Kleefstra syndrome (KLEFS) is a rare autosomal dominant disease characterized by abnormal facial appearance, language and motor delays, intellectual disability, and childhood hypotonia, and some patients may present epilepsy, gastroesophageal reflux disease, and sleep disorders (1, 2). KLEFS is classified into Kleefstra syndrome-1 (KLEFS1, OMIM: 610253) caused by terminal deletions of chromosome 9q34.3 or mutations of the Euchromatic Histone Lysine Methyltransferase 1 (EHMT1) gene (3, 4) and Kleefstra syndrome-2 (KLEFS2, OMIM: 617786) caused by mutations of the KMT2C gene (5). KLEFS1 was initially described in 1999. At present, there are more than 100 cases of KLEFS1 worldwide. Only 7 cases of KLEFS2 have been reported so far, but all lack long-term follow-up data. In the present case report, we reviewed the clinical manifestations and follow-up data of a case of KLEFS2 from 9 months to 10 years of age and analyzed the pathogenicity of frameshift mutations of the KMT2C gene, aiming to elucidate its therapeutic strategy, clinical outcome, and prognosis.

Kleefstra syndrome, caused by the haploinsufficiency of EHMT1, is characterized by intellectual disability, ASD, characteristic facial dysmorphisms, and other variable clinical features. In addition to EHMT1 mutations, de novo variants have been reported in four additional genes (MBD5, SMARCB1, NR1I3, and KMT2C). Although KLEFS1 cases have been reported, the number of KLEFS2 cases are rare and related long-term follow-up data are scant. This study reported a case of KLEFS2, providing a complete narrative of clinical features, genetic testing results, and long-term treatment and follow-up.

Koemans et al. (5–8) summarized the common features of seven patients with KLEFS2 aged 7–31 years, such as mild to severe intellectual disability, language/motor development delay, and behavioral disorders (e.g., autism spectrum disorder, ADHD, sleep problems, self-injurious behavior, and aggressive behavior). To note, three patients with KLEFS2 developed hypotonia and two developed epilepsy in childhood. Scoliosis, short stature, microcephaly, recurrent respiratory infections, phenylketonuria, hypospadias, dry skin, and hoarseness were reported in these cases as well. In the present report, the patient also developed language/motor development delay, ADHD, and dry skin. In addition, obvious feeding difficulty, persistent weight loss, and repeated eczema were not reported in previous cases. Notably, his physical development was improved to the normal level at the age of 3 years, but his intellectual development remained retarded.

In the present case, the nutritional intervention did not reverse weight loss after birth due to feeding difficulty and emaciation. Through reviewing his disease course and genetic testing data, we suggest that feeding difficulty and emaciation may be early-stage manifestations of KLEFS2, and patients with KLEFS2 may not benefit from nutritional interventions. Providing aripiprazole medication significantly increased body weight, especially the gain of abdominal fat. Nevertheless, this medication did not remarkably increase the height. Considering the close link between abdominal fat and visceral fat, the risk of long-term metabolic syndrome in KLEFS2 children should be well estimated. During the follow-up period, the liver function and the blood lipid profile of this case were normal, but the potential influence of KLEFS2 on metabolic diseases should be further evaluated.

Both KLEFS1 and KLEFS2 are caused by genetic defects involving histone methyltransferases. KMT2C, pathogenic gene of KLEFS2, is located on human chromosome 7q36. It encodes a histone methyltransferase that regulates gene transcription by altering chromatin structure. KMT2C induces transcriptional activation via mediating H3K4me1 and H3K4me3. A previous study has shown that the neuropathological mechanism of EHMT1 (KLEFS1) mutations in Drosophila is similar to those of KMT2C (KLEFS2). There is a molecular convergence between the KMT2C/D homolog TRR and the EHMT1 homolog G9a; this convergence plays a decisive role in gene regulatory networks responsible for normal neuronal development and function, especially for complex brain functions, such as learning and memory (5). In addition, four of the five potential target genes of TRR, such as REG-2, Acer, PCB, and Arc-1, are directly or indirectly implicated in the formation of memory. Among them, REG-2 and Acer can regulate circadian rhythm (9, 10). Arc is an immediate-early gene that is activated in neurons associated with learning and memory (11). Memory is a basic element of intelligence. In the present case report, the frameshift mutation in the KMT2C gene may cause an LOF effect, thus impairing his learning and memory. In addition, the intelligence of this case might be underestimated due to attention deficit, a condition arising from comprehensive developmental delay. Notably, the intelligence of this case gradually returned to normal with aging, suggesting that patients with KLEFS2 may be exempted from intellectual disability after a long term.

In this case, severe ADHD symptoms significantly influenced his sociability, and he was unable to study attentively in the classroom. Low-dose atomoxetine hydrochloride, methylphenidate hydrochloride, and aripiprazole did not remarkably alleviate ADHD symptoms, but induced adverse events, such as irascibility, appetite loss, and body weight fluctuation. It is suggested that patients with KLEFS2 may not be sensitive to ADHD medications. Impulsive behaviors and attention deficit, in this case, were significantly improved at the age of 5 years through behavioral interventions performed by his parents but aggravated again after these interventions were interrupted. Therefore, behavioral interventions should be regularly given for a long period until the patient with KLEFS2 grows up, which is challenging and requires support from the family and the society.

Dry skin has been reported in KLEFS2 cases. In addition to dry skin, the present case also suffered persistent ear eczema and an increased eosinophil count, which have never been reported in previous cases. In the present case, electroencephalography findings were normal and epileptic seizures were not detected. The patient should be further followed-up for other manifestations, such as epileptic seizure, hypotonia, and scoliosis.

During the 10-year follow-up from 9 months to 10 years in this case, his pathological growth was difficult to be intervened with and his severe ADHD symptoms significantly disrupted his development. For those who only presented developmental delay without specific facial features, KLEFS2 was easily misdiagnosed because genetic testing needs time. Therefore, genetic testing should be timely carried out for pediatric patients with early-stage feeding difficulties, weight loss, repeated eczema, distinctive facial features, and ADHD symptoms insensitive to medications.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Ethics Committee of Children's Hospital of Nanjing Medical University. Written informed consent was obtained from the participant's legal guardian for the publication of this case report. Written informed consent was obtained from the participant's legal guardian for the publication of any potentially identifiable images or data included in this article.

RL designed the study and helped in writing the manuscript. DW performed the experiments, interpreted data, and wrote the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by the Open Fund Program of Jiangsu Population Association (JSPA2019019).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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