Genetic and Phenotypic Spectrum of KBG Syndrome: A Report of 13 New Chinese Cases and a Review of the Literature

Functional investigation of a novel ANKRD11 frameshift variant identified in a Chinese family with KBG syndrome

KBG syndrome is a rare autosomal dominant condition characterized by multisystem developmental disorder, primarily caused by loss-of-function variants in ankyrin repeat domain-containing protein 11 (ANKRD11). Approximately 80 % of ANKRD11 variants associated with KBG syndrome, are frameshift and nonsense variants. Current insight into the pathogenesis of KBG syndrome resulting from ANKRD11 truncating variants remains limited. Here, we presented two members from a non-consanguineous Chinese pedigree both exhibiting characteristics fitting the KBG syndrome-associated phenotypic spectrum. Whole-exome sequencing identified a novel heterozygous frameshift variant in ANKRD11 (NM_013275.6, c.2280_2281delGT, p.Y761Qfs*20) in the proband. Sanger sequencing confirmed that the variant was inherited from her mother and co-segregated with KBG syndrome phenotype. In vitro functional assays revealed that the frameshift variant escaped nonsense-mediated mRNA decay, and resulting in a truncated protein with significantly increased expression levels compared to full-length ANKRD11. Immunofluorescence results demonstrated that truncated protein was predominantly expressed in the nucleus of HEK293 cells, while wild-type ANKRD11 was equally distributed in both the nucleus and cytoplasm. Moreover, the truncated protein significantly reduced CDKN1A/P21-promoter luciferase activity in comparison to wild-type ANKRD11 protein, as well as a remarkably decrease in the endogenous CDKN1A/P21 mRNA level in HEK293 cells. These findings suggest a loss of transcriptional activation function and potentially a dominant-negative mechanism. Overall, our study expands the mutational spectrum of ANKRD11 gene and provides new insights into the pathogenic mechanism of KBG syndrome caused by ANKRD11 truncating variants.

Epilepsy is an important feature of KBG syndrome associated with poorer developmental outcome

OBJECTIVE

The aim of this study was to describe the epilepsy phenotype in a large international cohort of patients with KBG syndrome and to study a possible genotype-phenotype correlation.

METHODS

We collected data on patients with ANKRD11 variants by contacting University Medical Centers in the Netherlands, an international network of collaborating clinicians, and study groups who previously published about KBG syndrome. All patients with a likely pathogenic or pathogenic ANKRD11 variant were included in our patient cohort and categorized into an "epilepsy group" or "non-epilepsy group". Additionally, we included previously reported patients with (likely) pathogenic ANKRD11 variants and epilepsy from the literature.

RESULTS

We included 75 patients with KBG syndrome of whom 26 had epilepsy. Those with epilepsy more often had moderate to severe intellectual disability (42.3% vs 9.1%, RR 4.6 [95% CI 1.7-13.1]). Seizure onset in patients with KBG syndrome occurred at a median age of 4 years (range 12 months - 20 years), and the majority had generalized onset seizures (57.7%) with tonic-clonic seizures being most common (23.1%). The epilepsy type was mostly classified as generalized (42.9%) or combined generalized and focal (42.9%), not fulfilling the criteria of an electroclinical syndrome diagnosis. Half of the epilepsy patients (50.0%) were seizure free on anti-seizure medication (ASM) for at least 1 year at the time of last assessment, but 26.9% of patients had drug-resistant epilepsy (failure of ≥2 ASM). No genotype-phenotype correlation could be identified for the presence of epilepsy or epilepsy characteristics.

SIGNIFICANCE

Epilepsy in KBG syndrome most often presents as a generalized or combined focal and generalized type. No distinctive epilepsy syndrome could be identified. Patients with KBG syndrome and epilepsy had a significantly poorer neurodevelopmental outcome compared with those without epilepsy. Clinicians should consider KBG syndrome as a causal etiology of epilepsy and be aware of the poorer neurodevelopmental outcome in individuals with epilepsy.

Deep phenotyping of the neuroimaging and skeletal features in KBG syndrome: a study of 53 patients and review of the literature

BACKGROUND

KBG syndrome is caused by haploinsufficiency of ANKRD11 and is characterised by macrodontia of upper central incisors, distinctive facial features, short stature, skeletal anomalies, developmental delay, brain malformations and seizures. The central nervous system (CNS) and skeletal features remain poorly defined.

