Kabuki syndrome (KS, OMIM#147920) is a rare syndrome characterized by distinct dysmorphic facial features, postnatal growth retardation, intellectual disabilities, skeletal abnormalities, and unusual dermatoglyphic patterns. It is usually detected sporadically and has a wide spectrum of clinical manifestations¹⁾. Recently identified as a genetic syndrome via whole exome sequencing²⁾, KS has been increasingly recognized in the primary care setting, and in many cases, its underlying etiology has been discovered. KS is diagnosed and managed by pediatricians. To enhance the understanding of patients with KS among pediatricians, this review focuses on the clinical and molecular genetic characteristics of these patients.

In 1969, Japanese geneticist Norio Niikawa encountered a young patient presenting with unusual facial features and mental retardation. Whether this was a never before diagnosed condition with a genetic underpinning was unclear. Over the next several years, Niikawa treated several other children with the same symptoms in his outpatient clinic, furthering support for a new disorder¹⁾. In 1981, two independent groups in Japan separately described this disorder in the Journal of Pediatrics³⁾. Because its characteristic facial features resembled the make-up of actors in Kabuki, the traditional Japanese theater, the term 'Kabuki make-up syndrome' was coined. The most remarkable aspects of KS are the atypical facial features, which usually prompt the clinician to consider this diagnosis⁴⁾. Patients often have long palpebral fissures with eversion of the lateral one-third of the lower eyelid, arched eyebrows with sparseness in the lateral one-third, short columella with a depressed nasal tip, prominent ears, other congenital anomalies, and various health problems⁴⁾.

KS was initially thought to be specific to Japanese individuals. Its estimated prevalence in Japan is 1/32,000⁵⁾. However, there are now reports of KS in a variety of ethnic groups, including Northern European, Brazilian, Vietnamese, Filipino, East Indian, Arabic, Chinese, Mexican, and African⁴⁾. The global incidence of KS is not known. Although KS is reportedly rare in Korea, its prevalence in this country may be underestimated because of under-recognition of the diagnosis and the paucity of diagnostic tools. Considering the many reports of other populations, the frequency of KS in the Korean population presumably approximates that of the Japanese population. Recently, a multicenter study⁶⁾ confirmed that number of patients newly diagnosed with KS has increased owing to high-quality comprehensive diagnostic services beginning in 2000. A population-based study is required to accurately determine whether the incidence of KS is rising in Korea.

KS is first identified by a pediatrician after physical examination of the patient and review of his or her medical history. However, clinical diagnosis of KS by pediatricians is often challenging because the phenotype tends to evolve over time, and characteristic facial features, such as long palpebral fissures with everted lower lids, become more evident during childhood¹⁾. Currently, there are no established consensus diagnostic criteria for KS. Traditionally, five cardinal manifestations have been defined⁵⁾: a 'peculiar face' (eversion of the lower lateral eyelid, arched eyebrows with the lateral one-third dispersed or sparse, a depressed nasal tip, and prominent ears; present in all patients), skeletal anomalies (deformed spinal column with or without sagittal cleft vertebrae and brachydactyly), dermatoglyphic abnormalities, mild to moderate mental retardation, and postnatal growth deficiency. Less frequent findings include visceral abnormalities, premature breast development in girls, and susceptibility to frequent infections. We recently phenotypically characterized 12 Korean patients with KS (Fig. 1)⁷⁾. All patients showed the characteristic facial dysmorphism, including long palpebral fissures, eversion of the lateral portion of the lower eyelids, and a broad nasal root, and 92% displayed intellectual disabilities. Irrespective of racial differences, the suggested minimal criteria for diagnosis of KS are long palpebral fissures with eversion of the lateral portion of the lower eyelids, broad and arched eyebrows with lateral sparseness, short nasal columella with a depressed nasal tip, prominent or cupped ears, and developmental delay or mental retardation. Most patients with KS have several of these features that collectively produce an overall presentation suggestive of KS⁸⁾.

The genetic etiology of KS was not elucidated until 2010. At this time, Ng et al.²⁾ reported that heterozygous mutations in KMT2D (MIM #602113) are the major genetic cause of KS. Subsequent mutation screenings in KS cohorts found a KMT2D nutation in 55.8% to 80% of patients^6,9). Most mutations were private de novo mutations; familial occurrence with an autosomal dominant pattern has also been reported¹⁰⁾. Most identified mutations are truncating, likely leading to haploinsufficiency of the KMT2D protein.

