Nell1-deficient mice have reduced expression of extracellular matrix proteins causing cranial and vertebral defects.

The mammalian Nell1 gene encodes a protein kinase C-beta1 (PKC-beta1) binding protein that belongs to a new class of cell-signaling molecules controlling cell growth and differentiation. Over-expression of Nell1 in the developing cranial sutures in both human and mouse induces craniosynostosis, the premature fusion of the growing cranial bone fronts. Here, we report the generation, positional cloning and characterization of Nell1(6R), a recessive, neonatal-lethal point mutation in the mouse Nell1 gene, induced by N-ethyl-N-nitrosourea. Nell1(6R) has a T-->A base change that converts a codon for cysteine into a premature stop codon [Cys(502)Ter], resulting in severe truncation of the predicted protein product and marked reduction in steady-state levels of the transcript. In addition to the expected alteration of cranial morphology, Nell1(6R) mutants manifest skeletal defects in the vertebral column and ribcage, revealing a hitherto undefined role for Nell1 in signal transduction in endochondral ossification. Real-time quantitative reverse transcription-PCR assays of 219 genes showed an association between the loss of Nell1 function and reduced expression of genes for extracellular matrix (ECM) proteins critical for chondrogenesis and osteogenesis. Several affected genes are involved in the human cartilage disorder Ehlers-Danlos Syndrome and other disorders associated with spinal curvature anomalies. Nell1(6R) mutant mice are a new tool for elucidating basic mechanisms in osteoblast and chrondrocyte differentiation in the developing skull and vertebral column and understanding how perturbations in the production of ECM proteins can lead to anomalies in these structures.

Efficient gene-driven germ-line point mutagenesis of C57BL/6J mice.

BACKGROUND: Analysis of an allelic series of point mutations in a gene, generated by N-ethyl-N-nitrosourea (ENU) mutagenesis, is a valuable method for discovering the full scope of its biological function. Here we present an efficient gene-driven approach for identifying ENU-induced point mutations in any gene in C57BL/6J mice. The advantage of such an approach is that it allows one to select any gene of interest in the mouse genome and to go directly from DNA sequence to mutant mice. RESULTS: We produced the Cryopreserved Mutant Mouse Bank (CMMB), which is an archive of DNA, cDNA, tissues, and sperm from 4,000 G1 male offspring of ENU-treated C57BL/6J males mated to untreated C57BL/6J females. Each mouse in the CMMB carries a large number of random heterozygous point mutations throughout the genome. High-throughput Temperature Gradient Capillary Electrophoresis (TGCE) was employed to perform a 32-Mbp sequence-driven screen for mutations in 38 PCR amplicons from 11 genes in DNA and/or cDNA from the CMMB mice. DNA sequence analysis of heteroduplex-forming amplicons identified by TGCE revealed 22 mutations in 10 genes for an overall mutation frequency of 1 in 1.45 Mbp. All 22 mutations are single base pair substitutions, and nine of them (41%) result in nonconservative amino acid substitutions. Intracytoplasmic sperm injection (ICSI) of cryopreserved spermatozoa into B6D2F1 or C57BL/6J ova was used to recover mutant mice for nine of the mutations to date. CONCLUSIONS: The inbred C57BL/6J CMMB, together with TGCE mutation screening and ICSI for the recovery of mutant mice, represents a valuable gene-driven approach for the functional annotation of the mammalian genome and for the generation of mouse models of human genetic diseases. The ability of ENU to induce mutations that cause various types of changes in proteins will provide additional insights into the functions of mammalian proteins that may not be detectable by knockout mutations.

Mouse bone marrow micronucleus test results do not predict the germ cell mutagenicity of N-hydroxymethylacrylamide in the mouse dominant lethal assay.

N-Hydroxymethylacrylamide (NHMA), a mouse carcinogen inactive in the Salmonella assay and mouse micronucleus (MN) assay, was tested for reproductive effects in a mouse continuous breeding study. In that study, increased embryonic deaths were observed after 13 weeks exposure of parental animals to NHMA via drinking water (highest dose, 360 ppm); the results indicated the possible induction of chromosome damage in germ cells of treated males. An additional mouse MN test was conducted using a 31-day treatment period to better match the dosing regimen used in the breeding study; the results were negative. Additional studies were conducted to explore the germ cell activity of NHMA. A male mouse dominant lethal study was conducted using a single intraperitoneal injection of 150 mg/kg NHMA; the results were negative. A follow-up study was conducted using fractionated dosing, 50 mg/kg/day for 5 days; again, no increase in dominant lethal mutations was observed. NHMA (180-720 ppm) was then administered to male mice in drinking water for 13 weeks, during which three sets of matings occurred. Two weeks after mating, females were killed and the uterine contents were analyzed. Large, dose-related increases in dominant lethal mutations were observed with increasing length of exposure. The magnitude of the increases stabilized after 8 weeks of treatment. However, the frequency of micronucleated peripheral blood erythrocytes was not elevated in mice treated for 13 weeks with NHMA in drinking water. Thus, NHMA appears to be unique in inducing genetic damage in germ cells but not somatic cells of male mice.