Results

Of the original 300 subjects, 282 (94%) were available and willing to have blood drawn for genotyping (table 1 shows the demographic details of the patients and genotype frequencies). There were 110 men and 182 women of similar mean age. More men than women had been smokers but fewer were current smokers possibly representing a survivor bias. The different categories of spinal degenerative disease had similar prevalence in both men and women. Apophyseal arthritis was present in virtually all subjects, osteophytes were present in approximately 70% with more than half showing some evidence of disc narrowing. The vitamin D receptor allele frequencies of “T” 63% and “t” 37% were comparable in men and women and similar to previously reported frequencies.

Figure 1 gives the prevalence of the various categories of spinal degenerative disease by presence, severity partition, and genotype. In general, osteophytosis was progressively more common and severe in the “tt” than in the “Tt” and “TT” subjects. There was a similar relation for the presence, but not the severity, of disc degeneration. Apophyseal involvement was present in all subjects with equal prevalence and a non-statistically significant trend to decreasing severity for “TT” versus “tt” with “Tt” in between.

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Figure 1

Allelic variation in the vitamin D receptor gene and spinal degenerative disease. For most categories there is a trend for decreasing presence and severity of spinal degenerative disease. However, this was not statistically significant at any site for the unadjusted data.

Table 2 shows the univariate and multivariate associations between age, smoking, and other lifestyle factors and genotype with the various categories of spinal degenerative disease. Figures for presence of apophyseal arthritis were not included because of lack of variation. The presence of osteophytosis was predicted, in multivariate analysis, by age and smoking history while its severity was predicted by these factors as well as body mass index, quadriceps strength, and VDR genotype. Figure 2 shows the unadjusted prevalence of osteophytes (both presence and severity) by smoking category. The presence of disc narrowing was predicted by age and VDR genotype, although only age was a significant predictor of its severity. Disc narrowing and osteophyte severity scores were correlated with each other (Spearman’sr=0.31, p<0.001). Forcing the disc narrowing score into the model for osteophyte severity led to a non-significant result for genotype (p=0.14) indicating that it is not possible, in this case, to assess whether the genotype association is mediated separately by either osteophytosis or disc narrowing. The genetic associations with spinal degenerative disease were not changed by adjustment for femoral neck bone mineral density (data not shown) suggesting that these associations are not mediated by bone density.

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Figure 2

Smoking status and spinal osteophytosis. For both presence (A) and severity (B) of osteophytosis the proportion with osteophytosis is increased in smokers compared with non- smokers with past smokers having an intermediate value. Data are presented unadjusted for potential confounders and the p values represent the difference between each category compared with non-smokers after excluding the other category.

Table 3 shows bone density stratified by site, sex, and genotype. No association is evident between genotype and bone density at either site in this sample. This lack of association persisted after multivariate adjustment for potential confounders (data not shown).

Discussion

This cross sectional population-based study of elderly men and women has shown, for the first time, an association between allelic variation in the vitamin D receptor and spinal degenerative disease. Taken as a whole the data suggest a linear decrease in the risk of spinal osteophytosis (both presence and severity) and presence of disc narrowing from “tt” to “TT”. The magnitude of the genetic association is substantial with those in the “TT” group having a 50–60% reduction in risk compared with the “tt” group with heterozygotes in between. Increasing age and body mass index were associated with increased prevalence and severity of spinal degenerative disease as were smoking and quadriceps strength for osteophytosis.

The genetic and lifestyle associations in this sample were most evident for osteophytosis and less clear for disc narrowing and apophyseal osteoarthritis. This is consistent with the two previous studies reporting an association at the knee where strong associations were reported with osteophytes but not with joint space narrowing.14 ,15 However, in our sample it is probable that at least some of this discrepancy is a result of definition difficulties within the various categories of spinal degenerative disease. Intra-observer variation was good for osteophytosis but was suboptimal for the other two categories particularly apophyseal osteoarthritis (where there was also lack of variation). The overall effect of this non-differential misclassification would be to weaken the strength of the associations between the various study factors and these two categories of spinal degenerative disease. This proposition would seem consistent with our data and indicates the need for the development of more reproducible methods of assessment for these categories perhaps using morphometry or a standard atlas.

The results from this study do not allow us to separate the associations between the VDR and osteophytes or disc narrowing as the associations were not independent of each other. Biologically, it is plausible that the VDR may have a direct effect on the pathophysiology of OA through either of these categories. It may have its effect through increased bone density leading to increased local damage to articular cartilage by differential impact loading. However, this does not seem probable in this case as adjustment for bone density did not change the associations (a finding similar to the above mentioned Dutch study15). However, bone density may not reflect more subtle changes in bone quality particularly subchondral stiffness, which may be important in the pathogenesis of this disease. The VDR is expressed in both osteoblasts21 and chondrocytes,22 both of which are found in osteophytes23 suggesting a possible role in osteophyte formation or progression, or both. Our results would suggest a greater role in osteophyte progression. In addition to the VDR being expressed in chondrocytes, vitamin D can also influence proteoglycan synthesis by articular chondrocytes in vitro.24 These findings suggest that the VDR may be involved in the pathophysiology of OA in bone or cartilage, or both, at a molecular level.

