Reverse genetics in the mouse and its application to the study of deafness

Forward Genetics in Apicomplexa Biology: The Host Side of the Story

Forward genetic approaches have been widely used in parasitology and have proven their power to reveal the complexities of host-parasite interactions in an unbiased fashion. Many aspects of the parasite’s biology, including the identification of virulence factors, replication determinants, antibiotic resistance genes, and other factors required for parasitic life, have been discovered using such strategies. Forward genetic approaches have also been employed to understand host resistance mechanisms to parasitic infection. Here, we will introduce and review all forward genetic approaches that have been used to identify host factors involved with Apicomplexa infections, which include classical genetic screens and QTL mapping, GWAS, ENU mutagenesis, overexpression, RNAi and CRISPR-Cas9 library screens. Collectively, these screens have improved our understanding of host resistance mechanisms, immune regulation, vaccine and drug designs for Apicomplexa parasites. We will also discuss how recent advances in molecular genetics give present opportunities to further explore host-parasite relationships.

Fewer GABAergic interneurons, heightened anxiety and decreased high-frequency electroencephalogram components in Bronx waltzer mice, a model of hereditary deafness

The homozygous Bronx waltzer mutation (bv/bv) results in the degeneration of most but not all of the primary auditory receptors, the inner hair cells and their afferent neurons, and leads to perceptive deafness. However, the influence of the mutation on the central nervous system (CNS) remains largely unclear. In this study, we have conducted behavioral, morphological and electrophysiological investigations to clarify the CNS dysfunction in bv/bv mice. These mutant mice exhibited heightened levels of anxiety with normal levels of motor activity. Immunohistochemical analysis revealed a significant reduction in parvalbumin-containing GABAergic interneurons in the anterior cingulate, somatosensory and auditory cortices of bv/bv mice. The current results suggest that interneuron development may be disrupted in the bv/bv cerebrum. Moreover, the high-frequency electroencephalogram components of the cortical activity, including the frequency range containing high gamma, were markedly decreased in bv/bv mice compared with controls, probably indicating a disturbance in cortical inhibitory function. Together, these results suggest that the cortical development of interneurons and their electrophysiological profiles are altered in bv/bv mice. We propose that these novel phenotypes identified in bv/bv mice provide new perspectives on the basic neuronal mechanisms of developmental disorders.

The deaf mouse mutant whirler suggests a role for whirlin in actin filament dynamics and stereocilia development

Stereocilia, finger-like projections forming the hair bundle on the apical surface of sensory hair cells in the cochlea, are responsible for mechanosensation and ultimately the perception of sound. The actin cytoskeleton of the stereocilia contains hundreds of tightly cross-linked parallel actin filaments in a paracrystalline array and it is vital for their function. Although several genes have been identified and associated with stereocilia development, the molecular mechanisms responsible for stereocilia growth, maintenance and organisation of the hair bundle have not been fully resolved. Here we provide further characterisation of the stereocilia of the whirler mouse mutant. We found that a lack of whirlin protein in whirler mutants results in short stereocilia with larger diameters without a corresponding increase in the number of actin filaments in inner hair cells. However, a decrease in the actin filament packing density was evident in the whirler mutant. The electron-density at the tip of each stereocilium was markedly patchy and irregular in the whirler mutants compared with a uniform band in controls. The outer hair cell stereocilia of the whirler homozygote also showed an increase in diameter and variable heights within bundles. The number of outer hair cell stereocilia was significantly reduced and the centre-to-centre spacing between the stereocilia was greater than in the wildtype. Our findings suggest that whirlin plays an important role in actin filament packing and dynamics during postnatal stereocilium elongation.