METHODS

CNS and/or skeletal imaging were collected from molecularly confirmed individuals with KBG syndrome through an international network. We evaluated the original imaging and compared our results with data in the literature.

RESULTS

We identified 53 individuals, 44 with CNS and 40 with skeletal imaging. Common CNS findings included incomplete hippocampal inversion and posterior fossa malformations; these were significantly more common than previously reported (63.4% and 65.9% vs 1.1% and 24.7%, respectively). Additional features included patulous internal auditory canal, never described before in KBG syndrome, and the recurrence of ventriculomegaly, encephalic cysts, empty sella and low-lying conus medullaris. We found no correlation between these structural anomalies and epilepsy or intellectual disability. Prevalent skeletal findings comprised abnormalities of the spine including scoliosis, coccygeal anomalies and cervical ribs. Hand X-rays revealed frequent abnormalities of carpal bone morphology and maturation, including a greater delay in ossification compared with metacarpal/phalanx bones.

CONCLUSION

This cohort enabled us to describe the prevalence of very heterogeneous neuroradiological and skeletal anomalies in KBG syndrome. Knowledge of the spectrum of such anomalies will aid diagnostic accuracy, improve patient care and provide a reference for future research on the effects of ANKRD11 variants in skeletal and brain development.

Mosaicism of a novel variant in the ANKRD11 gene in a child with a mild KBG phenotype: A case report

BACKGROUND

KBG syndrome is likely underdiagnosed because of mild and non-specific features in some affected patients especially before the upper permanent central incisors eruption at about the age of 7-8 years. Somatic mosaicisms are usually recognized in the parents only after a typically affected son is diagnosed with KBG syndrome. We describe for the first time the mosaicism of a novel variant in a child with a mild KBG phenotype.

CASE SUMMARY

Our patient presented at 24 mo of age with short stature, hand abnormalities, facial dysmorphism and mild developmental delay. Pituitary hypoplasia and central hypothyroidism were also detected. By next generation sequencing (NGS) analysis we found a novel deletion in the ANKRD11 gene (c.4880_4893del.), that can be classified as likely pathogenic for the syndrome, with the percentage of mutated allele of 36%. We considered this finding as causative of the mild and non-specific phenotype for KBG syndrome in our patient, as previously reported in adults. A heterozygous variant in HESX1 gene, classified as variant of uncertain significance, but suspected of causing pituitary hypoplasia and hormonal deficiency, was also found. The patient started levothyroxine and growth hormone treatment.

CONCLUSION

The increased use of NGS analysis may expand the phenotypic spectrum of KBG syndrome because it allows genetic diagnosis of somatic mosaicisms also in children.

KBG syndrome: Clinical features and molecular findings in seven unrelated Korean families with a review of the literature

BACKGROUND

KBG syndrome is a rare genetic disorder involving macrodontia of the upper central incisors, craniofacial, skeletal, and neurologic symptoms, caused either by a heterozygous variant in ANKRD11 or deletion of 16q24.3, including ANKRD11. Diagnostic criteria were proposed in 2007 based on 50 cases, but KBG syndrome remains underdiagnosed.

METHODS

Whole exome sequencing (WES) and array comparative genomic hybridization (array CGH) were conducted for genetic analysis and patient phenotypes were characterized based on medical records.

RESULTS

Eight patients from seven unrelated families were confirmed with KBG syndrome. All patients (8/8, 100%) had some degree of craniofacial dysmorphism and developmental delay or intellectual disabilities. Triangular face, synophrys, anteverted nostril, prominent ears, long philtrum, and tented upper lip, which are typical facial dysmorphism findings in patients with KBG syndrome, were uniformly identified in the eight patients participating in this study, with co-occurrence rates of 4/8 (50%), 4/8 (50%), 4/8 (50%), 4/8 (50%), 5/8 (62.5%), and 5/8 (62.5%), respectively. Various clinical manifestations not included in the diagnostic criteria were observed. Six patients had point mutations in ANKRD11, one had an exonic deletion of ANKRD11, and one had a 16q24.3 microdeletion. According to the ACMG guidelines, all mutations were classified as pathogenic. The c.2454dup (p.Asn819fs*1) mutation in Pt 4 was reported previously. The remaining variants (c.397 + 1G>A, c.226 + 1G>A, c.2647del (p.Glu883Argfs*94), and c.4093C>T (p.Arg1365Ter)) were novel.

CONCLUSION

The clinical and molecular features of eight patients from seven unrelated Korean families with KBG syndrome described here will assist physicians in understanding this rare genetic condition.