Familial isolated clubfoot is associated with recurrent chromosome 17q23.1q23.2 microduplications containing TBX4

Abstract

Clubfoot is a common musculoskeletal birth defect for which few causative genes have been identified. To identify the genes responsible for isolated clubfoot, we screened for genomic copy-number variants with the Affymetrix Genome-wide Human SNP Array 6.0. A recurrent chromosome 17q23.1q23.2 microduplication was identified in 3 of 66 probands with familial isolated clubfoot. The chromosome 17q23.1q23.2 microduplication segregated with autosomal-dominant clubfoot in all three families but with reduced penetrance. Mild short stature was common and one female had developmental hip dysplasia. Subtle skeletal abnormalities consisted of broad and shortened metatarsals and calcanei, small distal tibial epiphyses, and thickened ischia. Several skeletal features were opposite to those described in the reciprocal chromosome 17q23.1q23.2 microdeletion syndrome associated with developmental delay and cardiac and limb abnormalities. Of note, during our study, we also identified a microdeletion at the locus in a sibling pair with isolated clubfoot. The chromosome 17q23.1q23.2 region contains the T-box transcription factor TBX4, a likely target of the bicoid-related transcription factor PITX1 previously implicated in clubfoot etiology. Our result suggests that this chromosome 17q23.1q23.2 microduplication is a relatively common cause of familial isolated clubfoot and provides strong evidence linking clubfoot etiology to abnormal early limb development.

Figure 1

Clinical Findings in Patients with Chromosome 17q23.1q23.2 Microduplications (A) Chromosome 17q21.1q23.2 region showing 2.2 Mb duplication at chromosome 17: 55457520–57693617 (hg18 build of the UCSC genome browser) with increased log₂ ratio and copy-number state in two patients (5103-01 and 5377-01) involving 1119 and 1120 markers. Seventeen RefSeq genes are located within the interval, including TBX2 and TBX4. Paired segmental duplications flank the duplicated region (shown in orange). (B and C) Untreated bilateral clubfoot: patient 5103-01 in (B) and patient 5377-01 in (C). (D) Limb hypoplasia in proband with bilateral clubfoot (5103-01) and hypoplasia of the right leg in his affected father with unilateral right-sided clubfoot (5103-03). Both had been treated surgically after failing conservative therapy. (E) Broad, short, overlapping toes with nail hypoplasia in proband (5103-01).

Figure 2

Pedigrees of Four Isolated Clubfoot Families with Chromosome 17q23.1q23.2 CNVs Microduplications segregate with clubfoot with incomplete penetrance in three families (5377, 5103, and 5035), and microdeletions are present in two siblings with clubfoot (5046). Black affection status indicates clubfoot, and gray indicates developmental dysplasia of the hip. Dup indicates duplication, del indicates deletion, and WT indicates normal copy number. The twins in family 5035 are not identical.

Figure 3

Skeletal Abnormalities Associated with Chromosome 17q23.1q23.2 CNVs in Individuals with Isolated Familial Clubfoot All radiographic images are of patients with chromosome 17q23.1q23.2 microduplication except the image in (F), which is of an individual with a microdeletion. (A) Large distal fibular head (lateral malleolus) (arrowhead) and shortened calcaneus (arrow) in proband's father (5103-03) imaged at age 50. (B) Normal patella in proband (5103-01) at 7 years of age. (C) Short, broad first metacarpal (arrow) and triangular, tufted first phalanx (arrowhead) in foot of a proband (5377-01) at 3 years of age. (D) Bilateral hypoplastic lateral distal tibial epiphysis in proband (5103-01) at 7 years of age. (E) Pelvis of proband's father (5103-03) showing thickened ischium (white arrow), infra-acetabular axe-cut notches (black arrow), absent normal iliac flare with moth-eaten appearance (arrowhead), and tall, narrow ilium (black line). (F) Comparison image of pelvis of proband with microdeletion (5046-01) at 9 months of age showing hypoplastic ischium (white arrow), small infra-acetabular axe-cut notches (black arrow), and short, wide iliac bone (black line).