Janis Weis' group has been mapping genetic variants that make laboratory mice prone to severe Lyme arthritis. One of these variants is described in a paper that appeared in The Journal of Clinical Investigation earlier this year. The affected gene encodes the enzyme β-glucuronidase, which carries out a critical function in the lysosome. β-glucuronidase cooperates with other degradative enzymes in the lysosome to break down glycosaminoglycans (GAGs) into their individual sugar units, which are then removed from the lysosome and reused by the cells. GAGs are long chains of specific disaccharides located on the cell surface and within the extracellular matrix. GAGs are covalently (in proteoglycans) or noncovalently attached to proteins. GAGs are always being degraded and resynthesized by cells. Blocking any of the enzymes involved in GAG breakdown causes accumulation of GAG fragments, which are potentially detrimental to health. In humans, certain mutations in the β-glucuronidase gene lead to a rare condition called Sly syndrome.
Large amounts of GAGs are found in the joints, where they serve an important mechanical function. GAGs carry a high density of negative charge due to the presence of acidic sugars such as glucuronic acid, the target of β-glucuronidase, and the sulfate groups attached to most types of GAGs. The negative charge attracts cations, which in turn
attract large numbers of water molecules. The water within GAGs acts as a cushion that allows
the joints to withstand large compressive forces.
The key to the study was having strains of mice that differed in their susceptibility to Lyme arthritis. The C3H mouse strain develops severe joint inflammation during B. burgdorferi infection. On the other hand, the B6 strain develops mild joint inflammation when infected. Weis' group had earlier narrowed the locations of the genetic variations accounting for the different susceptibilities to several distinct segments within the mouse genome. They used the traditional techniques of mouse genetics, which involved numerous matings involving the C3H and B6 strains and their progeny (see this review for details). The authors focused on one of the segments, and with help from mouse genome sequence data that became available, they found a nucleotide difference within the Gusb (β-glucuronidase) gene that changed a single amino acid in the enzyme.
The investigators found that β-glucuronidase activity was mildly reduced in the infected C3H strain relative to the B6 strain. Staining tissue sections of infected mice with Alcian blue, a dye attracted to polyanions, revealed accumulation of GAGs in the joint tissues of infected C3H mice but not infected B6 mice, lending further support to the lesion in Gusb being responsible for severe inflammation. When a functional copy of the β-glucuronidase gene was stitched into the genome of C3H mice, B. burgdorferi infection no longer caused joint inflammation.
Does the same process occur in humans with Lyme
arthritis? One hint that β-glucuronidase influences the course of Lyme
arthritis is the finding from other labs that found that the concentration of the enzyme in joint fluid is higher in patients with Lyme arthritis than it is in healthy
uninfected individuals, although how the high enzyme levels are mechanistically linked
to arthritis remains unexplained.
So how does β-glucuronidase deficiency lead to severe Lyme arthritis? One possibility raised by the authors is that GAG fragments worsen tissue inflammation by stimulating Toll-like receptors, as shown in other studies (see this paper for an example).
The findings may also tell us something about rheumatoid arthritis (RA). The B6 strain ends up with a form of RA following injection with certain autoantibodies. One of Weis' mouse crosses generated a B6 strain with its Gusb gene and flanking regions swapped for the same region of the C3H strain. This strain developed severe arthritis when injected with the same autoantibodies and when infected with B. burgdorferi. Therefore, the pathologic processes leading to Lyme arthritis and RA share common steps, at least in laboratory mice. In humans, RA patients, like those with Lyme arthritis, have high levels of β-glucuronidase levels in their joint fluid.
The search for host factors affecting the development of Lyme arthritis goes on. Weis' group continue to identify genetic variants responsible for severe Lyme arthritis.
References
Bramwell KK, Ma Y, Weis JH, Chen X, Zachary JF, Teuscher C, & Weis JJ (2014). Lysosomal β-glucuronidase regulates Lyme and rheumatoid arthritis severity. The Journal of Clinical Investigation, 124 (1), 311-320 PMID: 24334460
Bramwell KK, Teuscher C, & Weis JJ (2014). Forward genetic approaches for elucidation of novel regulators of Lyme arthritis severity. Frontiers in Cellular and Infection Microbiology, 4 PMID: 24926442
Pancewicz S, Popko J, Rutkowski R, Knaś M, Grygorczuk S, Guszczyn T, Bruczko M, Szajda S, Zajkowska J, Kondrusik M, Sierakowski S, & Zwierz K (2009). Activity of lysosomal exoglycosidases in serum and synovial fluid in patients with chronic Lyme and rheumatoid arthritis. Scandinavian Journal of Infectious Diseases, 41 (8), 584-589 PMID: 19513935
Jiang D, Liang J, Fan J, Yu S, Chen S, Luo Y, Prestwich GD, Mascarenhas MM, Garg HG, Quinn DA, Homer RJ, Goldstein DR, Bucala R, Lee PJ, Medzhitov R, & Noble PW (2005). Regulation of lung injury and repair by Toll-like receptors and hyaluronan. Nature Medicine, 11 (11), 1173-1179 PMID: 16244651