A novel, in-frame KMT2B deletion in a patient with apparently isolated, generalized dystonia

Genotype-Phenotype Relations for Isolated Dystonia Genes: MDSGene Systematic Review

This comprehensive MDSGene review is devoted to 7 genes - TOR1A, THAP1, GNAL, ANO3, PRKRA, KMT2B, and HPCA - mutations in which may cause isolated dystonia. It followed MDSGene's standardized data extraction protocol and screened a total of ~1200 citations. Phenotypic and genotypic data on ~1200 patients with 254 different mutations were curated and analyzed. There were differences regarding age at onset, site of onset, and distribution of symptoms across mutation carriers in all 7 genes. Although carriers of TOR1A, THAP1, PRKRA, KMT2B, or HPCA mutations mostly showed childhood and adolescent onset, patients with GNAL and ANO3 mutations often developed first symptoms in adulthood. GNAL and KMT2B mutation carriers frequently have 1 predominant site of onset, that is, the neck (GNAL) or the lower limbs (KMT2B), whereas site of onset in DYT-TOR1A, DYT-THAP1, DYT-ANO3, DYT-PRKRA, and DYT-HPCA was broader. However, in most DYT-THAP1 and DYT-ANO3 patients, dystonia first manifested in the upper half of the body (upper limb, neck, and craniofacial/laryngeal), whereas onset in DYT-TOR1A, DYT-PRKRA and DYT-HPCA was frequently observed in an extremity, including both upper and lower ones. For ANO3, a segmental/multifocal distribution was typical, whereas TOR1A, PRKRA, KMT2B, and HPCA mutation carriers commonly developed generalized dystonia. THAP1 mutation carriers presented with focal, segmental/multifocal, or generalized dystonia in almost equal proportions. GNAL mutation carriers rarely showed generalization. This review provides a comprehensive overview of the current knowledge of hereditary isolated dystonia. The data are also available in an online database (http://www.mdsgene.org), which additionally offers descriptive summary statistics. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

KMT2B-related disorders: expansion of the phenotypic spectrum and long-term efficacy of deep brain stimulation

Heterozygous mutations in KMT2B are associated with an early-onset, progressive and often complex dystonia (DYT28). Key characteristics of typical disease include focal motor features at disease presentation, evolving through a caudocranial pattern into generalized dystonia, with prominent oromandibular, laryngeal and cervical involvement. Although KMT2B-related disease is emerging as one of the most common causes of early-onset genetic dystonia, much remains to be understood about the full spectrum of the disease. We describe a cohort of 53 patients with KMT2B mutations, with detailed delineation of their clinical phenotype and molecular genetic features. We report new disease presentations, including atypical patterns of dystonia evolution and a subgroup of patients with a non-dystonic neurodevelopmental phenotype. In addition to the previously reported systemic features, our study has identified co-morbidities, including the risk of status dystonicus, intrauterine growth retardation, and endocrinopathies. Analysis of this study cohort (n = 53) in tandem with published cases (n = 80) revealed that patients with chromosomal deletions and protein truncating variants had a significantly higher burden of systemic disease (with earlier onset of dystonia) than those with missense variants. Eighteen individuals had detailed longitudinal data available after insertion of deep brain stimulation for medically refractory dystonia. Median age at deep brain stimulation was 11.5 years (range: 4.5-37.0 years). Follow-up after deep brain stimulation ranged from 0.25 to 22 years. Significant improvement of motor function and disability (as assessed by the Burke Fahn Marsden's Dystonia Rating Scales, BFMDRS-M and BFMDRS-D) was evident at 6 months, 1 year and last follow-up (motor, P = 0.001, P = 0.004, and P = 0.012; disability, P = 0.009, P = 0.002 and P = 0.012). At 1 year post-deep brain stimulation, >50% of subjects showed BFMDRS-M and BFMDRS-D improvements of >30%. In the long-term deep brain stimulation cohort (deep brain stimulation inserted for >5 years, n = 8), improvement of >30% was maintained in 5/8 and 3/8 subjects for the BFMDRS-M and BFMDRS-D, respectively. The greatest BFMDRS-M improvements were observed for trunk (53.2%) and cervical (50.5%) dystonia, with less clinical impact on laryngeal dystonia. Improvements in gait dystonia decreased from 20.9% at 1 year to 16.2% at last assessment; no patient maintained a fully independent gait. Reduction of BFMDRS-D was maintained for swallowing (52.9%). Five patients developed mild parkinsonism following deep brain stimulation. KMT2B-related disease comprises an expanding continuum from infancy to adulthood, with early evidence of genotype-phenotype correlations. Except for laryngeal dysphonia, deep brain stimulation provides a significant improvement in quality of life and function with sustained clinical benefit depending on symptoms distribution.

