Genetic regulation of osteoarthritis: A QTL regulating cranial and caudal acetabular osteophyte formation in the hip joint of the dog (Canis familiaris)

Dogs, Canis familiaris, share more than 200 genetic disease phenotypes with humans [Patterson et al., 1982; Patterson, 2000]. Genes for specific diseases are often concentrated in purebred dogs (genetic isolates) due to founder effects, inbreeding and/or frequent use of ‘‘popular sires. ’’ As a result, recessive, interactive, or polygenic modes of inheritance are more readily investigated, leading to identification of Quantitative Trait Loci (QTLs) [Chase et al., 2002, 2004]. The recent completion of the sequence for the canine genome has facilitated the comparison of the dog genome with human and othermammalian genomes, allowing the further investigative analysis of canine QTLs in other mammalian systems, notably the human and mouse. Here we examine changes that involve pathological remodeling of bone, altered joint conformation, and osteophyte formation, visible radiographically as osteoarthritis (OA) [Olsson, 1971; Riser, 1973]. We have related such changes in the coxofemoral (hip) joint of Portuguese Water Dogs (PWDs) to genotypes defined by SSR markers and associated themwith a specific region (haplotype) of the canine genome (QTL). Radiographs and blood for DNA were collected from 431 PWDs through the Georgie Project [http://www.georgieproject. com, Karen Miller director; Chase et al., 2002]. The dogs, ranging in age from 1.7 to 17 years (median age, 6 years), included 171 males and 260 females and represented a crosssection of the entire PWD population in the USA. They trace their ancestry to 31 founders through ca. 25 generations and consanguinities range from 0 to 0.6 with a mean of 0.2 [Chase et al., 1999]. We have associated marker alleles with a few infrequently used founders using the consanguinity between that founder and all dogs known to carry the allele. Permutation tests are used to establish the significance of each association [Alroy et al., 2000]. DNA was isolated from blood and characterized by PCR amplification and electrophoretic identification of the alleles of simple sequence repeat geneticmarkers [Francisco et al., 1996; Mellersh et al., 2000; Chase et al., 2002, 2004]. Osteoarthritis (OA) was scored from ventrodorsal radiographs of the pelvis as illustrated in Figure 2 of Chase et al. [2002]. In all, 431 dogs were scored for subchondral sclerosis of the cranial acetabular margin, osteophytes of the caudal and cranial acetabular margins, and femoral head osteophytes (illustrated in Fig. 1). The severity of each of these phenotypes was scored on an ordinal scale from 0 (none) to 3 (most severe). Left and right hips were scored independently. Scores ranged from0 to 22 out of a possiblemaximumof 24 (12 leftþ 12 right). Fifty percent of the dogs had a score greater than 0. Methods for estimatingheritability (h), identifying theQTL and estimating its effect on the variation of specific phenotypes (R) have been described previously [Chase et al., 2004]. Estimation of QTL significance used permutations, as described in Chase et al. [2002]. In the PWD population OA is heritable (h1⁄4 30%). About half of the population show some degree of OA. OA is significantly correlated with the Norberg Angle (an indicator of joint laxity). However, whereas the Norberg Angle is significantly greater for the right than the left hip, there is no significant difference in the OA scores between the right and left joints. OA also is significantly correlated with the 4th Principal Component (PC) defined by variation in the skeletal metrics of the pelvis and limb bones. There is no significant correlation with other PCs. We had identified two QTLs on autosome 1 (CFA 1) associated with the Norberg Angle [Chase et al., 2004] and several QTLs associated with PC4 (unpublished data). In addition, we have identified two QTLs related to autoimmune Addison’s disease (unpublished data). We analyzed these QTLs (11 in all) for association with OA. One QTL identified by the SSR marker, FH2320 on CFA 3 and associated with PC4, also was significantly associated withOA (P 0.002, corrected for pedigree effects and number of QTLs tested [Chase et al., 2002]). Table I presents the relevant region of the canine genome and its syntenic counterparts on the human and mouse genomes. We have characterized the effects of this QTL in greater detail. It accounts for about 16% of the OA variation (R1⁄4 16.4%), and involves primarily cranial and caudal acetabular marginal osteophytes. This same QTL affects PC4 through its effect on the ischial tuberosity, the width of which is segregating in the PWD population (R1⁄4 7.5% for the trait residual after removing the effects of PCs 1, 2, and 3). This QTL also