Serum levels of PIICP, PIIANP, and PIIBNP are decreased in patients with an endemic osteochondropathy, Kashin-Beck disease

Kashin-Beck disease (KBD) is an endemic, chronic, and deformative osteoarthropathy that mainly occurs in children aged 5–15 years old [1], and it was named such by the international medical community [2]. The disease is known for the formation of multi-joint hyperplasia bone changes [3]. KBD has a high prevalence in the broad diagonal belt from northern-east to southern-west in China. And the disease can also be found in Siberia and a few areas in North Korea. Most of the endemic areas are located in the cold and arid regions of warm and humid areas [4]. Although KBD has been studied over 160 years, the etiology is still unclear. There are three main etiological hypotheses, namely biogeochemical theory (mainly selenium deficiency), fusarium toxin (mainly T-2 toxin) poisoning theory of food, and drinking water organism poisoning theory.

Articular cartilage has an important role in cushioning the joints of skeleton, and it is mainly composed of type II collagen and various proteoglycans including aggrecan. Changes in the quality and quantity of type II collagen both are the direct cause of loss of their normal biomechanical properties [5], and these changes are closely related to KBD [6]. Type II collagen is synthesized by chondrocytes. The immature protein contains three extra domains: a signal peptide and N-terminal and C-terminal propeptide domains when propeptide regions upon cleavage allow mature collagen molecules to be incorporated into the extracellular matrix [7]. As C-terminal propeptide (PIICP) are released only during synthesis of the new molecules, its production is known to reflect the rate of type II collagen synthesis and the cartilage metabolism [8‐10]. IIA and IIB procollagen N-terminal propeptide (PIIANP, PIIBNP) is the N-terminal non-helical structure cleaved when type II collagen precursor forms type II collagen, which also reflects the anabolic status of articular cartilage [11, 12]. Now, with the rapid development of modern molecular biology technology, the study of products in the process of type II collagen synthesis has also advanced.

The diagnosis of KBD usually adopts the X-ray examination (mainly hand images); the deformation of the interphalangeal joint is the basic characteristic of KBD. The different degrees of shortening of the fingers, limbs, and body are the main gist in diagnosing and classifying the severity of Kashin-Beck disease [13]. And when the disease could be diagnosed, the cartilage was already damaged badly and clinical signs and symptoms appear. The currently available treatments for KBD are limited to nonspecific interventions, pharmacologic management of the symptoms, and surgical treatment for severe adult KBD [14‐16]. Non-steroidal anti-inflammatory drugs and analgesics are heavily used to alleviate patients’ pain because these agents are inexpensive. However, these agents are accompanied by a high risk of adverse events [14]. Therefore, the effective early diagnosis of KBD before the cartilage was damaged badly is an important step toward improving the management of this disease.

As mentioned above, measuring the level of some molecules related to type II collagen synthesis in the serum could reflect the degree of cartilage metabolism, and it might be helpful to understand the pathogenesis of KBD. Therefore, the aim of this study was to identify changes in the serum levels of PIICP, PIIANP, and PIIBNP in KBD patients and explore the possibility of these indicators as diagnostic biomarkers of KBD.

KBD is an endemic, chronic, degenerative osteoarthropathy, which is characterized primarily by epiphyseal cartilage and articular cartilage deep chondrocyte necrosis and secondary hyperplasia and repair changes to pathological features, leading to osteochondral dysplasia, secondary degenerative joint disease [17]. So, the early stage of KBD is manifested as osteochondrosis, and then, these changes develop continuously, ultimately progresses to OA [18]. Therefore, the research on biomarker of OA is helpful for the research of KBD.

PIICP is the product of type II collagen synthesis; type II collagen is specifically expressed in cartilage tissues, so PIICP can specifically reflect cartilage metabolism and indicate the occurrence of OA. Sugiyama et al. conducted a 4-year follow-up of 172 early OA women with knee pain, and the data of 110 patients with successful follow-up showed that there was a mild positive correlation between body mass index and baseline PIICP level. At the same time, the degree of joint space stenosis was positively correlated with baseline PIICP level. Therefore, the level of PIICP in synovial fluid can predict the imaging progress of early knee OA [19]. Kobayashi et al. [20] detected the level of PIICP in synovial fluid of OA patients and found that the level of PIICP increased in early and middle stages of OA and decreased in the late stage of OA. The reason may be due to the damage of articular cartilage in the early stage of OA is not serious, but the compensatory increase of chondrocytes and enhancement of synthetic function are very obvious, so the level of PIICP elevates. In the late stage, the cartilage matrix is severely damaged, both the chondrocytes and the synthesis ability decreased, therefore the level of PIICP also decreased. In this study, we found that the serum levels of PIICP in KBD patients were significantly lower than those in internal and external control groups. The possible reason was that KBD mostly occurred in children and adolescents; with the growing of patients and the progress of disease, the cartilage matrix damaged more seriously and the synthesis ability decreased apparently. Therefore, serum concentrations of PIICP in adult KBD patients should be lower than those in normal subjects.

Type II collagen is synthesized as procollagen. During its maturation, there is a cleavage of C-terminal (PIICP) and N-terminal (PIINP) extension propeptides, which can serve as markers of cartilage synthesis. The N-terminal propeptide exist two variants (IIA/IIB), which arise from the alternative splicing of COL2A1 gene. These two procollagen forms differ from each other in the presence of exon 2 and in the distribution of their expression. Type IIA (PIIANP) contains exon 2 and is expressed by chondroprogenitor cells, whereas type IIB (PIIBNP) is devoid of exon 2 and is expressed in differentiated chondrocytes [21‐23]. Although Sharif reported that serum PIIANP in knee OA progressors was higher than that in OA non-progressors [24], most of the previous studies have demonstrated that serum PIIANP decreased in knee OA patients compared to controls [25‐30]. PIIBNP is believed to be the only procollagen expressed during the formation of type II collagen in healthy adult human cartilage [31]. Hayashi et al. found that PIIBNP can inhibit osteoclast survival and bone resorption via signal transduction through the αVβ3 integrins [32]. They proposed that PIIBNP may play a role in vivo in protecting cartilage from osteoclast invasion and also could be a new therapeutic strategy for decreasing bone loss. In this study, we observed that the serum levels of PIIANP and PIIBNP in KBD patients were significantly lower than those in the internal and external controls, suggesting a lack of type II collagen synthesis, resulting in deficiency of cartilage repair, further leading to the rapid development of KBD.

Some limitations are present in this study. Because in rural areas, men 40–60 years of age go out to work more, the survey population therefore included a relatively small number of healthy men. In addition, the mean age of patients with KBD is larger than previous studies. Meanwhile, we also needed to increase the number of patients to be verified in the follow-up experiment.

In this study, the serum levels of PIICP, PIIANP, and PIIBNP in the KBD group were significantly lower than those in the internal and external control groups, but there was no significant difference between the internal control group and the external control group. The results preliminarily indicated that these three indicators could reflect the abnormal synthesis of type II collagen in KBD patients. And the results also suggested that these indicators might be used as potential biomarkers for the diagnosis of KBD.