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

KBG Syndrome in 16 Indian Individuals

We aimed to describe the clinical and genetic characteristics of 16 individuals with KBG syndrome (KBGS) from 13 Indian families. We retrospectively analyzed the clinical details of individuals with KBGS harboring a likely pathogenic/pathogenic variant in ANKRD11. We also analyzed their facial gestalt using Face2Gene and recorded the top three differential disorders suggested by the application. The most frequent clinical features observed in our cohort were as follows: learning and intellectual disability-14/15 (93%), skeletal abnormalities-14/15 (93%), postnatal short stature-13/15 (87%), brachydactyly-11/15 (73%), and characteristic facial appearance-13/15 (87%). We identified 12 single nucleotide variants (SNVs), including six recurrent and six novel variants, and a copy number variant in the 16q24.3 region encompassing ANKRD11 gene. The novel variants were as follows: p.(Gln1236Ter), p.(Asp884ThrfsTer93), p.(Arg1466GlyfsTer87), p.(Tyr2056Ter), p.(Leu955TrpfsTer22), and p.(Lys766ArgfsTer10). The identified SNVs in ANKRD11 clustered around exon 9. We observed a high concordance of Face2Gene in predicting KBGS.

Identification of a novel frameshift variation in ANKRD11: a case report of KBG syndrome

Background

KBG syndrome (KBGS, OMIM: 148050) is a rare genetic disorder characterized by macrodontia, short stature, skeletal abnormalities, and neurological manifestations. The objective of this study is to investigate a case of KBG syndrome caused by a novel frameshift mutation in ANKRD11.

Methods and results

We present the case of an 18-year-old Chinese male exhibiting characteristic features including a triangular face, micrognathia, hypertelorism, macrodontia, bushy eyebrows, prominent ears, short stature, low hairline, delayed cognitive development, and scoliosis. Whole exome sequencing identified a novel frameshift variant in the ANKRD11 gene which ultimately led to the diagnosis of KBG syndrome.

Conclusion

In this study we have identified a previously unreported frameshift variant (NM_013275.6:c.2589dup) in ANKRD11 that causes KBG syndrome. This finding expands both the molecular and clinical spectrum of this rare genetic disease.

Child with KBG syndrome

We report a male child with developmental delay, microcephaly and facial dysmorphism in the form of a turri-brachycephaly-shaped skull, triangular face, posteriorly rotated lop ears, thick bushy eyebrows, synophrys, long deep philtrum and prominent incisors. The mobile application Face2Gene was used to screen the patient's facial photographs for known syndromes. The application suggested a high likelihood of KBG syndrome. Exome sequencing confirmed this by identifying a heterozygous pathogenic known mutation p.Tyr1406Ter in the ANKRD11 gene. Thus, the diagnosis of KBG syndrome was suggested only by the Face2Gene application, and the confirmation required DNA testing such as exome sequencing analysis.

Insights into the ANKRD11 variants and short-stature phenotype through literature review and ClinVar database search

Ankyrin repeat domain containing-protein 11 (ANKRD11), a transcriptional factor predominantly localized in the cell nucleus, plays a crucial role in the expression regulation of key genes by recruiting chromatin remodelers and interacting with specific transcriptional repressors or activators during numerous biological processes. Its pathogenic variants are strongly linked to the pathogenesis and progression of multisystem disorder known as KBG syndrome. With the widespread application of high-throughput DNA sequencing technologies in clinical medicine, numerous pathogenic variants in the ANKRD11 gene have been reported. Patients with KBG syndrome usually exhibit a broad phenotypic spectrum with a variable degree of severity, even if having identical variants. In addition to distinctive dental, craniofacial and neurodevelopmental abnormalities, patients often present with skeletal anomalies, particularly postnatal short stature. The relationship between ANKRD11 variants and short stature is not well-understood, with limited knowledge regarding its occurrence rate or underlying biological mechanism involved. This review aims to provide an updated analysis of the molecular spectrum associated with ANKRD11 variants, investigate the prevalence of the short stature among patients harboring these variants, evaluate the efficacy of recombinant human growth hormone in treating children with short stature and ANKRD11 variants, and explore the biological mechanisms underlying short stature from both scientific and clinical perspectives. Our investigation indicated that frameshift and nonsense were the most frequent types in 583 pathogenic or likely pathogenic variants identified in the ANKRD11 gene. Among the 245 KBGS patients with height data, approximately 50% displayed short stature. Most patients showed a positive response to rhGH therapy, although the number of patients receiving treatment was limited. ANKRD11 deficiency potentially disrupts longitudinal bone growth by affecting the orderly differentiation of growth plate chondrocytes. Our review offers crucial insights into the association between ANKRD11 variants and short stature and provides valuable guidance for precise clinical diagnosis and treatment of patients with KBG syndrome.

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.