Previous studies showed that DDH is a polygenic malformation disease [4,16]. And most of the DDH gene studies are candidate gene association studies at present, but family linkage analysis can find main effect genes of DDH. Linkage analysis studies can detect variations that are potentially related to DDH through pedigree analysis, although the results require confirmation through a series of experiments using different populations and animal models. Feldman et al  found that CX3CR1 co-segregated with DDH families by linkage analysis and Li et al  verified the pathogenicity of the gene in sporadic cases subsequently, which identified a pathogenic gene systematically. The possible pathogenic genes currently identified by family linkage analysis are TENM3, HSPG2, ATP2B4 and PTGFR gene, but none of them have been validated in the sporadic DDH.
Teneurin 3 belongs to a highly conserved family of proteins and is necessary for various functions, including cell adhesion, cytoskeleton interaction, and calcium binding [17,18]. Previous studies revealed a novel mutation in TENM3 (OMIM * 610083) , which co-segregated in all severely affected members in a three-generation family from Philadelphia. This mutation also delayed development of the left acetabulum and left glenoid fossa, as observed by Alcian blue staining in 8-week-old knock-in mutant mice . Feldeman et al found that Matrix metallopeptidase 13 (MMP13，OMIM * 600108), which is related to chondrogenesis , was overexpressed in femur-derived bone cells of knock-in mice . Previous studies showed that TENM3 is related to cartilage formation and expressed in prechondrogenic mesenchymal cells.
HSPG2 (OMIM * 142461) encodes the perlecan protein, a heparan sulfate proteoglycan , which localizes in basement membranes, vascular structures, cartilage, and osteogenic tissues  and participates in cellular proliferation, differentiation, and migration . In vitro studies showed that perlecan regulates chondrocyte differentiation, which plays an important role in cartilage development . Previous studies showed that Safranin-O staining of the cartilage and expression of the typical collagen network were decreased in perlecan-deficient mice , which may have affected the occurrence of DDH. Therefore, HSPG2 may cause DDH by affecting cartilage growth.
Heterozygous variation in ATP2B4 (OMIM * 108732) was observed in a Saudi family pedigree, with HSPG2 shared by all three affected individuals in the family, as revealed by whole exome sequencing . Using in silico analysis, Basit et al found that ATP2B4 expression was regulated by HSPG2 and involved various transcription factors . Previous studies showed that ATP2B4 participated in the regulation of bone homeostasis through calcium signaling . The expression level of ATP2B4 was increased during cellular senescence in human mesenchymal stem cells; these cells show the potential to differentiate into chondrocytes, which are components of the hip [25,26]. Thus, ATP2B4 may play an important role in hip joint formation.
PTGFR (OMIM * 600563) , also known as FP, belongs to the G protein-coupled receptor family of seven transmembrane-spanning receptors. PTGFR plays an important role in chondrocyte differentiation and cartilage development [27-29]. PGF2α exerts its biological activities by binding to its receptor, PTGFR . Previous studies showed that PGF2α stimulated the expression of cartilage marker genes in a rat cell line and human articular chondrocytes [30,31]. Studies also revealed that PTGFR participates in chondrocyte hypertrophy differentiation by regulating Bmp signaling . Therefore, PTGFR may affect hip joint development as a receptor of PGF2α, which promotes cartilage formation.
In this study, we did not observe an association between these four variations and DDH; however, the association between other sites in these four genes and DDH remains unclear. The 250 sporadic cases evaluated in this study were from a Chinese Han population, indicating the possibility of a population-specific result. The relationship between these variations and DDH in other populations requires further analysis.