T>A polymorphism, so we aimed to investigate its phenotypic consequences. Genotyping of this mutation in the 350 Duroc barrows revealed the existence of seven individuals homozygous (AA) for the nonsense mutation. These AA pigs had a normal weight despite the fact that mild citrullinemia often involves impaired growth. Sequencing of the region surrounding the mutation in TT, TA and AA individuals revealed that the A substitution in the second position of the codon (c.944T>A) is in complete linkage disequilibrium with a C replacement (c.943T>C) in the first position of the codon. This second mutation would compensate for the potentially damaging effect of the c.944T>A replacement. In fact, this is the most probable reason why pigs with homozygous AA genotypes at the 944 site of the ASS1 coding region are alive. Our results illustrate the complexities of predicting the consequences of nonsense mutations on gene function and phenotypes, not only because of annotation issues but also owing to the existence of genetic mechanisms that sometimes limit the penetrance of highly harmful mutations.

Changes in disease gene frequency over time with differential genotypic fitness and various control strategies

A spreadsheet model was constructed to describe the change in allelic frequency over time for a lethal recessive mutation in an animal population. The model allowed relative fitness to differ between genotypes, between sexes, and over time. Whereas a lethal recessive allele is naturally eliminated very slowly from a population, a small selective disadvantage of the heterozygote results in a large increase in the rate of elimination. With selective advantage of the heterozygote through linkage with a production trait or pleiotropy, the allele is never naturally eliminated but tends toward a stable equilibrium frequency. The model was used to investigate various alternative control programs based on the detection of heterozygotes by genotyping and their exclusion from breeding. The programs (genotyping males only, genotyping males and 50% of females, and genotyping all breeding animals) were modeled for various initial heterozygote frequencies, and the results were described in terms of the number of generations, number of tests, and number of culls required to reduce the heterozygote frequency to a predefined level. The model can be used to compare the feasibility and cost of various control strategies and to illustrate clearly to breeders the expected outcomes, as well as the danger of prematurely terminating a control program when there is a selective advantage of the heterozygote.

Yantar, a conserved arginine-rich protein is involved in Drosophila hemocyte development

To identify novel factors involved in Drosophila hematopoiesis, we screened a collection of lethal recessive mutations that also affected normal hemocyte composition in larvae. We present the characterization of the gene yantar ( ytr) for which we isolated null and hypomorphic mutations that were associated with severe defects in hemocyte differentiation and proliferation; ytr is predominantly expressed in the hematopoietic tissue during larval development and encodes an evolutionary conserved protein which is predominantly localized in the nucleus. The hematopoietic phenotype in ytr mutants is consistent with a defect or block in differentiation of precursor hemocytes: mutant larvae have enlarged lymph glands (LGs) and have an excess of circulating hemocytes. In addition, many cells exhibit both lamellocyte and crystal cell markers. Ytr function has been preserved in evolution as hematopoietic specific expression of the Drosophila or mouse Ytr proteins rescue the differentiation defects in mutant hemocytes.

Fragilitas ossium: a new autosomal recessive mutation in the mouse

Fragilitas ossium, fro, is an often lethal recessive mutation that was discovered in a randombred stock of mice after treatment with the chemical mutagen tris(1-aziridinyl) phosphine-sulphine. The fro/fro mice have osteoporosis-bending and fracture of the long bones-that is associated with apparently normal cartilage growth. The roentgenological and pathological features are similar to those of the severe, often lethal recessive form of human osteogenesis imperfecta.

Haplodiploid sex ratios and the mutation rate

Deviations from a 1:1 sex ratio in species with haplodiploid genetics are well known and sociobiologists have produced various explanations1–8, all of which ignore the effect of mutations on haploid viability. Lethal recessive mutations can significantly increase male (haploid) mortality and their effect should be estimated to provide the appropriate null hypothesis with which to compare observed sex ratios before considering any more complex, sociobiological hypothesis. Prospective maternal care4 and local mate competition1,2 will also affect sex ratio, but increased haploid mortality due to mutation rate is the only effect common to all haplodiploid species. Using emergence data for some parasitoid species, the rate of mutation to lethal recessives is estimated to be about 0.035 per genome per generation giving a differential haploid mortality of 10%.

Normal production of cartilage glycosaminoglycan in mice homozygous for the chondrodysplasia gene

AbstractBiochemical analysis of a lethal recessive mutation affecting chondrogenesis in mice further delineated the mechanism by which this pleiotropic gene acts. Cartilage from late fetuses homozygous for chondrodysplasia (cho/cho) were terminally labeled in vitro with Na235SO4, [14C] glucosamine, and [14C] glucose. Rates of precursor incorporation; tissue contents of uronic acid, amino sugar, and protein; and molecular weight of glycosaminoglycan were determined and found normal compared with control littermate cartilage. Samples from mutants, agitated overnight in distilled water and assayed for loss of incorporated 35S, showed between 47 and 77% of control activity. The rinse water from mutant cartilage contained correspondingly more 35S than did control. Chromatographic analysis of both tissue‐bound and water‐diffusible macro‐molecules confirmed that glycosaminoglycan and protein backbone were intact. Because the molecular content and rates of precursor incorporation were normal the results suggested that the cho gene does not alter net synthesis of glycosaminoglycan. The observation that water facilitates extraction of a glycosaminoglycan‐protein complex of normal molecular weight suggests that the defect in chondrogenesis results from defective interaction of proteoglycan and some other matrix constituent. In view of a previous report that collagen fibrils of mutant cartilage are atypically large and natively banded the site of chondrogenic defect might be the interaction between proteoglycan and collagen.