TBX4 mutations (small patella syndrome) are associated with childhood-onset pulmonary arterial hypertension

Abstract

Background: Childhood-onset pulmonary arterial hypertension (PAH) is rare and differs from adult-onset disease in clinical presentation, with often unexplained mental retardation and dysmorphic features (MR/DF). Mutations in the major PAH gene, BMPR2, were reported to cause PAH in only 10-16% of childhood-onset patients. We aimed to identify more genes associated with childhood-onset PAH.

Methods: We studied 20 consecutive cases with idiopathic or heritable PAH. In patients with accompanying MR/DF (n=6) array-comparative genomic hybridisation analysis was performed, with the aim of finding common deletion regions containing candidate genes for PAH. Three patients had overlapping deletions of 17q23.2. TBX2 and TBX4 were selected from this area as candidate genes and sequenced in all 20 children. After identifying TBX4 mutations in these children, we subsequently sequenced TBX4 in a cohort of 49 adults with PAH. Because TBX4 mutations are known to cause small patella syndrome (SPS), all patients with newly detected TBX4 mutations were screened for features of SPS. We also screened a third cohort of 23 patients with SPS for PAH.

Results: TBX4 mutations (n=3) or TBX4-containing deletions (n=3) were detected in 6 out of 20 children with PAH (30%). All living patients and two parents with TBX4 mutations appeared to have previously unrecognised SPS. In the adult PAH-cohort, one TBX4 mutation (2%) was detected. Screening in the cohort of (predominantly adult) SPS patients revealed no PAH.

Conclusions: These data indicate that TBX4 mutations are associated with childhood-onset PAH, but that the prevalence of PAH in adult TBX4 mutation carriers is low.

Keywords: Clinical genetics; Copy-number; Developmental; Molecular genetics; Pulmonary hypertension.

Figure 1

Position of deletions in 17q23.2 and mutations in TBX4. (A) Position of breakpoints in 17q23.2 deletions in three childhood-onset PAH patients compared with eight patients with 17q23.2 deletions reported in the literature. Childhood-onset PAH patient 1: 55.4–57.6 Mb; childhood-onset PAH patient 2: 56.7–58.7 Mb; childhood-onset PAH patient 3:55.6–57.6 Mb; Ballif: patients 1, 3–7: 55.4–57.6 Mb; Ballif: patient 2: 54.8–57.6 Mb; Nimmakayalu: one patient:53.7–57.5 Mb; SRO, smallest region of overlap. (B) The TBX4 protein, represented by a blue bar, with the dark part representing the T-box. The positions of TBX4 mutations in three childhood-onset PAH patients (mutations in boxes) and one adult-onset patient (p.W77R) are depicted on the lower side of the protein bar. The positions of TBX4 mutations in SPS patients from our SPS cohort (mutations in boxes, grey background if reported before in the literature), and from patients reported in the literature (no boxes) are depicted on the upper side of the protein bar. The mutation R250W was detected de novo in two unrelated SPS patients.

Figure 2

Pelvic and lower limb malformations characteristic for small patella syndrome. Radiographs and photographs showing variable expression of pelvic and lower limb malformations associated with TBX4 deletions (A) and mutations (B) in five childhood-onset PAH patients (patients 1–5) and in two mutation carrier parents (M, mother, F, father). Radiographs of the pelvis showing long femoral necks, especially in patient 1 (black arrows), axe-cut notches (black arrows in patient 4) and lack of or abnormal ossification of the ischio-pubic junction (unblackened arrows in patient 4) in all patients. x-rays of left knees showing small patellae, and x-rays and photographs of left feet, showing long toes in patient 3, and a large gap between digits I and II and relatively long second and third rays in all patients.