The tight-skin (Tsk) mutation is closely linked to B2m on mouse chromosome 2

Murine animal models of systemic sclerosis

Animal models of systemic connective tissue diseases have provided valuable insights into the causative mechanisms and the pathogenesis of these diseases, and have provided the means to test potentially useful therapeutic interventions. Although numerous animal models for systemic sclerosis (SSc) have been described, the most extensively studied are murine. One advantage of murine animal models is the large body of genetic information available for the mouse that is not available for other species. No animal model described to date reproduces precisely all manifestations of SSc. However, all animal models display tissue fibrotic changes similar to those present in SSc. The prudent interpretation of the results obtained from the study of animal models has provided substantial and valuable information about the pathogenesis of the human disease.

Association of microsatellite markers near the fibrillin 1 gene on human chromosome 15q with scleroderma in a Native American population

OBJECTIVE

To localize disease genes for scleroderma, or systemic sclerosis (SSc), in a population of Choctaw Native Americans with a high prevalence of SSc, in which there is evidence of a possible founder effect.

METHODS

A candidate gene approach was used in which microsatellite alleles on human chromosomes 15q and 2q, homologous to the murine tight skin 1 (tsk1) and tsk2 loci, respectively, were analyzed in Choctaw SSc cases and race-matched normal controls for possible disease association. Genotyping first-degree relatives of the cases identified potential disease haplotypes, and haplotype frequencies were obtained by expectation-maximization and maximum-likelihood estimation methods. Simultaneously, the ancestral origins of contemporary Choctaw SSc cases were ascertained using census and historical records.

RESULTS

A multilocus 2-cM haplotype was identified on human chromosome 15q homologous to the murine tsk1 region, which showed a significantly increased frequency in SSc cases compared with controls. This haplotype contains 2 intragenic markers for the fibrillin 1 (FBN1) gene. Genealogical studies demonstrated that the SSc cases were distantly related, and their ancestry could be traced back to 5 founding families in the mid-eighteenth century. The probability that the SSc cases share this haplotype due to familial aggregation effects alone was calculated and found to be very low. There was no evidence of any microsatellite allele disturbances on chromosome 2q in the region homologous to the tsk2 locus or the region containing the interleukin-1 family.

CONCLUSION

A 2-cM haplotype on chromosome 15q that contains FBN1 is associated with scleroderma in Choctaw Native Americans from Oklahoma. This haplotype may have been inherited from common founders about 10 generations ago and may contribute to the high prevalence of SSc that is now seen.

B-cell deficiency does not abrogate development of cutaneous hyperplasia in mice inheriting the defective fibrillin-1 gene

Tight-skin (TSK) mouse, the experimental model for scleroderma, develops cutaneous hyperplasia, cardiac hypertrophy, pulmonary emphysema and autoimmunity against scleroderma target autoantigens. The cutaneous hyperplasia is associated with the accumulation of microfibrils and elastic fibers in the middle and deep dermis. Fibrillin-1 (Fbn-1) is a major component of the 10-12 nm microfibrils found in the extracellular matrix. In this study we report the identification of a genetic marker in the Fbn-1 gene that can distinguish the mutant phenotype. TSK mice exhibit an unique polymorphism in the Fbn-1 gene. RNA analysis, PCR analysis and sequence determination of the mutant gene showed that the Fbn-1 gene polymorphism is due to intragenic duplication of a segment of the gene coding for 3.0 Kb of mRNA sequence (10 Kb of the genome). Histological analysis of skin samples from F1 progeny obtained by crossing TSK mice with JH-/-, RAG2-/- or vit/vit showed a significant correlation between the inheritance of the defective Fbn-1 gene and the development of cutaneous hyperplasia. Further, our results also show that in mice deficient in mature B cells inheriting the defective Fbn-1 gene, development of cutaneous hyperplasia is not abrogated. Thus, production of autoantibodies or the presence of mature B lymphocytes do not play an integral role in the pathogenesis of cutaneous hyperplasia.

A tandem duplication within the fibrillin 1 gene is associated with the mouse tight skin mutation

Mice carrying the Tight skin (Tsk) mutation have thickened skin and visceral fibrosis resulting from an accumulation of extracellular matrix molecules. These and other connective tissue abnormalities have made Tskl + mice models for scleroderma, hereditary emphysema, and myocardial hypertrophy. Previously we localized Tsk to mouse chromosome 2 in a region syntenic with human chromosome 15. The microfibrillar glycoprotein gene, fibrillin 1 (FBN1), on human chromosome 15q, provided a candidate for the Tsk mutation. We now demonstrate that the Tsk chromosome harbors a 30- to 40-kb genomic duplication within the Fbn1 gene that results in a larger than normal in-frame Fbn1 transcript. These findings provide hypotheses to explain some of the phenotypic characteristics of Tskl + mice and the lethality of Tsk/Tsk embryos.

Of mice and Marfan: genetic linkage analyses of the fibrillin genes, Fbn1 and Fbn2, in the mouse genome

The fibrillin genes, FBN1 and FBN2, encode large extracellular matrix glycoproteins involved in the structure and function of microfibrils. Mutations in FBN1 are found in patients with Marfan syndrome, a heritable connective tissue disease that primarily affects the cardiovascular, ocular, and skeletal systems. We extended the studies of these genes by determining their chromosomal position in the mouse genome. Restriction fragment length polymorphisms (RFLPs) between the progenitors of an interspecific backcross involving AEJ/Gn and Mus spretus mice were used to establish the segregation patterns of the murine homologs, Fbn1 and Fbn2, in the backcross progeny. The results position Fbn1 between the B2m and Illa genes on mouse Chromosome (Chr) 2 and establish its candidacy for the Tight skin (Tsk) mutation. The results position Fbn2 between the D18Mit35 and Pdgfrb loci in the central region of mouse Chr 18. Fbn2 maps near three mutations [bouncy (bc), plucked (pk), and shaker with syndactyly (sy)] and may be a candidate for the pk mutation.