Identification of a novel de novo KMT2B variant in a Greek dystonia patient via exome sequencing genotype-phenotype correlations of all published cases

Mutations in Lysine-Specific Histone Methyltransferase 2B gene (KMT2B) have been reported to be associated with isolated and complex early-onset generalized dystonia. We describe clinico-genetic features on a Greek patient with a novel de novo variant and demonstrate the phenotypic spectrum of KMT2B variants. We performed whole exome sequencing (WES), in a Greek patient with sporadic generalized dystonia. Additionally, we performed a systematic review of all published cases with KMT2B variants. The patient presented with isolated and mild generalized dystonia. We identified a novel splice site variant that was confirmed by Sanger sequencing and was not found in parents. This is the first reported KMT2B variant, in the Greek population. This case report further highlights the growing trend of identifying genetic diseases previously restricted to few cases in many different ethnic groups worldwide via exome sequencing. In the systematic review, we evaluated the mutation pathogenicity in all previously reported cases to investigate possible phenotype-genotype correlations. Greater mutation numbers in different populations will be important and mutation-specific functional studies will be essential to identify the pathogenicity of the various KMT2B variants.

Radiofrequency ablation for DYT-28 dystonia: short term follow-up of three adult cases

Mutations in the lysine methyltransferase 2B (KMT2B) gene have recently been reported to be associated with childhood‐onset generalized dystonia. There have been no studies investigating ablative treatments for the management of this disorder. Three patients underwent either a staged unilateral pallidotomy and contralateral pallidothalamic tractotomy (19‐year‐old man, 2‐year follow‐up), a unilateral pallidothalamic tractotomy (34‐year‐old man, 6‐month follow‐up) or a simultaneous unilateral pallidothalamic tractotomy and ventro‐oral thalamotomy (29‐year‐old man, 6‐month follow‐up). The average total patient score on the Burke‐Fahn‐Marsden Dystonia Rating Scale‐Movement Scale improved from 39.5 to 13.2 (66.6%) after the procedures. No significant complications were identified. Ablative treatments appear to be a promising alternative surgical option for generalized dystonia with KMT2B mutation.

Failure to thrive - an overlooked manifestation of KMT2B-related dystonia: a case presentation

Background

KMT2B-related dystonia is a recently described form of childhood onset dystonia that may improve with deep brain stimulation. Prior reports have focused on neurologic features including prominent bulbar involvement without detailing general health consequences that may result from orolingual dysfunction. We describe a family with novel KMT2B mutation with several members with failure to thrive to highlight this non-neurologic, but consequential impact of mutation in this gene.

Case presentation

We present a case of a 15-year old female who was admitted and evaluated for failure to thrive. On exam, she had severe speech dysfluency, limited ability to protrude the tongue, and generalized dystonia involving the oromandibular region, right upper and left lower extremity with left foot inversion contracture. The proband and her parents underwent whole genome sequencing. A previously undescribed variant, c.4960 T > C (p.Cys1654Arg), was identified in the KMT2B gene in the proband and mother, and this variant was subsequently confirmed in two maternal cousins, one with failure to thrive. Literature review identified frequent reports of prominent bulbar involvement but failure to thrive is rarely mentioned.

Conclusion

Failure to thrive is a common pediatric clinical condition that has consequences for growth and development. In the presence of an abnormal neurologic exam, a search for a specific underlying genetic etiology should be pursued. With this case series, we highlight an unusual potentially treatable cause of failure to thrive, reinforce the importance of precise molecular diagnosis for patients with failure to thrive and an abnormal neurologic exam, and underscore the importance of cascade screening of family members.

Update on KMT2B-Related Dystonia

PURPOSE OF REVIEW

To summarize the molecular and clinical findings of KMT2B-related dystonia (DYT-KMT2B), a newly identified genetic dystonia syndrome.

RECENT FINDINGS

Since first described in 2016, 66 different KMT2B-affecting variants, encompassing a set of frameshift, nonsense, splice-site, missense, and deletion mutations, have been reported in 76 patients. Most mutations are de novo and expected to mediate epigenetic dysregulation by inducing KMT2B haploinsufficiency. DYT-KMT2B is characterized phenotypically by limb-onset childhood dystonia that tends to spread progressively, resulting in generalized dystonia with cranio-cervical involvement. Co-occuring signs such as intellectual disability are frequently observed. Sustained response to deep brain stimulation (DBS), including restoration of independent ambulation, is seen in 93% (27/29) of patients. DYT-KMT2B is emerging as a prevalent monogenic dystonia. Childhood-onset dystonia presentations should prompt a search for KMT2B mutations, preferentially via next-generation-sequencing and genomic-array technologies, to enable specific counseling and treatment. Prospective multicenter studies are desirable to establish KMT2B mutational status as a DBS outcome predictor.

Identification of Novel KMT2B Variants in Chinese Dystonia Patients via Whole-Exome Sequencing

Background: Dystonia is a movement disorder with high clinical and genetic heterogeneity. Recently mutations in lysine-specific histone methyltransferase 2B (KMT2B) gene have been reported to be associated with early-onset progressive dystonia.

