Numerous factors, including trauma, alcohol, steroids, coagulation disorders, sickle cell disease and fat embolism, have been reported to be implicated in the destruction of the femoral head (1). Avascular necrosis of the femoral head (ANFH), as a consequence of impaired blood supply, is a common type II collagenopathy that is associated with collagen type II α1 chain (COL2A1) mutations (2,3). The primary clinical manifestations of ANFH are limping gait, discrepancy in leg length and pain on exertion, which have a substantial effect on the quality of life of individuals (4,5). Although the exact pathogenesis of ANFH remains to be elucidated, evidence obtained from idiopathic osteonecrosis in twin studies indicates that genetic factors may have a vital role in the development of ANFH (6,7). Additionally, genetic studies have demonstrated that various single nucleotide mutations in COL2A1 are associated with an increased risk of ANFH. Based on the data obtained from familial ANFH patients in three Chinese families, a study conducted by Liu et al (2) revealed that a G to A transition in exon 50 (c.3665G>A) or exon 30 (c.2306G>A) in COL2A1 led to the replacement of glycine (Gly) with serine at codon 1,170 in the Gly-X-Y domain among patients with ANFH, which did not occur in healthy individuals. Furthermore, a study conducted by Li et al (8) revealed that a novel p.Gly630Ser mutation in the COL2A1 gene may have led to ANFH and Legg-Calve-Perthes disease (LCPD) in a Chinese family.

The COL2A1 gene, which is localized at 12q13, encodes the necessary element for type II collagen, characterized by three homologous α1-peptide chains (9). Type II collagen is predominantly expressed in the hyaline cartilage, as well as the vitreous body and nucleus pulposus (10). Type II collagen is also an essential component of collagen in the connective tissue; the biomechanical strength of this tissue is primarily dependent on the amount and extent of collagen fibril crosslinking (11). Type II collagen is composed of three homologous α1 peptide chains (homotrimers) that twist together to form a triple-stranded helix. The triple helical domain contains ~300 amino acids, and Gly followed by two other amino acids (AAs) constitute the repeated (Gly-X-Y)_n sequence. If Gly in the specific Gly-X-Y repeat is replaced by other AAs, the structure of the type II collagen is destroyed (8,12). Over 200 mutations have been identified in the COL2A1 gene, and the majority of these mutations have been associated with the replacement of Gly residues in the triple helix domain (13). In recent decades, an increasing number of studies have indicated that mutations in the COL2A1 gene are associated with familial or inherited ANFH (8,14–20). However, at present, no mutation hotspot has been confirmed. The present study was performed to identify potential mutations in the COL2A1 gene that may be implicated in ANFH among a unique three-generation Chinese family, and to conduct a review of the literature.

The present study identified a heterozygous c.3508G>A mutation in the COL2A1 gene, which resulted in inherited ANFH in a large Chinese family. Unexpectedly, one family member with ANFH (II-1) had a normal genotype compared with the other affected family members; further investigation into the identification of the possible mutated genes is required. In recent decades, >200 mutations have been identified in the COL2A1 gene, including single substitution, splice-site mutations and partial deletions (2,3,8). Type II collagen is a major component of the articular cartilage, which reduces articular friction and absorbs load pressure during movement. COL2A1 mutations have been associated with various human disorders, which are collectively termed type II collagenopathies and include spondyloepiphyseal dysplasia congenita, Kniest dysplasia, Stickler dysplasia, otospondylomegaepiphyseal dysplasia and spondyloepiphyseal dysplasia with premature onset arthrosis (23–26).

The c.3508G>A transition identified in the present study resulted in the replacement of Gly by serine at codon 1,170 in the affected family members, leading to a hip disorder that manifested as inherited ANFH. This did not occur in control family members. Liu et al (2) revealed that a c.3665G>A transition (p.G1170S) or c.2306G>A transition (p.G717S) in the COL2A1 gene led to ANFH in three Chinese families. However, the differing histopathology between inherited and sporadic ANFH and the histopathological characteristics of the mutant subjects has yet to be further discussed (2). All subjects with ANFH from the three families were confirmed to have different COL2A1 mutations. However, in the present study, no similar mutations were detected in one of the family members (subject II-1), which strongly indicates that acquired factors may have an irreversible role in the pathogenesis of ANFH. Additionally, it was reported that a c.4148G>A (18) or c.1774G>A mutations (19) in the COL2A1 gene may also lead to familial ANFH. Notably, evidence from clinical studies has demonstrated that a single base substitution in the COL2A1 gene is also implicated in the development of LCPD (8,14,25) and premature hip OA, which are two other types of type II collagenopathies (16,27). However, it remains to be determined whether these diseases occur separately or simultaneously in a single family with COL2A1 mutations. Although a c.3508G>A transition was not reported to be associated with idiopathic osteonecrosis of the femoral head (iONFH) in sporadic Japanese patients, studies have indicated that a c.3508G>A mutation may be the root cause of familial Ionfh (3). However, it remains unclear whether the G>A transition (p.G1170S) is the hotspot mutation in COL2A1 that leads to inherited hip disorders in Chinese individuals (3). Hence, this requires further investigation.

Type II collagen is predominantly expressed in the matrix and chondrocytes. The morphology and density of chondrocytes vary according to the different depths of normal articular cartilage surface, which is divided into three zones: Superficial zone (10–20%), middle zone (40–60%), and deep zone (30%) (11). One histological study revealed that the deep zone in inherited ANFH dramatically increased to 70%, from 30% in normal samples, and the superficial zone dramatically decreased to 10% (17). As mature chondrocytes are primarily distributed in the superficial zone, these results indicated that the number of mature chondrocytes is significantly decreased in inherited ANFH. Furthermore, although mutated type II collagen was reported to be secreted into the matrix, the non-uniform arrangement and distribution of the pathological collagen indicated that the biomechanical characteristics of the pathological cartilage was destroyed. As endochondral ossification occurs in infant development of the femoral head, the biomechanical properties of the subchondral bone may also be markedly weakened (27). These features are notably different from sporadic ANFH, which begins with subchondral bone necrosis and does not affect the articular cartilage (28–30). No marked differences between inherited and sporadic ANFH were demonstrated in the present study. This may due to the different biopsy locations or the small sample size.

The process of type II collagen synthesis and assembly has been reported to be impaired in chondrocytes with a COL2A1 mutation (31,32). The pro-α1 peptide chain encoded by COL2A1 forms a triple-helix structure within the endoplasmic reticulum and Golgi bodies of chondrocytes, which integrates into numerous fibrin networks following secretion from the extracellular matrix (10,11). The Gly-X-Y triple-helix motif is crucial for the proper crosslinking of the pro-α1 peptide chain to form functional type II collagen. The deletion of sequences for maintaining the Gly-X-Y triple-helix domain lead to type II collagenopathies (33,34). Mortier et al reported that there are numerous excessive post-translational modifications in type II collagen in individuals carrying a Gly-substituted mutation (35). These excessively modified type II collagens prevent the protein from folding into its fundamental triple-helical domain structure (35). In addition, an increase in hydroxymethyl may cause local structural damage and a loosened super helix structure, resulting in pathological alterations (8,11). However, why this mutation only appears to affect the hip joint remains unknown and suggests that other biomechanical factors may be involved, which require further investigation.

In conclusion, the present study identified a heterozygous c.3508G>A mutation in COL2A1 that was implicated in familial ANFH, and was compatible with autosomal dominant inheritance. The present study may provide a novel molecular basis for genetic counseling and the molecular diagnosis of ANFH. Further studies are required to investigate the potential mechanism underling COL2A1 mutation-induced inherited ANFH.