Heterozygous deletions in a second gene, KDM6A (MIM #30012⁸⁾, are another cause of KS¹⁰⁾. In 2012, Lederer et al.¹¹⁾ reported the association of heterozygous whole or large partial deletion of KDM6A (observed in three patients) with a broad phenotypic spectrum, ranging from typical KS to a milder clinical presentation. A detection rate of 12.5% highlighted the usefulness of array-comparative genomic hybridization as a method for differential diagnosis in KMT2D-negative patients¹⁰⁾. In 2013, point mutations in the KDM6A coding region in KMT2D mutation-negative KS individuals were also identified as KS-related¹²⁾. In 2014, splice-site mutations in KDM6A were reported in Japanese and Korean patients^7,13). Despite the rarity of KDM6A mutations in KS patients, KDM6A sequencing should be considered in all KMT2D mutation-negative KS patients. To date, the genetic basis of KS remains unknown in 20% to 45% of patients⁹⁾, and further investigation is therefore needed. The role of histone modifications in KS also requires clarification, because the proteins encoded by both KMT2D and KDM6A are histone modifiers.

To date, 12 Korean patients with suspected KS in whom direct Sanger sequencing or exome sequencing was performed have been shown to express KMT2D (n=11) and KDM6A (n=1) variants (Table 1). All mutations were de novo, and each patient had a unique mutation. The 11 KMT2D mutations included five small insertions or deletions causing frameshifts, four nonsense mutations, and two missense mutations (Fig. 2). Ten (90.9%) were novel, and nine (81.8%) resulted in a truncated or shortened protein product. The mutations identified were relatively evenly distributed across exons 14 to 46 of the KMT2D gene, without a definite hot spot. However, four mutations were located in exon 39, which contains several regions that suggest the presence of a mutational hot spot (i.e., that encode long polyglutamine tracts)⁷⁾.

The high detection rate of KMT2D and KDM6A mutations (92.3 %) is expected owing to the strict criteria used to establish a clinical diagnosis⁶⁾. Therefore, exhaustive mutation analysis of KMT2D and KDM6A may be a useful first step toward early diagnosis and counseling of Korean patients with KS. About 140 different mutations in the KMT2D gene have been reported in patients with KS (http://www.hgmd.org). Of these, 54 are nonsense mutations, 39 are small deletion mutations, 20 are missense mutations, 13 are small insertion mutations, nine are splicing mutations, three are small indel mutations, one is a gross deletion mutation, and one is a complex rearrangement. Including the 10 novel mutations reported in Korea, there are 57 nonsense, 42 small deletions, 22 missense, and 15 small insertions mutations (Fig. 3).

KMT2D was identified and characterized in 1997. It encodes a histone H3 lysine 4 (H3K4)-specific methyltransferase needed for H3K4 di- and trimethylation, a hallmark of an active transcription state¹⁴⁾. H3K4 methyltransferases act in stable multiprotein complexes that contain shared components as well as distinct components that denote the specific function of the complex¹⁰⁾. KMT2D proteins contain a catalytic domain termed the SET domain, as do other histone methyltransferases, which are evolutionarily homologous to the Drosophila trithorax protein. The Su(var)3-9 and enhancer of zeste (SET) domains are also highly conserved in evolution¹⁵⁾. The KMT2D complex ensures the transcription of constitutively expressed genes via H3K4 trimethylation. As part of the ASC-2 complex (ASCOM), KMT2D facilitates its binding to target genes¹⁶⁾. Actions of ASCOM include the removal of repressive epigenetic marks, polycomb displacement, and positioning of activating methylation marks¹⁷⁾.

KDM6A encodes a histone H3 lysine-27 (H3K27) demethylase¹⁴⁾. It is important for general chromatin remodeling and interacts with KMT2D in a conserved SET-1-like complex that trimethylates H3K4. KDM6A and KMT2D interact with each other and are both trithorax-like proteins¹⁸⁾. KMT2D methylates H3K4, and KMD6A demethylates H3K27, which activates transcription; conversely loss of KMT2D or KDM6A function represses transcription¹⁹⁾. Little is known of the genes transcriptionally regulated by KMT2D complexes during development. Determination of the underlying pathogenesis of KS, including the functions of the proteins in these complexes during cell differentiation and growth is required¹⁰⁾.

