Kathleen A. Mills

Genetic and physical maps of human chromosome 4 based on dinucleotide repeats

Characterization of inherited variations within tandem arrays of dinucleotide repeats has substantially advanced the construction of genetic maps using linkage approaches over the last several years. Using a backbone of 10 newly identified microsatellite repeats on human chromosome 4 and 6 previously identified short tandem repeat element polymorphisms, we have constructed several genetic maps and a physical map of human chromosome 4. The genetic and physical maps are in complete concordance with each other. The genetic maps include a 15-locus microsatellite-based linkage map, a framework map of high support incorporating a total of 39 independent loci, a 25-locus high-heterozygosity, easily used index map, and a gene-based comprehensive map that provides the best genetic location for 35 genes mapped to chromosome 4. The 16 microsatellite markers are each localized to one of nine regions of chromosome 4, delineated by a panel of somatic cell hybrids. These results demonstrate the utility of PCR-based repeat elements for the construction of genetic maps and provide a valuable resource for continued high-resolution mapping of chromosome 4 and of genetic disorders to this chromosome.

Mouse chromosome 8

Genetic and physical mapping of the mouse genome is important for studying genome organization and evolution, identifying genes at or near existing mouse mutations, and establishing mouse models for human diseases. Several interesting mutations whose gene products have not yet been identified have been mapped to mouse Chromosome (Chr) 8 (Lyon and Searle 1989). Mutations such as nervous (nr), tottering (tg) and hydrocephalus-3 (hy-3) affect neurological development, and mutations such as oligosyndactylism (Os), amputated (am), quinky (Q), and neonatal anemia (Nan) affect early embryonic development (as well as other functions). As more molecular clones are mapped to Chr 8, it will be possible to identify candidate genes that are altered by the mutations. The purpose of this report is to provide a comprehensive summary of genetic and cytogenetic mapping data that will aid in the development of consensus maps, and to show the homologous relationships between mouse Chr 8 and human chromosomes.

Genetic mapping near the myd locus on mouse Chromosome 8

Myodystrophy (myd), an autosomal recessive mutation of the mouse characterized by progressive weakness and dystrophic muscle histology, maps to the central portion of Chromosome (Chr) 8 (Lane et al. J. Hered 67, 135, 1976). This portion of Chr 8 contains the genes for a mitochondrial uncoupling protein (Ucp) and kallikrein (Kal3), which map to distal 4q in the human, providing evidence for a segment of homology. Characteristics of the myd phenotype coupled with this homology suggest that myd may be a mouse homolog of facioscapulohumeral muscular dystrophy (FSHD), which maps to human 4q35. We have confirmed and expanded the region of mouse 8-human 4 homology by generating a map of Chr 8 in an interspecific backcross of C57BL/6J and a partially inbred strain derived from M. spretus. The map is comprised of the genes for Ucp, coagulation factor XI (Cf11), and chloride channel 5 (Clc5), all of which have homologs on distal human 4q, 15 microsatellite loci, and the membrane cofactor protein pseudogene (Mcp-ps). To place myd in the genetic map, 75 affected progeny from an intersubspecific backcross of animals heterozygous for myd with Mus musculus castaneus were genotyped with Chr 8 microsatellite loci. The mutation maps between D8Mit30 and D8Mit75, an interval that is flanked by genes with human homologs at distal 4q. These results are consistent with the possibility that myd is the mouse homolog of FSHD.

Construction of a human chromosome 4 YAC pool and analysis of artificial chromosome stability

In order to construct a human chromosome 4-specific YAC library, we have utilized pYAC4 and a mouse/human hybrid cell line HA(4)A in which the only human chromosome present is chromosome 4. From this cell line, approximately 8Mb of chromosome 4 have been cloned. The library includes 65 human-specific clones that range in size from 30kb to 290kb, the average size being 108kb. In order to optimize the manipulation of YAC libraries, we have begun to investigate the stability of YACs containing human DNA in yeast cells; these studies will also determine if there are intrinsic differences in the properties of chromosomes containing higher eukaryotic DNAs. We are examining two kinds of stability: 1] mitotic stability, the ability of the YAC to replicate and segregate properly during mitosis, and 2] structural stability, the tendency of the YAC to rearrange. We have found that the majority of YACs examined are one to two orders of magnitude less stable than authentic yeast chromosomes. Interestingly, the largest YAC analyzed displayed a loss rate typical for natural yeast chromosomes. Our results also suggest that increasing the length of an artificial chromosome improves its mitotic stability. One YAC that showed a very high frequency of rearrangement by mitotic recombination proved to be a mouse/human chimera. In contrast to studies using total human DNA, the frequency of chimeras (i.e., mouse/human) in the YAC pool appeared to be low.

Nlrp12 mutation causes C57BL/6J strain-specific defect in neutrophil recruitment

The inbred mouse strain C57BL/6J is widely used in models of immunological and infectious diseases. Here we show that C57BL/6J mice have a defect in neutrophil recruitment to a range of inflammatory stimuli compared with the related C57BL/6N substrain. This immune perturbation is associated with a missense mutation in Nlrp12 in C57BL/6J mice. Both C57BL/6J and NLRP12-deficient mice have increased susceptibility to bacterial infection that correlates with defective neutrophil migration. C57BL/6J and NLRP12-deficient macrophages have impaired CXCL1 production and the neutrophil defect observed in C57BL/6J and NLRP12-deficient mice is rescued by restoration of macrophage NLRP12. These results demonstrate that C57BL/6J mice have a functional defect in NLRP12 and that macrophages require NLRP12 expression for effective recruitment of neutrophils to inflammatory sites.

The sarcolemma in the largemyd mouse

In the Largemyd mouse, dystroglycan is incompletely glycosylated and thus cannot bind its extracellular ligands, causing a muscular dystrophy that is usually lethal in early adulthood. We show that the Largemyd mutation alters the composition and organization of the sarcolemma of fast‐twitch skeletal muscle fibers in young adult mice. Costameres at the sarcolemma of the tibialis anterior muscle of Largemyd mice contain reduced levels of several membrane cytoskeletal proteins, including dystrophin and β‐spectrin. In the quadriceps, longitudinally oriented costameric structures tend to become thickened and branched. More strikingly, proteins of the dystrophin complex present between costameres in controls are absent from Largemyd muscles. We propose that the absence of the dystrophin complex from these regions destabilizes the sarcolemma of the Largemyd mouse and thereby contributes to the severity of its muscular dystrophy. Thus, the positioning of sarcolemmal proteins may have a profound effect on the health of skeletal muscle. Muscle Nerve, 2004

The molecular genetics of human facioscapulohumeral muscular dystrophy and the myodystrophy mouse model

Facioscapulohumeral dystrophy is an autosomal dominant muscular dystrophy, the gene for which is localized to 4q35. It appears to be caused by deletion of tandem repeats that do not contain an expressed sequence. One current hypothesis is that the deletion affects expression of a centromeric gene (not yet identified) through a position effect. The mouse mutant, myodystrophy (myd, is a candidate model for facioscapulohumeral dystrophy. Myd has a progressive muscular dystrophy and maps to a segment of mouse chromosome 8 that is syntenic with human 4q31–4q35.