Exome Sequence Identifies RIPK4 as the Bartsocas- Papas Syndrome Locus

Abstract

Pterygium syndromes are complex congenital disorders that encompass several distinct clinical conditions characterized by multiple skin webs affecting the flexural surfaces often accompanied by craniofacial anomalies. In severe forms, such as in the autosomal-recessive Bartsocas-Papas syndrome, early lethality is common, complicating the identification of causative mutations. Using exome sequencing in a consanguineous family, we identified the homozygous mutation c.1127C>A in exon 7 of RIPK4 that resulted in the introduction of the nonsense mutation p.Ser376X into the encoded ankyrin repeat-containing kinase, a protein that is essential for keratinocyte differentiation. Subsequently, we identified a second mutation in exon 2 of RIPK4 (c.242T>A) that resulted in the missense variant p.Ile81Asn in the kinase domain of the protein. We have further demonstrated that RIPK4 is a direct transcriptional target of the protein p63, a master regulator of stratified epithelial development, which acts as a nodal point in the cascade of molecular events that prevent pterygium syndromes.

A novel RIPK4–IRF6 connection is required to prevent epithelial fusions characteristic for popliteal pterygium syndromes

“…This likely explains the intraoral and esophageal fusions and the partial attachment of the limbs to the body wall in RIPK4 − / − mice (Figures 3b, d and f). Similar to the hypoplastic external genitalia observed in humans affected by PPS, 8,9 the genital tubercle was lacking in RIPK4 − / − mice (Figures 3f and 4a). …”

“…10,11 In addition, IRF6 (interferon regulatory factor 6) and RIPK4 have been implicated in PPS. 8,9,12 We found that similar to IRF6 mutants, 10,11,13 RIPK4 − / − mice 1 have a defective skin barrier function and cleft palate, and RIPK4 − / − keratinocytes fail to differentiate in vitro. In Xenopus, we show that IRF6 controls RIPK4 expression and that RIPK4 can rescue developmental defects imposed by IRF6 dysfunction, thereby identifying a novel genetic interaction between IRF6 and RIPK4.…”

“…To address the functional properties of RIPK4 mutants identified in BPS, 8,9 we cloned four of these mutants: one introducing a frameshift and premature stop codon in the kinase domain (hRIPK4ins777a), one introducing a stop at the end of the intermediate domain (hRIPK4S376X), and two missense mutants in the kinase domain (hRIPK4I121N and hRIPK4T184I) (Figure 5f). HaCaT cells were transduced with these Flag-tagged, dox-inducible hRIPK4 constructs together with an NF-κB reporter.…”

“…The autosomal dominant form of PPS (MIM: 119500) is caused by mutations in the IRF6 gene, 12,29 whereas the autosomal-recessive form of PPS, also referred to as BPS (MIM: 263650), was linked to inactivating RIPK4 mutations. 8,9 Despite the strong resemblance of these two syndromes, 30 no molecular interconnection between IRF6 and RIPK4 was known.…”

“…RIPK4 was found to be mutated in humans affected by an autosomal-recessive form of popliteal pterygium syndrome (PPS), also known as Bartsocas-Papas syndrome (BPS). 8,9 Similar to RIPK4 − / − mice, BPS patients show epithelial fusions, skin webs at flexural surfaces, atresia of the oral cavity, syndactily and alopecia, and they die perinatally. Similar to RIPK4 − / − mice, transcription factor IRF6 − / − mice display fusion of the esophagus and defective epidermal differentiation.…”