A series of vectors that simplify mammalian gene targeting

A deafness mutation isolates a second role for the tectorial membrane in hearing

Alpha-tectorin (encoded by Tecta) is a component of the tectorial membrane, an extracellular matrix of the cochlea. In humans, the Y1870C missense mutation in TECTA causes a 50- to 80-dB hearing loss. In transgenic mice with the Y1870C mutation in Tecta, the tectorial membrane's matrix structure is disrupted, and its adhesion zone is reduced in thickness. These abnormalities do not seriously influence the tectorial membrane's known role in ensuring that cochlear feedback is optimal, because the sensitivity and frequency tuning of the mechanical responses of the cochlea are little changed. However, neural thresholds are elevated, neural tuning is broadened, and a sharp decrease in sensitivity is seen at the tip of the neural tuning curve. Thus, using Tecta(Y1870C/+) mice, we have genetically isolated a second major role for the tectorial membrane in hearing: it enables the motion of the basilar membrane to optimally drive the inner hair cells at their best frequency.

Targeted mutagenesis of the Hira gene results in gastrulation defects and patterning abnormalities of mesoendodermal derivatives prior to early embryonic lethality

The Hira gene encodes a nuclear WD40 domain protein homologous to the yeast transcriptional corepressors Hir1p and Hir2p. Using targeted mutagenesis we demonstrate that Hira is essential for murine embryogenesis. Analysis of inbred 129Sv embryos carrying the null mutation revealed an initial requirement during gastrulation, with many mutant embryos having a distorted primitive streak. Mutant embryos recovered at later stages have a range of malformations with axial and paraxial mesendoderm being particularly affected, a finding consistent with the disruption of gastrulation seen earlier in development. This phenotype could be partially rescued by a CD1 genetic background, although the homozygous mutation was always lethal by embryonic day 11, with death probably resulting from abnormal placentation and failure of cardiac morphogenesis.

A targeted deletion in alpha-tectorin reveals that the tectorial membrane is required for the gain and timing of cochlear feedback

alpha-tectorin is an extracellular matrix molecule of the inner ear. Mice homozygous for a targeted deletion in a-tectorin have tectorial membranes that are detached from the cochlear epithelium and lack all noncollagenous matrix, but the architecture of the organ of Corti is otherwise normal. The basilar membranes of wild-type and alpha-tectorin mutant mice are tuned, but the alpha-tectorin mutants are 35 dB less sensitive. Basilar membrane responses of wild-type mice exhibit a second resonance, indicating that the tectorial membrane provides an inertial mass against which outer hair cells can exert forces. Cochlear microphonics recorded in alpha-tectorin mutants differ in both phase and symmetry relative to those of wild-type mice. Thus, the tectorial membrane ensures that outer hair cells can effectively respond to basilar membrane motion and that feedback is delivered with the appropriate gain and timing required for amplification.

A zinc finger truncation of murine WT1 results in the characteristic urogenital abnormalities of Denys-Drash syndrome

The Wilms tumor-suppressor gene, WT1, plays a key role in urogenital development, and WT1 dysfunction is implicated in both neoplastic (Wilms tumor, mesothelioma, leukemias, and breast cancer) and nonneoplastic (glomerulosclerosis) disease. The analysis of diseases linked specifically with WT1 mutations, such as Denys-Drash syndrome (DDS), can provide valuable insight concerning the role of WT1 in development and disease. DDS is a rare childhood disease characterized by a nephropathy involving mesangial sclerosis, XY pseudohermaphroditism, and/or Wilms tumor (WT). DDS patients are constitutionally heterozygous for exonic point mutations in WT1, which include mutations predicted to truncate the protein within the C-terminal zinc finger (ZF) region. We report that heterozygosity for a targeted murine Wt1 allele, Wt1(tmT396), which truncates ZF3 at codon 396, induces mesangial sclerosis characteristic of DDS in adult heterozygous and chimeric mice. Male genital defects also were evident and there was a single case of Wilms tumor in which the transcript of the nontargeted allele showed an exon 9 skipping event, implying a causal link between Wt1 dysfunction and Wilms tumorigenesis in mice. However, the mutant WT1(tmT396) protein accounted for only 5% of WT1 in both heterozygous embryonic stem cells and the WT. This has implications regarding the mechanism by which the mutant allele exerts its effect.

Human repeat-mediated integration of selectable markers into somatic cell hybrids

We describe a strategy to introduce preferentially the dominant selectable marker neoR into the human chromosome within a monochromosome hybrid cell line. Integration of a construct containing the marker is mediated by human-specific repeat elements that promote multilocus human-specific integration with a single targeting vector. We tested two classes of repeat elements: the Alu family of SINE repeats and the Line1 repeat family. We show that Alu sequences alone are insufficient to direct human-specific integration but when used in combination with a Line1 element, or when only Line1 elements are included, integration of the vector into the human component of a monochromosome somatic cell hybrid is favored. The vectors also carry sequences that facilitate mapping and selective cloning of the targeted region. This strategy provides a means to generate selectable human subchromosomal fragments that can be used for localization of genes through positional cloning and, more important, for the identification of functional units through DNA transfer.