Disorganization is a completely dominant gain-of-function mouse mutation causing sporadic developmental defects.

Disorganization (Ds) is an exceptional mutation because of its diverse and profound developmental effects. Although other mouse mutations produce similar congenital defects, extreme pleiotropism, random occurrence, developmental independence of multiple defects, and type of anomaly make Ds unique. Examples of developmental defects include cranioschisis, rachischisis, thoracoschisis, exencephaly, hamartomas, and anomalies of appendages, digestive, genital and urinary tracts, sense organs, limbs and girdles, tail and pharynx. No other mutation in the mouse has such broad effects. Ds is therefore an important model for studying not only the genetic control of lineage determination and pattern formation, but also the occurrence of sporadic congenital defects. To characterize the effects of gene dosage, we examined the viability and phenotype of Ds homozygotes and the phenotype of +/+/Ds trisomic fetuses. Occurrence of homozygotes was tested by intercrossing Ds/+ heterozygotes, typing genetic markers that flank Ds, and examining homozygotes for morphological abnormalities. Not only were Ds homozygotes found in their expected frequency, homozygotes were not more severely affected than heterozygotes. Trisomies provide a direct test for determining whether Ds is a gain-of-function mutation. Trisomic fetuses were derived by crossing Ds/Ds homozygous mice to hybrid mice that were heterozygous for two related Robertsonian translocations. Two trisomic fetuses had developmental defects characteristic of Ds mice. Together these results demonstrate that Ds is a completely dominant, gain-of-function mutation.

The cardiac actin locus (Actc-1) is not on mouse chromosome 17 but is linked to beta 2-microglobulin on chromosome 2.

A restriction fragment variant and recombinant inbred strains were used to show that the cardiac actin locus (Actc-1) is closely linked to beta 2-microglobulin (B2m) and several other loci on chromosome 2 of the mouse. Close linkage of Actc-1 and B2m in both man and mouse provides another example of a chromosomal segment that has been conserved since the divergence of the lineages leading to these two species.

A complex of serine protease genes expressed preferentially in cytotoxic T-lymphocytes is closely linked to the T-cell receptor alpha- and delta-chain genes on mouse chromosome 14.

A complex of genes encoding serine proteases that are preferentially expressed in cytotoxic T-cells was shown to be closely linked to the T-cell receptor alpha- and delta-chain genes on mouse chromosome 14. A striking difference in recombination frequencies among linkage crosses was reported. Two genes, Np-1 and Tcra, which fail to recombine in crosses involving conventional strains of mice, were shown to recombine readily in interspecific crosses involving Mus spretus. This difference in recombination frequency suggests chromosomal rearrangements that suppress recombination in conventional crosses, recombination hot spots in interspecific crosses, or selection against recombinant haplotypes during development of recombinant inbred strains. Finally, a mutation called disorganization, which is located near the serine protease complex, is of considerable interest because it causes an extraordinarily wide variety of congenital defects. Because of the involvement of serine protease loci in several homeotic mutations in Drosophila, disorganization must be considered a candidate for a mutation in a serine protease-encoding gene.

Two-hit model for sporadic congenital anomalies in mice with the disorganization mutation.

Congenital anomalies have complex etiologies involving both genetic and nongenetic components. Many are sporadic, without obvious evidence for heritability. An important model for these anomalies is a mutation in laboratory mice that is called "disorganization" (Ds), which functions as a variable autosomal dominant and leads to a wide variety of congenital anomalies involving many developmental processes and systems. Variable expressivity, asymmetrical manifestations, and low penetrance suggest that somatic events determine the location and nature of these anomalies. A statistical analysis suggests that occurrence of anomalies in mice with the Ds mutation follows a Poisson distribution. These results suggest that congenital anomalies in mice with the Ds mutation occur independently of each other. We propose that Ds causes a heritable predisposition to congenital anomalies and that Ds and appropriate somatic events combine to compromise normal development. We also propose that some sporadic, nonheritable congenital anomalies involve somatic mutations at Ds-like loci. Ds may therefore serve not only as a model for developmental anomalies in cell fate and pattern formation but also for complex developmental traits showing variable expressivity, low penetrance, and sporadic occurrence in mice and humans.

Zinc finger protein gene complexes on mouse chromosomes 8 and 11.

