Clinical findings in patients with RAP 1 A / RAP 1 B mutations Symptom Frequency in KSA RAP 1 A patient RAP 1

3 5 8 5 jci.org Volume 125 Number 9 September 2015 Introduction De novo dominant germline mutations in lysine (K)–specific methyltransferase 2D (KMT2D; formerly MLL2, ALR; ref. 1) or lysine (K)–specific demethylase 6A (KDM6A) cause Kabuki syndrome (KS; MIM 147920 and 300867; refs. 2–4), a developmental disorder characterized by a recognizable facial gestalt, intellectual disability, and short stature as well as cardiac, renal, and skeletal malformations (5). KMT2D mutations are the major cause of KS, present in up to 75% of patients (6), while KDM6A mutations account for roughly 5% of cases (7). Consequently, for the residual 20% of patients, the genetic cause of the disorder remains unresolved. KMT2D encodes a methyltransferase of the trithorax group, responsible for histone 3 lysine 4 (H3K4) diand trimethylation (8), which is a hallmark of active transcription states that counteract the influence of the repressive polycomb group proteins (9). KMT2D interacts closely with several proteins, building a multiprotein complex that also includes the proteins RBBP5 and KDM6A (10, 11). KDM6A is a histone 3 lysine 27 (H3K27) demethylase, responsible for polycomb mark removal (12), a crucial step in the complex function of the KMT2D-containing complex, also known as ASCOM. The majority of mutations found in KMT2D and KDM6A in patients with KS are heterozygous truncating mutations that completely abolish enzyme activity (4). It has been shown that truncating mutations in KMT2D lead to nonsense-mediated mRNA decay (NMD) and significantly reduced KMT2D protein levels (13). In the case of KDM6A, whole-gene deletions have been described. Hence, mutations in the known KS genes most likely result in haploinsufficiency. Although altered The genetic disorder Kabuki syndrome (KS) is characterized by developmental delay and congenital anomalies. Dominant mutations in the chromatin regulators lysine (K)–specific methyltransferase 2D (KMT2D) (also known as MLL2) and lysine (K)–specific demethylase 6A (KDM6A) underlie the majority of cases. Although the functions of these chromatin-modifying proteins have been studied extensively, the physiological systems regulated by them are largely unknown. Using wholeexome sequencing, we identified a mutation in RAP1A that was converted to homozygosity as the result of uniparental isodisomy (UPD) in a patient with KS and a de novo, dominant mutation in RAP1B in a second individual with a KS-like phenotype. We elucidated a genetic and functional interaction between the respective KS-associated genes and their products in zebrafish models and patient cell lines. Specifically, we determined that dysfunction of known KS genes and the genes identified in this study results in aberrant MEK/ERK signaling as well as disruption of F-actin polymerization and cell intercalation. Moreover, these phenotypes could be rescued in zebrafish models by rebalancing MEK/ERK signaling via administration of small molecule inhibitors of MEK. Taken together, our studies suggest that the KS pathophysiology overlaps with the RASopathies and provide a potential direction for treatment design. RAP1-mediated MEK/ERK pathway defects in Kabuki syndrome