Suggested evaluation and management procedures for patients with KS according to organ system are summarized in review articles^5,9).

Genetic changes causing KS have been recently discovered via exome sequencing, resulting in improved clinical diagnosis and more accurate genotype-phenotype correlations. However, correlations between mutation type and the phenotype of KS patients have yet to be definitively established. In a screen of 81 KS patients with or without KMT2D or KTM6A mutations, Miyake et al.²⁸⁾ observed the following. First, cleft lip/palate was more frequent in the mutation-positive group. Second, developmental delay and intellectual disabilities were detected in all patients with mutations but only in some patients without mutations. Third, blue sclera, lower lip pits, spine/rib abnormalities, hip joint dislocation, umbilical hernia, kidney abnormalities, cryptorchidism, liver/spleen abnormalities, premature thelarche, neonatal hyperbilirubinemia, and anemia were observed only in the mutation-positive group. Fourth, high arched eyebrows, short fifth fingers, and hypotonia in infancy were more common in individuals with KMT2D mutations compared with KDM6A mutations, whereas the opposite was true for short stature. Fifth, patients with truncating mutations more frequently had prominent ears and hypotonia and a more atypical facial appearance than patients without truncating-type mutations.

Patients with KDM6A mutations display a variety of phenotypes, ranging from typical KS to a milder clinical presentation^11,12). Banka et al.¹³⁾ noted that patients with KDM6A mutations have hypotonia and feeding difficulties during infancy and poor postnatal growth and short stature. They suggested that hypertrichosis, long halluces, and large central incisors may be useful indicators of an underlying KDM6A mutation, even if facial dysmorphism is variable. Developmental delay and learning disabilities are generally moderate to severe in boys but mild to moderate in girls with KDM6A mutations¹³⁾. We recently described a female patient with a KDM6A mutation who presented with typical facial dysmorphism, including long palpebral fissures, lateral eversion of the lower eyelid, short nasal septum, severe mental retardation, profound short stature, cardiac abnormality, recurrent infection, and Chiari malformation⁷⁾.

As KS is a multisystem disorder, people with KS may require various diagnostic and screening tests, assessments, referrals, and multidisciplinary interventions at different stages of their lives. Early diagnosis of the disease is crucial in regard to patient management of his or her associated medical problems and for prompt family and genetic counseling, and serves as a starting point for therapeutic interventions. Most reported KMT2D and KDM6A mutations are de novo mutations found in sporadic cases. In a few cases, an affected person is believed to have inherited the mutation from an affected parent. KS caused by mutations in the KMT2D gene is inherited in an autosomal dominant manner, whereas KS caused by mutations in the KDM6A gene is inherited in an X-linked inheritance manner. The KDM6A gene is located on the X chromosome, which is one of the two sex chromosomes. In women, a mutation in one of the two copies of the gene in each cell is sufficient to cause KS. In men, a mutation in the only copy of the gene causes the disorder. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Recently, a genetically engineered mouse model of KS with a heterozygous deletion in the gene encoding the lysine-specific methyltransferase 2D (Kmt2d) was developed. These mice (termed Kmt2d+/bGeo) have impaired methyltransferase function³⁷⁾. In vitro reporter alleles demonstrated reduced acetylation of histone H4 and histone H3 lysine K4 trimethylation (H3K4me3) in fibroblasts derived from Kmt2d+/bGeo embryos. Oral administration of the histone deacetylase inhibitor AR-42 to either young (1-2 months old) or adult (5-6 months old) Kmt2d+/bGeo mice eliminated the structural and functional deficits in the dentate gyrus in association with restoration of H3K4 trimethylation³⁷⁾. H3K4me3 deficiency in the dentate gyrus granule cell layer of Kmt2d+/bGeo mice correlated with reduced neurogenesis and hippocampal memory defects in an in vivo analysis, suggesting that KS could be treated by modulating the epigenetic machinery.

Considering the increasing number of patients newly diagnosed with KS owing to advanced comprehensive diagnostic services, clinicians need to be aware of KS and to look for a combination of abnormal medical conditions in patients with KS to prevent further complications. A greater understanding of patients with KS will allow physicians to better provide primary care to the patients and their families. It will also facilitate the detection of KS mutations, which depends on the accuracy of clinical diagnosis and the inclusion or exclusion of atypical cases. A population-based study is needed to accurately estimate the incidence of pediatric KS in Korea.