Methods: We performed whole-exome sequencings (WES) in a cohort of early-onset dystonia patients from China. Bioinformatics analysis and cosegregation testings were conducted to select candidate causal variants. The effects of identified variants were classified according to the American College of Medical Genetics and Genomics (ACMG) standards and guidelines.

Results: Three novel KMT2B variants were identified, including p.Q1359^* in patient 1, p.R1487AfsTer7 in patient 2, and p.R152W in patient 3. Among these variants, the nonsense variant p.Q1359^* and the frameshift variant p.R1487AfsTer7 showed high pathogenicity and were rated as pathogenic according to the ACMG guideline. Regarding the phenotypes of these two patients with pathogenic variants, patient 2 showed the similar presentation as reported whereas patient 1 seemly harbored the atypical presentations, including later onset age, atypical sites of onset and milder degree of dystonia.

Conclusions: We further report three dystonia patients with novel variants in KMT2B and expand the spectrums of genotype and phenotype of KMT2B.

Frequency and phenotypic spectrum of KMT2B dystonia in childhood: A single-center cohort study

BACKGROUND

Childhood-onset dystonia is often genetically determined. Recently, KMT2B variants have been recognized as an important cause of childhood-onset dystonia.

OBJECTIVE

To define the frequency of KMT2B mutations in a cohort of dystonic patients aged <18 years at onset, the associated clinical and radiological phenotype, and the natural history of disease.

METHODS

Whole-exome sequencing or customized gene panels were used to screen a cohort of 65 patients who had previously tested negative for all other known dystonia-associated genes.

RESULTS

We identified 14 patients (21.5%) carrying KMT2B variants, of which 1 was classified as a variant of unknown significance. We also identified 2 additional patients carrying pathogenic mutations in GNAO1 and ATM. Overall, we established a definitive genetic diagnosis in 23% of cases. We observed a spectrum of clinical manifestations in KMT2B variant carriers, ranging from generalized dystonia to short stature or intellectual disability alone, even within the same family. In 78.5% of cases, dystonia involved the lower limbs at onset, with later caudocranial generalization. Eight patients underwent pallidal DBS with a median decrease of Burke-Fahn-Marsden Dystonia Rating Scale-Motor score of 38.5% in the long term. We also report on 4 asymptomatic carriers, suggesting that some KMT2B mutations may be associated with incomplete disease penetrance.

CONCLUSIONS

KMT2B mutations are frequent in childhood-onset dystonia and cause a complex neurodevelopmental syndrome, often featuring growth retardation and intellectual disability as additional phenotypic features. A dramatic and long-lasting response to DBS is characteristic of DYT-KMT2B dystonia. © 2019 International Parkinson and Movement Disorder Society.

Novel mutations in KMT2B offer pathophysiological insights into childhood-onset progressive dystonia

Rapid progress has recently been made in the elucidation of the genetic basis of childhood-onset inherited generalized dystonia (IGD) due to the implementation of genomic sequencing methodologies. We identified four patients with childhood-onset IGD harboring novel disease-causing mutations in lysine-specific histone methyltransferase 2B gene (KMT2B) by whole-exome sequencing. The main focus of this paper is to gain novel pathophysiological insights through understanding the molecular consequences of these mutations. The disease course is mostly progressive, evolving from lower limbs into generalized dystonia, which could be associated with dysarthria, dysphonia, intellectual disability, orofacial dyskinesia, and sometimes distinct dysmorphic facial features. In two patients, motor performances improved after bilateral implantation of deep brain stimulation in the globus pallidus internus (GPi-DBS). Pharmacotherapy with trihexyphenidyl reduced dystonia in two patients. We discovered three novel KMT2B mutations. Our analyses revealed that the mutation in patient 1 (c.7463A > G, p.Y2488C) is localized in the highly conserved FYRC domain of KMT2B. This mutation holds the potential to alter the inter-domain FYR interactions, which could lead to KMT2B instability. The mutations in patients 2 and 3 (c.3596_3697insC, p.M1202Dfs*22; c.4229delA, p.Q1410Rfs*12) lead to predicted unstable transcripts, likely to be subject to degradation by non-sense-mediated decay. Childhood-onset progressive dystonia with orofacial involvement is one of the main clinical manifestations of KMT2B mutations. In all, 26% (18/69) of the reported cases have T2 signal alterations of the globus pallidus internus, mostly at a younger age. Anticholinergic medication and GPi-DBS are promising treatment options and shall be considered early.

Review of the phenotype of early-onset generalised progressive dystonia due to mutations in KMT2B

In 2016, two research groups independently identified microdeletions and pathogenic variants in the lysine-specific histone methyltransferase 2B gene, KMT2B in patients with early-onset progressive dystonia. KMT2B-dystonia (DYT28) is emerging as an important and frequent cause of childhood-onset progressive generalised dystonia and is estimated to potentially account for up to 10% of early-onset generalised dystonia. Herein, we review variants in KMT2B associated with dystonia, as well as the clinical phenotype, treatment and underlying disease mechanisms. Furthermore, in context of this newly identified condition, we summarise our approach to the genetic investigation of paediatric dystonia.