Type I Collagen Racemization and Isomerization and the Risk of Fracture in Postmenopausal Women: The OFELY Prospective Study
作者:Patrick Garnero、Paul Cloos、E. Sornay-Rendu、Per Qvist、Pierre D. Delmas
DOI:10.1359/jbmr.2002.17.5.826
日期:——
The Asp1211 residue of the1209AHDGGR1214 sequence of the C‐terminal cross‐linking telopeptide of type I collagen (CTX) can undergo spontaneous post‐translational modifications, namely, racemization and isomerization, which result in the formation of four isomers: the native form (α‐L) and three age‐related forms, that is, an isomerized form (β‐L), a racemized form (α‐D), and an isomerized/racemized (β‐D) form. Previous studies have suggested that changes in the pattern of type I collagen racemization/isomerization, which can be assessed in vivo by measuring the degradation products of the CTX isoforms, may be associated with alterations of bone structure. The aim of this study was to examine prospectively the value of the different urinary CTX isoforms and their related ratio in the prediction of osteoporotic fractures in 408 healthy untreated postmenopausal women aged 50‐89 years (mean, 64 years) who were part of the OFELY cohort. During a median 6.8 years follow‐up, 16 incident vertebral fractures and 55 peripheral fractures were recorded in 65 women. The baseline levels of the four CTX isoforms in women who subsequently had a fracture were compared with those of the 343 women who did not fracture. At baseline, women with fractures had increased levels of ratios of native α‐L‐CTX to age‐related isoforms (β‐L, α‐D, and β‐D) compared with controls (p < 0.01). In logistic regression analysis after adjustment for age, prevalent fractures, and physical activity, women with levels of α‐L/β‐L, α‐L/α‐D, and α‐L/β‐D‐CTX ratios in the highest quartile had a 1.5‐ to 2‐fold increased risk of fractures compared with women with levels in the three lowest quartiles with relative risk (RR) and 95% CI of 2.0 (1.2‐3.5), 1.8 (1.02‐2.7), and 1.5 (0.9‐2.7), respectively. Adjustment of α‐L/β‐L and α‐L/α‐D‐CTX ratios by the level of bone turnover assessed by serum bone alkaline phosphatase (ALP)‐ or femoral neck bone mineral density (BMD) decreased slightly the RR, which remained significant for the α‐L/β‐L‐CTX ratio (RR [95%] CI, 1.8 [1.1‐3.2] after adjustment for bone ALP, 1.8 [1.03‐3.1] after adjustment for BMD, and 1.7 [0.95‐2.9] after adjustment for both bone ALP and BMD). Women with both high α‐L/β‐L‐CTX ratio and high bone ALP had a 50% higher risk of fracture than women with either one of these two risk factors. Similarly, women with both increased CTX ratio and low femoral neck BMD (T score < −2.5) had a higher risk of fracture with an RR (95% CI) of 4.5 (2.0‐10.1). In conclusion, increased urinary ratio between native and age‐related forms of CTX, reflecting decreased degree of type I collagen racemization/isomerization, is associated with increased fracture risk independently of BMD and partly of bone turnover rate. This suggests that alterations of type I collagen isomerization/racemization that can be detected by changes in urinary CTX ratios may be associated with increased skeletal fragility.
类型I胶原蛋白(CTX)C端交联肽段的1209AHDGGR1214序列中的Asp1211残基可以经历自发的转译后修饰,即消旋化和异构化,导致四种异构体的形成:天然形式(α-L)和三种与年龄相关的形式,即异构化形式(β-L)、消旋化形式(α-D)以及异构化/消旋化形式(β-D)。先前的研究表明,类型I胶原蛋白的消旋化/异构化模式的变化可以通过测量CTX异构体的降解产物在体内进行评估,并可能与骨结构的改变相关。本研究的目的是前瞻性地检查不同尿液CTX异构体及其相关比率在预测408名年龄在50至89岁(均值64岁)的健康未治疗绝经后女性骨质疏松性骨折中的价值,这些女性是OFELY队列的一部分。在中位6.8年的随访中,共记录到65名女性发生了16例椎体骨折和55例外周骨折。随后发生骨折的女性的四种CTX异构体的基线水平与343名未发生骨折女性的基线水平进行了比较。基线时,与对照组相比,发生骨折的女性的天然α-L-CTX与年龄相关异构体(β-L、α-D和β-D)的比率水平显著增加(p < 0.01)。在调整年龄、已有骨折和身体活动后的逻辑回归分析中,处于最高四分位的女性,其α-L/β-L、α-L/α-D和α-L/β-D-CTX比率的水平相较于处于最低三个四分位的女性,骨折风险增加了1.5至2倍,相对风险(RR)及95%可信区间(CI)分别为2.0(1.2-3.5)、1.8(1.02-2.7)及1.5(0.9-2.7)。通过评估血清骨碱性磷酸酶(ALP)或股骨颈骨密度(BMD)水平来调整α-L/β-L和α-L/α-D-CTX比率稍微降低了相对风险,但对α-L/β-L-CTX比率的显著性影响仍然存在(RR [95%] CI, 1.8 [1.1-3.2]在调整骨ALP后、1.8 [1.03-3.1]在调整BMD后、以及1.7 [0.95-2.9]在同时调整骨ALP和BMD后)。同时拥有高α-L/β-L-CTX比率和高骨ALP的女性,其骨折风险比仅拥有其中一个风险因素的女性高50%。同样,具有高CTX比率和低股骨颈BMD(T评分 < −2.5)的女性,其骨折风险更高,RR(95% CI)为4.5(2.0-10.1)。总之,尿液中天然和年龄相关形式CTX之间的比率增加,反映了类型I胶原蛋白消旋化/异构化程度的降低,与骨折风险增加独立于BMD且部分与骨重建率相关。这表明,通过尿液CTX比率的变化可以检测到的类型I胶原蛋白的异构化/消旋化的改变可能与增加的骨骼脆性有关。