The direction of the genetic effect on spinal degenerative disease in this study is in the same direction as would be expected for fracture given the effect seen for bone mass in other studies.11This finding was somewhat unexpected given the reported inverse association between osteoporosis and OA that we and others have previously published on including in this particular sample.16 ,17 ,25 However, our finding is in the same direction and of similar magnitude to that reported recently by Uitterlinden et al 15 who studied a comparable elderly group to our own. Keen et al,14 in a younger sample with early arthritis reported an association in the opposite direction. These variations between studies may be caused by a number of potential reasons in addition to the differing age groups. The first is that of population stratification. This seems unlikely in all three studies as cases and controls were selected in a nested fashion from within ethnically homogeneous cohorts. Secondly, there may be variations in gene environment interactions in the different communities resulting in variations in disease expression within the various genotypes. The other two studies do not report on gene environment interactions suggesting this may be a potential explanation. However, linkage disequilibrium is a more likely reason for the discrepancies between studies. The vitamin D receptor gene is located on chromosome 12q. Other potential candidate genes for OA that are also located in this region include type 2 collagen (COL2A1) and insulin-like growth factor type 1. In particular, COL2A1 maps within 920 kilobases from the VDR.26 The degree of linkage disequilibrium may vary between population groups9 thus, despite the biological plausibility outlined above, it remains possible that the VDR is not directly involved in the genesis of OA but is a marker of other gene(s).

The VDR was not associated with bone mass in this sample possibly because of sample size considerations where considerably larger samples are required to demonstrate an association between VDR and bone density. This lack of sample size is contributed to by the fact that 38% of the study group are male in whom association with the VDR genotype has not been reported. In the whole sample from the cohort who have been genotyped the association with BMD is weak (unpublished data) perhaps reflecting the high prevalence of spinal degenerative disease of widely varying severity in these elderly subjects with its resultant artefactual changes in bone mass particularly at the spine.17

This study has also shed new light on the relation between lifestyle factors and spinal degenerative disease. As expected, the prevalence of all types of OA increases with age and our findings are consistent with this for each of the three categories. Body mass index was associated with osteophytosis particularly severity suggesting consistent with a contributory effect of mechanical loading on the spine. Quadriceps strength was also associated with severity of osteophytosis suggesting that physical activity may also play a part in its development or progression, or both, at the spine. The associations with body mass index and quadriceps strength were independent of one another. In contrast with most other studies, smoking considerably increased the risk of osteophytosis in this sample. This was most evident for current smokers but there was also an intermediate increase in the risk for past smokers suggesting a duration effect. This association persisted and even increased after adjustment for potential confounders such as age and body mass index so it appears real. Previous studies at other sites have found conflicting results. Two American studies found a modest 20% decrease in the risk of knee OA.27 ,28However, a British study found an increased risk of OA at most sites although this was only statistically significant for Heberden’s disease and the confidence limits for the knee estimate overlapped those from the above two studies.29 Taken as a whole these findings suggest that smoking may have site specific effects on osteophytosis. Furthermore, smokers in our total cohort had lower BMD at the spine18 while the smokers in this sample had higher spinal BMD (p=0.03) consistent with an association between smoking and osteophytosis and possibly indicating that osteophytosis is more common in this random sample than in the total cohort. This evidence of selection bias was somewhat unexpected as our sample were selected by computer generated random numbers and is very similar to the whole cohort in terms of age, body mass index, smoking, and self reported OA.30 There have, however, been no other studies that have examined this question in relation to the spine so our findings need to be confirmed in other populations. Potential mechanisms for this association are not clear but our analysis suggests that it is unlikely to be mediated by changes in body weight, age or physical activity and may thus be a direct effect of smoking. The presence of additive gene environment interaction in our sample is also of clinical significance. The statistical significance of both environmental and particularly genetic factors increased after adjustment for the other. This is presumably a result of a reduction in the “noise” introduced by random variation in genetic and lifestyle factors when each is considered separately. However, the present sample did not have sufficient power to study multiplicative interaction suggesting that further studies will need to be considerably larger and consider both lifestyle and genetic factors. Furthermore, the clinical application of these results would suggest that both factors will need to be considered for more accurate assessment of risk of disease at both the individual and population level and that prevention may be better targeted to high risk groups.

In conclusion, both genetic and lifestyle factors are associated with the presence and severity of spinal degenerative disease in our elderly sample. There were site specific differences in associations at the spine that may be because of misclassification of disease status or may indicate possible environmental and genetic differences in the pathophysiology of spinal degenerative disease. Further studies are required to confirm these findings in different population samples and to further explore potential aetiological mechanisms particularly gene environment interaction.