The etiology of idiopathic congenital talipes equinovarus: a systematic review

Congenital talipes equinovarus (CTEV) is a foot deformity characterized by hindfoot varus, forefoot (metatarsus) adductus, an augmented midfoot arch (cavus), and equinus. This pediatric malformation can be classified according to its clinical presentation. It can be secondary or syndromic when its presentation is associated with another congenital disease (20% of cases). However, it may also occur as an isolated birth defect with no other malformations (80% of cases), which introduces the concept of idiopathic CTEV (ICTEV). The etiology of CTEV is largely unknown. Secondary CTEV is usually a manifestation of distal arthrogryposis (DA), congenital myotonic dystrophy, myelomeningocele, or other congenital diseases. While the clinical presentation may be similar to the idiopathic form, secondary CTEV seems to derive from neuromuscular [1] and fetal abnormalities [2] involved in its etiopathogenesis, thus making ICTEV and syndromic CTEV rather different in clinical presentation, treatment, and proposed etiopathogenetic mechanism [3, 4].

ICTEV is one of the most common pediatric deformities. The epidemiological studies published over the last 55 years suggest a birth prevalence in the range of 0.5 to 2.0 cases/1000 live births, which results in an estimated 7–43 cases of clubfoot/year/million population, depending mainly on the birth rate [5]. The higher prevalence seems to be associated with social-demographic, genetic, and environmental risk factors, which explain its prevalence among low- and middle-income countries [5] and closed societies like the Maori population [6]. It affects males more than females [7] with a male-to-female ratio of 2:1, which is similar across different ethnic groups [8‐11]. Kruse et al. proposed a reason for the gender difference in 2008 [12], but the phenotypic variability in affected individuals is still unknown.

Several treatments have been proposed throughout the centuries, but today, the gold-standard treatment is the Ponseti method [13, 14]. In syndromic cases, current evidence supports the Ponseti method or other more invasive surgical procedures [15]. The aim of this review is to analyze the available literature to provide an update on the evidence related to ICTEV etiology.

We conducted this systematic review according to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [16]. Two medical electronic databases (PubMed and Science Direct) were searched by a single author (CE) on March 20, 2018. The research string used was “(clubfoot OR congenital talipes equinovarus OR clubfeet) AND (pathology OR embryology OR etiology OR etiopathogenesis OR genetics OR pathophysiology).” A total of n = 1590 articles were found. After excluding duplicates, n = 974 articles were selected.

The initial titles and abstracts were screened using the following inclusion criteria: studies of any level of evidence reporting clinical or preclinical results published in the last 20 years (1998–2018) and dealing with the etiology of ICTEV. Exclusion criteria were articles written in other languages or studies with a focus on secondary/syndromic CTEV, such as distal arthrogryposis, myelomeningocele, and Moebius syndrome. We also excluded all the remaining duplicates, articles dealing with other topics, with poor scientific methodology, or without an accessible abstract.

At the end of the first screening, we selected n = 76 articles that were eligible for full-text reading. After reading the full text, we ultimately selected n = 48 articles that satisfy the criteria. A PRISMA [16] flowchart of the selection and screening method is provided in Fig. 1. Reference lists from the selected papers were also screened.

The included articles [3, 4, 12, 17‐62] mainly focus on genetic research [17, 21‐23, 25, 27, 29‐31, 37‐50, 52, 54‐56, 58, 60, 61], epidemiological studies [12, 19, 20, 24, 32, 33, 35, 36, 53], MRI analysis [26, 51, 59], and histological histochemical analysis [18, 24, 28, 57] in ICTEV patients. Two previous reviews [3, 4, 62] reporting a significant analysis of the current evidence and research prospective were also included since they are part of the present evidence of the topic. The main findings of the included articles except for the two reviews were summarized (Table 1.)

ICTEV has historically been linked to several risk factors: oligohydramnios, smoking, parental age, parental education, parity, maternal anxiety or depression, alcohol use, and season of birth. A previous epidemiological study based on the difference in prevalence in different communities suggested that environmental factors have a role in pathogenesis. In 2014, a twin study done in Denmark surveyed 34,485 twins and found evidence of a role of environmental factors. The authors concluded that the presence of a genetic role in the development of the disease was not enough to explain the results. Therefore, they reported strong evidence of the presence of environmental factors to explain the statistical analysis [32]. Another study from 2013 [20] was conducted in a rural area of Turkey and showed that parental consanguineous marriage was associated with a higher risk of ICTEV. Even though the sample investigated was small, this result may support an etiology based on multiple genes and environmental factors.

Even though the role of environmental factors has been confirmed by several studies, all the proposed factors except for smoking were not significantly associated with ICTEV, which was linked to DNA oxidative damage caused by tobacco smoking [33‐36]. A meta-analysis examined 172 reports containing cumulative data for 173,687 individuals and 11,674,332 unaffected controls published from 1959 to 2010. The analysis looked at the effects of smoking during pregnancy and showed that 15,673 individuals with CTEV and maternal smoking had an OR of 1.28 (95% CI 1.10–1.47) [34].