Two murine homologs of the Drosophila Krüppel gene, a member of the gap class of developmental control genes that encode a protein with zinc fingers, were mapped to mouse chromosomes 8 and 11 by using somatic cell hybrids and an interspecific backcross. Surprisingly, both genes were closely linked to two previously mapped, Krüppel-related zinc finger protein genes, suggesting that they are part of gene complexes.

The murine retinoblastoma homolog maps to chromosome 14 near Es-10.

Restriction fragment length variants have been exploited to map genetically Rb-1, the murine homolog of the human retinoblastoma gene. Rb-1 localized to mouse chromosome 14 on the basis of results from analysis of somatic cell hybrids. In an interspecific backcross involving Mus spretus, Rb-1 and the murine homolog of the human esterase D gene (ESD), which we refer to here as Esd, were inseparable. Furthermore, the strain distribution patterns of Rb-1 and Es-10 are the same in 31 of 32 recombinant inbred strains. Close linkage of the chromosome 14 morphological marker hairless (hr) to Rb-1 is also implied. These results localize Rb-1 on the mouse linkage map and provide close genetic markers to follow Rb-1 in somatic as well as in germline genetic experiments. Additionally, the results suggest that Es-10 is the murine homolog of ESD and provide further evidence for linkage conservation during mammalian evolution.

Embryonic expression of Lim-1, the mouse homolog of Xenopus Xlim-1, suggests a role in lateral mesoderm differentiation and neurogenesis.

cDNAs encoded by the mouse homolog (Lim-1) of the Xenopus LIM-class homeobox gene Xlim-1 have been isolated from an 8.5-day mouse embryo cDNA library. Nucleotide and deduced amino acid sequences show a high degree of identity with Xlim-1 in the LIM and homeodomains, and 85% identity over the whole protein. An interspecific back-cross has been used to show close linkage of Lim-1 to the endogenous proviral marker Mpmv-4 on mouse chromosome 11. Whole mount in situ hybridization studies have been carried out on mouse embryos between 6.5 and 10.5 days. In mid- to late-streak stage embryos, Lim-1 is expressed in a restricted region of mesoderm in the primitive streak, with the highest level of signal at the anterior. At 7.5 days, transcripts can be seen in a horseshoe-shaped pattern in the periphery of the node, as well as along both sides of the immediately adjacent notochord. In addition, transcripts are present in presumptive lateral and intermediate mesoderm. Later, expression becomes progressively restricted to intermediate mesoderm, the nephrogenic cords, and eventually mesonephric ducts and tubules. By 10.5 days Lim-1 transcripts also appear in restricted regions of the central nervous system (CNS) that are associated with sensory function. The lateral diencephalon, hindbrain, and presumed commissural neurons in the dorsal spinal cord all show Lim-1 expression. In the adult, Lim-1 is expressed in the cerebellum/medulla and kidney, and at very low levels in the cerebrum. These data suggest that in the mouse embryo Lim-1 plays a role in early mesoderm formation and later specification of a differentiated phenotype in subsets of cells of the mesonephros and sensory neurons of the CNS.

Genetic maps of mouse chromosome 17 including 12 new anonymous DNA loci and 25 anchor loci.

An interspecific backcross between lab mice and Mus spretus was used to construct a multilocus map of Chromosome 17 consisting of 12 new anonymous loci and 9 anchor loci. In addition, 7 anonymous DNA loci were added to the Chr 17 map for the BXD strains. Although we were able to identify readily the most likely gene order in the interspecific backcross, we found no evidence for an unambiguous gene order using the BXD recombinant inbred strains. Comparison of the interspecific backcross map and the BXD RI strain map revealed evidence in the interspecific backcross for a longer total genetic length, enhanced recombination distal to H-2, a segment showing suppressed recombination, and strong interference.

A family of type I keratin genes and the homeobox-2 gene complex are closely linked to the rex locus on mouse chromosome 11.

Type I and type II keratins are major constituents of intermediate filaments that play a fundamental role in the cytoskeletal network. By using both somatic cell hybrids and conventional and interspecific linkage crosses, several genes encoding type I keratins, including the epidermal keratin K10, were shown to be closely linked to the homeobox-2 complex and the rex locus on mouse chromosome 11. The absence of crossovers between type I keratin-encoding genes and rex (N = 239), a locus affecting hair development, raises the possibility that mutations at rex and neighboring loci affecting skin and hair development involve type I keratin genes.