Location of the 9257 and ataxia mutations on mouse chromosome 18

Palatal development in Twirler mice

OBJECTIVE

The development of the secondary palate of Twirler (Tw) mice was characterized, and a quantitative coronal plane evaluation of the width and length of the craniofacial regions of homozygous mutant embryos was conducted.

RESULTS AND CONCLUSIONS

The secondary palatal shelves were retarded in growth and the timing of elevation from a vertical to horizontal position. The homozygous Tw embryos, when compared with their wild-type littermates, also had a significantly wider midface. It is not surprising, therefore, that the short palatal shelves in a wide midface resulted in cleftings of the secondary palate observed in all homozygous Tw mice. These findings are consistent with similar studies in humans and other murine models.

Construction of a long-range YAC physical map spanning the 10-cM region between the markers D18Mit109 and D18Mit68 on mouse proximal chromosome 18

Four yeast artificial chromosome (YAC) contigs, physically approximately 8 Mb, have been constructed spanning a 10-cM region on mouse proximal chromosome 18 and include the sites of 21 known genes, including those near the twirler (Tw) locus and the recently isolated Niemann-Pick type C1 (npc1) gene, formerly designated as the spm locus. This physical map consists of 49 YAC clones that cover roughly 15% of the chromosome. The physical order of 38 microsatellite sequence-tagged sites (STSs) could be assembled and confirmed based on their presence or absence in individual YACs, from proximal D18Mit109 through distal D18Mit68. These YACs provide an important resource for the further characterization and identification of known and unknown genes. The physical map has been integrated with our previously published genetic linkage map and showed an average genetic to physical distance of cM/Mb > 1.1.

The Twirler mouse, a model for the study of cleft lip and palate

Twirler (Tw) is a semidominant mutation in the mouse affecting the embryonic development of the midfacial region. Most heterozygous Tw mice, +/-, become obese at adulthood with a concomitant decrease in fertility. Homozygous mice have clefts of the midfacial region and a disrupted nasal cavity. Midfacial clefts included clefts of the palate combined with either unilateral or bilateral clefts of the lip. The clefts of the lip were either complete or incomplete. The palatal shelves in Tw/Tw were very much reduced. Apart from these defects, homozygous Tw looked normal, and were born alive, although they reportedly die within 24 h after birth. It is proposed that the Twirler model can be used to improve understanding of the genetic mechanisms involved in the normal development of the midfacial region.

Murine model of Niemann-Pick C disease: mutation in a cholesterol homeostasis gene

An integrated human-mouse positional candidate approach was used to identify the gene responsible for the phenotypes observed in a mouse model of Niemann-Pick type C (NP-C) disease. The predicted murine NPC1 protein has sequence homology to the putative transmembrane domains of the Hedgehog signaling molecule Patched, to the cholesterol-sensing regions of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and SREBP cleavage-activating protein (SCAP), and to the NPC1 orthologs identified in human, the nematode Caenorhabditis elegans, and the yeast Saccharomyces cerevisiae. The mouse model may provide an important resource for studying the role of NPC1 in cholesterol homeostasis and neurodegeneration and for assessing the efficacy of new drugs for NP-C disease.

The laminin alpha chains: expression, developmental transitions, and chromosomal locations of alpha1-5, identification of heterotrimeric laminins 8-11, and cloning of a novel alpha3 isoform

Laminin trimers composed of alpha, beta, and gamma chains are major components of basal laminae (BLs) throughout the body. To date, three alpha chains (alpha1-3) have been shown to assemble into at least seven heterotrimers (called laminins 1-7). Genes encoding two additional alpha chains (alpha4 and alpha5) have been cloned, but little is known about their expression, and their protein products have not been identified. Here we generated antisera to recombinant alpha4 and alpha5 and used them to identify authentic proteins in tissue extracts. Immunoprecipitation and immunoblotting showed that alpha4 and alpha5 assemble into four novel laminin heterotrimers (laminins 8-11: alpha4beta1gamma1, alpha4beta2gamma1, alpha5beta1gamma1, and alpha5beta2gamma1, respectively). Using a panel of nucleotide and antibody probes, we surveyed the expression of alpha1-5 in murine tissues. All five chains were expressed in both embryos and adults, but each was distributed in a distinct pattern at both RNA and protein levels. Overall, alpha4 and alpha5 exhibited the broadest patterns of expression, while expression of alpha1 was the most restricted. Immunohistochemical analysis of kidney, lung, and heart showed that the alpha chains were confined to extracellular matrix and, with few exceptions, to BLs. All developing and adult BLs examined contained at least one alpha chain, all alpha chains were present in multiple BLs, and some BLs contained two or three alpha chains. Detailed analysis of developing kidney revealed that some individual BLs, including those of the tubule and glomerulus, changed in laminin chain composition as they matured, expressing up to three different alpha chains and two different beta chains in an elaborate and dynamic progression. Interspecific backcross mapping of the five alpha chain genes revealed that they are distributed on four mouse chromosomes. Finally, we identified a novel full-length alpha3 isoform encoded by the Lama3 gene, which was previously believed to encode only truncated chains. Together, these results reveal remarkable diversity in BL composition and complexity in BL development.