Genes involved in the metabolism of smoke-derived products may also contribute to the development of birth defects. Therefore, N-acetylation genes including NAT1, NAT2, and other related genes were screened for association analysis. Polymorphisms in the NAT2 gene cause decreased acetylation activity and have been associated with TEV. This suggests a deficit in the biotransformation of aromatic amines and the accumulation of DNA adducts, leading to a potential toxic effect and the development of TEV [27].

Hecht et al. [27] examined the variants of the NAT2 gene in 56 ICTEV multiplex families, 57 trios with a positive family history, and 160 simplex individuals. They reported a slight decrease in the expected number of homozygotes for the normal NAT2 allele in the Hispanic simplex trios. Significantly, a slow NAT2 acetylator phenotype was detected among the ICTEV patients, suggesting that slow acetylation may be a risk factor for ICTEV.

The etiology of ICTEV remains unknown as stated in recent reviews [3, 4, 72]. Many theories have been developed, but no one has clarified the major roles in the pathogenesis of idiopathic clubfoot. Recent studies have focused on the interaction between genetics and environmental factors, showing a multifactorial identity of the disease. Today, this remains the most validated theory.

A recent paper [73] reported a genetic analysis on a spontaneous autosomal recessive mouse model of peroneal muscular atrophy (PMA). It was used to understand the underlying developmental causes of ICTEV. The PMA mutation was mapped, and several candidate genes were identified, of which LIMK1 was upregulated in mutant mice. Collison et al. also reported that in chickens, LIMK1 upregulation can cause sciatic nerve defects and a TEV phenotype [73]. Further studies should be conducted using these models.

The years of research using the candidate gene approach has provided us more knowledge on the possible pathways involved in ICTEV pathogenesis, but it has failed to find a major gene causing the disease. The literature illustrates the great heterogeneity of the genetic causes of ICTEV. The candidate approach has probably not recognized the real amount of various causative variants and has likely underestimated the phenotypical and genotypical variants. The reported studies were also done using also different technical approaches, such as genome-wide association analysis (GWAS), linkage analysis, the technique of copy number variation, and whole exome sequencing. These next-generation genetic analyses should lead future studies on ICTEV etiology. Collaborative multicenter studies involving large populations might be a necessary step to shed light on the etiology of this complex disease. ICTEV inheritance is most often considered complex, with more than 75% of all cases reporting no family history [17, 53]. Thus, a large-scale GWAS study might reveal interesting results.

The filamin B (FLNB) gene encodes a member of the filamin family. The encoded protein interacts with glycoprotein Ib alpha as part of the process of repairing vascular injuries. The platelet glycoprotein Ib complex includes glycoprotein Ib alpha and binds the actin cytoskeleton. In 2016, Yang et al. performed WES sequencing and Sanger sequencing to identify and validate disease-causing mutations in a three-generation pedigree and 53 sporadic patients with ICTEV, respectively. A c.4717G>T (p.D1573Y) mutation in the FLNB gene, which co-segregated with ICETV, was identified in the pedigree. Two additional novel missense mutations in the same gene, c.1897A>G (p.M633V) and c.2195A>G (p.Y732C), were identified in the 53 sporadic patients, thus providing evidence of the involvement of the FLNB gene in ICTEV [54].

In 2014, Zhang et al. performed a GWAS study of the DNA of 396 isolated clubfoot patients and 1000 controls of European descent. The DNA was genotyped for > 600,000 single nucleotide polymorphisms (SNPs) to identify novel genes for ICTEV. The variants selected were then replicated with an independent cohort of 370 isolated clubfoot cases and 363 controls of European descent. The study found a strong association with the disease for an intergenic SNP on chromosome 12q24.31 between NCOR2 and ZNF664 (rs7969148, OR = 0.58, p = 1.25 × 10^–5), which was significant on replication (combined OR = 0.63, p = 1.90 × 10^–7). However, additional suggestive SNPs (Hox Genes, PITX1, TBX4, FOXN3, SORCS1, and MMP7/TMEM123) in the identified pathways were not significant in the replication phase [55]. With the aid of a new animal model, next-generation studies may have the potential to identify genes underlying the phenotype and elucidate the inheritance pattern and penetrance of the disorder [3].

The available literature on the etiology of ICTEV presents major limitations in terms of great heterogeneity and lack of high-profile studies. Although many studies have focused on the genetic background of the disease, there is a lack of consensus on one or multiple targets. Recent evidence shows a major role of both genetic and environmental factors. Thus far, smoking is the major environmental factor supported by recent evidence. The etiology of ICTEV is probably multifactorial and associated with multiple gene alterations, and large multi-center studies are required to investigate them. Further large international collaborative studies using next-generation sequencing technology in ICTEV patients are strongly encouraged.