No major contribution of the TGFBR1‐ and TGFBR2‐mediated pathway to Kabuki syndrome

Kabuki syndrome (KS, OMIM 147920), also known as Kabuki make-up syndrome or Niikawa–Kuroki syndrome, is a multiple congenital anomaly/mental retardation syndrome of hitherto unknown cause [Wessels et al., 2002; Matsumoto and Niikawa, 2003]. The prevalence in the Japanese population has been estimated to be 1/32,000, and 1/86,000 in Australia and New Zealand [Adam and Hudgins, 2005]. Its complex phenotype combines characteristic facial features with dermatoglyphic, skeletal, and visceral anomalies, postnatal growth retardation and mental retardation. The single most striking feature that prompts clinical diagnosis is the particular facial appearance. Several lines of evidence point to a genetic etiology of KS, with a presumably autosomal dominant mode of inheritance [Adam and Hudgins, 2005; Armstrong et al., 2005]. Until recently, although numerous cytogenetic anomalies were identified in KS, no single abnormality was found repeatedly [Adam and Hudgins, 2005]. A number of studies [Hoffman et al., 2005; Sanlaville et al., 2005; Schoumans et al., 2005; Turner et al., 2005] questioned the potential role of a 8p microduplication, initially described in 6 unrelated patients with KS [Milunsky and Huang, 2003]. Candidate gene approaches have as yet not been undertaken to challenge the genetic etiology of KS. Based on the following reasoning, we hypothesized that the transforming growth factor b receptor (TGFBR)1 and TGFBR2 pathway might explain some of the phenotypic complexity observed in KS. TGFBR1 and TGFBR2 mutations have recently been shown to cause a disorder related to Marfan syndrome (Loeys-Dietz aneurysm syndrome or LDAS), featuring characteristic craniofacial, cardiovascular, skeletal anomalies, and developmental delay [Loeys et al., 2005]. Some of the key features of this new syndrome are also frequently present in KS patients, notably skeletal anomalies such as joint laxity and scoliosis, cleft palate and abnormal dentition, blue sclerae, heart anomalies, and finally developmental delay. In support of our reasoning, it was suggested that the interferon regulatory factor (IRF)-6 pathway could be implicated in KS [Armstrong et al., 2005]. The rationale behind this was that some overlapping features exist between KS and disorders caused by IRF-6 mutations, such as the Van der Woude syndrome (VWS) and the popliteal pterygium syndrome (PPS) [Kondo et al., 2002]. All IRFs including IRF-6 have a domain that interacts with obligatory components of the TGFBR-induced response named Smads [Moustakas and Heldin, 2003]. This so called Smad-IRF (SMIR) domain is a mutational hotspot in VWS and PPS [Kondo et al., 2002]. Therefore, biochemical and genetic interactions link the TGFBR1/TGFBR2with the IRF-6 pathway, such that impaired TGFBR-mediated signaling could also result in reduced IRF-6 function [Qing et al., 2004]. Since typical LDAS malformations such as aorta aneurysms are rare ( 1/62) in KS patients [Niikawa et al., 1988], we expected a different mutational mechanism, for instance partial loss of function mutations, insertions or deletions in TGFBR1/2. Based on these grounds, we screened 14 typical Kabuki patients for mutations in the coding region and adjacent splice sites, and for intragenic insertions