Asymmetric Lower-Limb Malformations in Individuals with Homeobox PITX1 Gene Mutation

Abstract

Clubfoot is one of the most common severe musculoskeletal birth defects, with a worldwide incidence of 1 in 1000 live births. In the present study, we describe a five-generation family with asymmetric right-sided predominant idiopathic clubfoot segregating as an autosomal-dominant condition with incomplete penetrance. Other lower-limb malformations, including patellar hypoplasia, oblique talus, tibial hemimelia, developmental hip dysplasia, and preaxial polydactyly, were also present in some family members. Genome-wide linkage analysis with Affymetrix GeneChip Mapping 10K mapping data from 13 members of this family revealed a multipoint LOD(max) of 3.31 on chromosome 5q31. A single missense mutation (c.388G-->A) was identified in PITX1, a bicoid-related homeodomain transcription factor critical for hindlimb development, and segregated with disease in this family. The PITX1 E130K mutation is located in the highly conserved homeodomain and reduces the ability of PITX1 to transactivate a luciferase reporter. The PITX1 E130K mutation also suppresses wild-type PITX1 activity in a dose-dependent manner, suggesting dominant-negative effects on transcription. The propensity for right-sided involvement in tibial hemimelia and clubfoot suggests that PITX1, or pathways involving PITX1, may be involved in their etiology. Implication of a gene involved in early limb development in clubfoot pathogenesis also suggests additional pathways for future investigations of idiopathic clubfoot etiology in humans.

Update on Clubfoot: Etiology and Treatment

“…While multiple theories have been proposed to explain the etiology of clubfoot [9,23,34], it is becoming increasingly clear that genetic factors play a major role [28,30]. A recent study points toward a polygenic threshold model for clubfoot inheritance where multiple genes and environmental factors will be found to play a role [42].…”

“…Many theories have been proposed to explain the etiology of idiopathic clubfoot including vascular deficiencies [34], environmental factors, in utero positioning [23], abnormal muscle insertions [9], and genetic factors [28,30]. While it is becoming more clear that clubfoot is multifactorial in origin, genetic factors clearly play a role as suggested by the 33% concordance of identical twins and the fact that nearly 25% of all cases are familial [44].…”

“…The importance of genes involved in early limb development was recently shown by the identification of a rare mutation in the transcription factor PITX1 in a large family with idiopathic clubfoot [28]. PITX1 is the first gene implicated in clubfoot that explains the specific involvement of the foot, since PITX1 is expressed nearly exclusively in the hindlimb and is responsible for rapid evolutionary changes in pelvic morphology in lower vertebrates [58].…”

Deletions in PITX1 cause a spectrum of lower-limb malformations including mirror-image polydactyly

“…16 In humans, a missense mutation (p.E130K) in PITX1 has been shown to segregate with dominantly inherited clubfoot with reduced penetrance in a large kindred, together with unilateral tibial hemimelia and preaxial polydactyly in one individual in this family. 7 The mutation is located in the DNA-binding domain of the protein and revealed reduced transactivation in a reporter gene assay.…”

“…7 In this family penetrance was reduced and phenotype expression was variable, ranging from complete absence of congenital abnormalities to clubfoot, tibial hemimelia, and duplicated great toes in a single family member. This was the first indication that a broader spectrum of malformations might be due to PITX1 dysfunction.…”