Using targeted large deletions and high-efficiency N-ethyl-N-nitrosourea mutagenesis for functional analyses of the mammalian genome

The Human Genome Project has generated nucleotide sequences from an estimated 80,000 to 100,000 genes, only a small fraction of which have a known role. Nucleotide sequence information alone is insufficient to predict gene function. One of the most powerful ways of revealing gene function, as demonstrated in bacteria, worms, yeast, and flies, is to generate mutations and characterize them at both the phenotypic and the molecular levels. Given the physiological and anatomical parallels between mouse and human, genotype-phenotype relationships established in mice can be extrapolated to human syndromes. A new method is described for functional genetic analyses in the mouse that uses loxP/Cre engineering to generate coat color-tagged large deletions. The haploid regions can then be dissected by mutagenesis with N-ethyl-N-nitrosourea in phenotype-driven screens to obtain functional information on genes in any desired region of the mouse genome.

Cochlear histopathology of the mutant bustling mouse, BUS/Idr

The inner ear of mutant bustling mice, BUS/Idr, was examined histopathologically. LM examinations revealed an age-dependent degeneration of the auditory organ of Corti in BUS homozygotes, but not heterozygotes. Cochlear base-to-apex gradient in severity of the degeneration was noted. First signs of degeneration were found in the outer hair cells of the cochlear basal turn at about 3 weeks of age, followed by degeneration of the spiral ganglion cells which occurred slowly. As examined by SEM, stereociliary derangements of both the inner and outer hair cells were apparent in homozygotes as early as after 10 days. No normal arrays of stereocilia were found in homozygotes examined at 10 days through 6 months. The results of immunohistochemical examinations suggest that the sensory cells of the Corti's organ of homozygotes are structurally once normally innervated. No significant difference was found in the expression of protooncogene c-mos in the CNS between BUS homozygotes and control mice. We propose that BUS mice be categorized as a member of the so-called "waltzer-shaker" mutants group.

Role of mouse germ-cell mutagenesis in understanding genetic risk and in generating mutations that are prime tools for studies in modern biology

Highlights are presented on (1) the role mouse germ-cell mutagenesis has played in assessing the genetic harm from radiations and chemicals, and (2) the contributions to the field of modern biology that are being made by the products of this research--the propagated mutations. Among the numerous findings in radiation mutagenesis were the humped dose-effect curve for spermatogonial stem cells, the major differences between the sexes and between germ-cell stages of each sex in both yield and nature of mutations, the dose-rate effect, which provided the first evidence for repair of mutational (or premutational) damage, the augmenting effect of certain regimes of dose fractionation, and many others. Chemical mutagenesis studies that followed revealed at least three patterns of mutation yield and demonstrated that germ-cell stage--much more than the nature of the chemical--governs the nature of the DNA lesions induced. Two "supermutagens," one for intragenic mutations and one for deletions and other rearrangements, have become very useful in the manufacture of mutations for specific purposes. The mutations propagated from radiation- and chemical-mutagenesis experiments are providing prime resources for basic studies in genome organization, gene structure, and function. DNA lesions that involve specific loci have made possible increasingly detailed characterization of extensive deletion complexes that facilitate high-intensity physical and functional mapping within them. Numerous loci associated with interesting developmental anomalies have been identified and have become accessible to positional cloning. Several of the genes accessed with the aid of induced mutations (deletions, other rearrangements, and point mutations) are furnishing prime reagents for elucidating human disease conditions.

Genetic deafness

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Close linkage of the olfactory marker protein gene to the mouse deafness mutation shaker-1

One thousand sixty-six progeny have been generated from a backcross segregating for the mouse deafness mutation, shaker-1 (sh-1). One thousand fifty-two mice were analyzed for a protein polymorphism segregating for the distal flanking marker, beta-globin (Hbb), and 13 recombinants between Hbb and sh-1 were identified. One thousand eight mice were analyzed for a restriction fragment length polymorphism segregating for the proximal flanking marker, tyrosinase (c), and 54 recombinants between c and sh-1 were identified, completing a panel of 67 recombinant mice from the backcross in the vicinity of the sh-1 mutation. This panel allows the identification of markers closely linked to the sh-1 mutation that may act as start points for a chromosomal walk to the gene. One such marker, the olfactory marker protein gene (Omp), is recombinant with sh-1 in only one mouse from the recombinant panel. Thus, the Omp gene lies 0.1 cM from sh-1, on average, a distance of 200 kb. Haplotype analysis indicates that Omp lies proximal to sh-1.