Genomic locus on chromosome 1 regulates susceptibility to spontaneous arthritis in mice deficiency of IL-1RA

Abstract

Background: To understand the role of genetic factors on chromosome 1 in the regulation of spontaneous arthritis in mice deficient in IL-1 receptor antagonist protein (IL_1RA), we previously used speed congenic breeding to transfer the QTL region from DBA/1(-/-) mice that are resistant to spontaneous arthritis into BALB/c(-/-) mice which are susceptible. We were able to establish two congenic strains which exhibited a delayed onset and reduced severity of disease. In this study, we asked a different set of questions. How will the QTL region from BALB/c(-/-) interact with the rest of the genome in the DBA/1(-/-) background? Will the DBA/1(-/-) mice become susceptible to spontaneous arthritis if the QTL genomic region on chromosome 1 was replaced with the genomic fragment of the same region from BALB/c(-/-)? We conducted the congenic breeding with the similar procedure as that of congenic strains with BALB/c(-/-) background.

Result: Instead of BALB/c(-/-), DBA/1(-/-) was used as the recurrent parent while BALB/c(-/-) was used as the donor parent. By the 6(th) generation we determined that all of the chromosomes in the progeny were of DBA/1(-/-) origin with the exception of the QTL portion of chromosome 1 which is heterozygous of BALB/c(-/-) and DBA/1(-/-) origin. We then intercrossed selected mice to produce homozygous strains containing the homozygous genomic region of BALB/c(-/-) on chromosome 1, while the rest of genome are homozygous DBA/1(-/-). This strain was observed for the development of spontaneous arthritis. Up to 9 weeks of age, both congenic strain and DBA/1(-/-) did not develop arthritis. However, after 9 weeks, the congenic strain started to exhibit signs of arthritis, while the DBA/1(-/-) remained free from disease.

Conclusion: The result indicates a strong influence of genetic factor(s) on the QTL of chromosome 1 on the susceptibility to spontaneous arthritis. Identification of genetic factors within this QTL region in the future will significantly enhance our understanding of molecular mechanism of spontaneous arthritis.

Figure 1

Genomic region of the QTL on chromosome 1 based three congenic strains. The top panel represents the location of initial mapping of the QTL for arthritis previously identified using the F2 generation. The numbers of the left of the vertical bar indicates the LOD score. The bottom panel is a diagrammatic representation of the transferred genomic regions (Red boxes) from DBA/1^−/− into BALB/c^−/− after the generation of 2 congenic strains. The genomic region from DBA/1 in BALB.D1-1^−/− is flanked by two markers, D1Mit55 and D1Mit209. The transferred genomic regions (Black boxes) from DBA/1^−/− into BALB/c^−/− in congenic strain BALB.D1-2^−/− is flanked by marker D1Mit359 and the distal end of the chromosome. The two red cycled white boxes indicate where the borders of the QTL are not clearly defined. The genomic region from BALB/c in D1BALB-1^−/− is flanked by markers, D1Mit400/506 at the distal end.

Figure 2

Arthritis in congenic strains DBA.B-1 ^−/− (left Average score (severity), right Incidence) relative to BALB/c ^−/− and DBA/1 ^−/− mice. BALB/c^−/− mice reached maximum levels of disease incidence and severity earlier than DBA.B-1^−/− mice, which did not reach their maximum levels until week 18. Disease severity measurements are expressed as a percentage of the maximal total score.

Figure 3

Comparison of arthritis severity in a DBA/1 ^−/− (A, B) and Congenic DBA.B-1 ^−/− mice (C, D). Panels A and C are cross section of the ankle joint. Panels B and D are higher power views to illustrate the development of an early erosion in the DBA.B-1^−/− mouse whereas the comparable area of the DBA/1^−/− mouse shows only synovitis without erosive disease.

Figure 4

Comparison of Safranin-O/Fast-Green staining of mouse articular cartilage surface of hind joints, showing the proteoglycan stained by Safranin-O (red) in cartilage matrix in a 16 week old DBA/1 ^−/− mouse (A,B) and a congenic DBA.B-1 ^−/− mouse (C,D). Arrow indicates the decreased Safranin-O staining in articular cartilage in congenic DBA.B-1^−/− mouse joint (C,D), as compared to that observed in a DBA/1^−/− mouse (A,B).

Figure 5

Production of cytokines by spleen cells from 4 month old mice. Cells were harvested from individual mice of each strain and cultured with CD3/CD28 stimulation beads. Culture supernatants were harvested after 48 hours and assayed for the cytokines indicated. Each bar represents the mean and standard deviation of 3 biologic replicates. Differences between groups were calculated by Student’s T test.