Over the period from 2015 to 2018 in Apulia, Italy, 124,017 births were recorded and 209 cases of clubfoot were found, accounting for an incidence rate of 1.7/1,000 and a prevalence rate of 1.6/1,000. Based on collected data, the highest incidence of congenital clubfoot (1.9/1,000) occurred in 2016 (60 cases/31,681 births) and 2018 (56 cases/29,399 births). The lowest incidence rate (1.4/1,000) was observed in 2014 (43 cases/32,161 births) (Table 1).

Over the studied period, Foggia, BAT (Andria-Barletta-Trani), Bari, and Lecce recorded an increase in the incidence of clubfoot, whereas a decrease was seen in Brindisi and Taranto (Table 2). The trend of the annual prevalence of congenital clubfoot from 2015 to 2018 in the six Apulian provinces is displayed in Fig. 1.

Annual prevalence rate of congenital clubfoot in the six Apulian provinces

Six families of genes have been reported to have an etiopathogenetic role on congenital clubfoot.

The strongest evidence for the role of genetics regards the PITX1-TBX4 pathway, needed for normal hindlimb development [18, 19]. The PITX1 gene is responsible for rapid changes in pelvic morphology in lower vertebrates and it is involved in foot morphogenesis, being expressed almost exclusively in the hindlimb. In isolated clubfoot phenotypes, dominant segregating mutation in PITX1 [20], inherited microduplications of TBX [4, 21,22,23,24] and copy number variants [25] have been described.

In addition to PITX1-TBX4, the involvement of Homeobox (HOX) genes has also been reported. The HOX genes encompass four groups of genes (HOXA-D) controlling limb development throughout the axial and appendicular skeleton [26]. HOXD12 and HOXD13 single nucleotide polymorphisms (SNPs) have been associated with idiopathic clubfoot [27].

Recently, HOXC microdeletions have been shown to overlap with a noncoding region upstream of HOXC13. A missense SNP in HOXC11 in a family with an isolated form of clubfoot and a missense SNP in HOXC12 in clubfoot patients have been reported [28]. The role of insulin-like growth factor binding protein (IGFBP3), HOXD13 gene and genes regulating caspase activity have also been evaluated [3].

FSTL5 is a gene involved in embryonic and postnatal development and it is also a modulator of transforming growth factor beta and bone morphogenetic protein signaling [29].

Studies in mice have shown that FSTL5 is expressed in cartilage cells during later stages of embryonic hind limb development but its abnormalities do not correlate with overt clubfoot-like deformity. This suggests that in the pathogenesis of idiopathic talipes equinovarus, FSTL5 mutations affect other cell lines such as neural cells.

The SHOX gene is part of a large family of homeobox genes, which act during early embryonic development to control the formation of body structures being essential for the growth and maturation of long bones.

One copy of the SHOX gene is located on each sex chromosome in the pseudo-autosomal region. Microduplications of the pseudo-autosomal chromosome region Xp22.33 (Par1) containing SHOX have been found in about 1% of clubfoot patients [30].

The PMA (peroneal muscular atrophy) mouse is a good animal model for disorders like arthrogryposis multiplex congenita or congenital clubfoot deformity.

In PMA mice, the peroneal branches of the sciatic nerves are absent. The defect is related to reduced growth of sciatic nerve lateral motor column (LMC) neurons and to an upregulation of LIM-domain kinase 1 (Limk1).

Genetic analyses showed that the mutation acts in the EphA4–Limk1–Cfl1/cofilin–actin pathway [31].

Alterations in muscle fibers and a growth disorder with a disproportionate amount of type I fibers in the posterior and medial muscle groups were found, suggesting the presence of an abnormality in neural development. Involvement of the tendon sheaths of the finger flexors and posterior tibial tendons can also be found, which shows signs of cellular hypoplasia with smaller cells and less cytoplasmic volume. [9]

Newborns with idiopathic talipes equinovarus show calf muscle hypoplasia at birth, suggesting the involvement of genes related to muscle development. Accordingly, alterations of genes encoding for muscle proteins (MYH3, TPM2, TNNT3, TNNI2, and MYH8) cause congenital contractures. Other genes involved are those encoding myosin heavy chains 3 and 8 (MYH3, MYH8), troponin I and T (TNNI2, TNNT3), and tropomyosin (TPM2). [32]

Filamin B (FLNB) is a protein that binds actin in a dynamic structure. FLNB missense mutations have been associated with isolated clubfoot.

Genetic analyses revealed mutations in genes involved in various cellular processes, including proliferation, apoptosis, differentiation, and extracellular matrix formation and remodeling. Genes in the collagen family have also been linked to idiopathic congenital talipes equinovarus. Mutations in genes encoding the ECM proteins COL9A1, COL9A2, COL9A3, COMP and MATN3, as well as the transmembrane glycoprotein involved in matrix organization, SLC26A2, have been associated with clubfoot. Mutations in peroxisomal biogenesis (PEX) factors, including PEX26, are also included in the pathogenesis of clubfoot [33].