The Genetic Landscape of Complex Childhood‐Onset Hyperkinetic Movement Disorders

ABSTRACT Background and Objective The objective of this study was to better delineate the genetic landscape and key clinical characteristics of complex, early‐onset, monogenic hyperkinetic movement disorders. Methods Patients were recruited from 14 international centers. Participating clinicians completed standardized proformas capturing demographic, clinical, and genetic data. Two pediatric movement disorder experts reviewed available video footage, classifying hyperkinetic movements according to published criteria. Results One hundred forty patients with pathogenic variants in 17 different genes (ADCY5, ATP1A3, DDC, DHPR, FOXG1, GCH1, GNAO1, KMT2B, MICU1, NKX2.1, PDE10A, PTPS, SGCE, SLC2A1, SLC6A3, SPR, and TH) were identified. In the majority, hyperkinetic movements were generalized (77%), with most patients (69%) manifesting combined motor semiologies. Parkinsonism‐dystonia was characteristic of primary neurotransmitter disorders (DDC, DHPR, PTPS, SLC6A3, SPR, TH); chorea predominated in ADCY5‐, ATP1A3‐, FOXG1‐, NKX2.1‐, SLC2A1‐, GNAO1‐, and PDE10A‐related disorders; and stereotypies were a prominent feature in FOXG1‐ and GNAO1‐related disease. Those with generalized hyperkinetic movements had an earlier disease onset than those with focal/segmental distribution (2.5 ± 0.3 vs. 4.7 ± 0.7 years; P = 0.007). Patients with developmental delay also presented with hyperkinetic movements earlier than those with normal neurodevelopment (1.5 ± 2.9 vs. 4.7 ± 3.8 years; P < 0.001). Effective disease‐specific therapies included dopaminergic agents for neurotransmitters disorders, ketogenic diet for glucose transporter deficiency, and deep brain stimulation for SGCE‐, KMT2B‐, and GNAO1‐related hyperkinesia. Conclusions This study highlights the complex phenotypes observed in children with genetic hyperkinetic movement disorders that can lead to diagnostic difficulty. We provide a comprehensive analysis of motor semiology to guide physicians in the genetic investigation of these patients, to facilitate early diagnosis, precision medicine treatments, and genetic counseling. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

Conclusions: This study highlights the complex phenotypes observed in children with genetic hyperkinetic movement disorders that can lead to diagnostic difficulty. We provide a comprehensive analysis of motor semiology to guide physicians in the genetic investigation of these patients, to facilitate early diagnosis, precision medicine treatments, and genetic counseling. Hyperkinetic movement disorders (HMDs) encompass a broad spectrum of complex diseases, frequently associated with motor disability. Hyperkinesia is characterized by involuntary, excessive movements or abnormal muscle activity during active movement. 1 A number of different hyperkinetic phenotypes are described, including dystonia, chorea, athetosis, myoclonus, tremor, tics, and stereotypies. Accurate terminology is essential to facilitate diagnosis, therapeutic choices, and communication between professionals.
Despite recent major advances in molecular genetics, children presenting with suspected genetic HMDs often remain a diagnostic challenge. Many of them have a combined hyperkinetic motor semiology that can be difficult to accurately categorize, even for the experienced movement disorder specialist. Associated comorbidities, such as developmental delay and other neurological or systemic features, frequently coexist, causing complex HMD phenotypes that are difficult to recognize.
The differential diagnosis is often broad, given the phenotypic overlap with hypoxic-ischemic encephalopathy, autoimmune diseases, infection, stroke, and metabolic disorders. For many complex genetic HMDs, there are currently no disease biomarkers or radiological clues to aid the diagnostic odyssey. In addition, with growing genetic heterogeneity and phenotypic pleiotropy, it has become increasingly difficult to predict genotype from clinical phenotype. Nonetheless, in this genomic era, the importance of accurate and detailed endophenotyping to assist correct interpretation of genomic variants cannot be underestimated.
In this study, we aim to better characterize the genetic landscape of complex childhood HMDs, with a focus on disorders without either blood and urine biomarkers or extensive structural abnormalities of the basal ganglia. Through this work, we provide a comprehensive analysis of motor semiology, as well as detailed information about associated key neurological and systemic features, to better guide and interpret genetic investigations, thereby facilitating accurate diagnosis, prognostication, and future genetic counseling.

Study Design
Nineteen international tertiary pediatric movement disorder centers were invited to participate in the study, of which 14 contributed patients (Supplementary Data in Appendix S1).

Ethical Approval and Consent
Appropriate ethical approvals were in place at all participating centers (Supplementary Data in Appendix S1). Written informed consent was obtained for publication of video footage.

Study Inclusion Criteria
Individual patient criteria included: (1) patient's age at motor symptom onset <18 years; (2) video footage of entire body (head, neck, trunk, and limbs) available for expert clinical review; and (3) confirmation of genetic etiology as defined by American College of Medical Genetics and Genomics guidelines. 2 For each genetic disorder, the reported literature confirms that (1) HMD is the main motor feature with either one (pure HMD phenotype) or more (combined HMD phenotype) of the following movement phenotypes: dystonia, chorea, athetosis, myoclonus, or tremor; (2) HMD is not purely paroxysmal; (3) HMD is commonly associated with other neurological, neuropsychiatric, or systemic comorbidities; (4) there are no highly predictive blood or urine biomarkers for the gene in question; and (5) there is no major basal ganglia structural abnormality (or only subtle findings) on neuroimaging for the gene in question.

Data Acquisition
Video footage was obtained during clinic appointments, as part of routine clinical care, and was considered to be representative of the HMD by the treating clinician. Only videos showing footage of the entire body were selected for review. Videos were reviewed by two child neurologists with expertise in movement disorders (B.P.-D. and M.A.K.). The observed HMD semiology was classified according to published consensus guidelines. 1 The body distribution was recorded as focal, segmental, or generalized. Other coexisting non-HMDs, such as parkinsonism and ataxia, were also recorded.
Clinical data were collected through an anonymized standardized proforma to record age at assessment, sex, ethnicity, consanguinity, other affected family members, causative gene and mutation, mode of inheritance, and gene-related phenotype. We also gathered data regarding age at HMD onset, disease evolution, paroxysmal or fluctuating HMD symptoms, triggers, other neurological or systemic features, and response to treatment. The functional impact of HMD on daily living activities (DLAs) was scored (Supplementary Data in Appendix S1).

Statistical Analysis
The SPSS v.24.0 (IBM Corp., Armonk, N.Y., USA) statistical package was used to calculate means, standard deviations, and ranges. Based on small sample size, the Mann-Whitney U test was chosen to compare continuous variables (ie, age at disease onset) between different phenotypic groups. Differences were reported as significant when P < 0.05.

Results
Retrospective data analysis was undertaken on 140 patients with complex genetic HMD (Table S1 in Appendix S1). The mean age of HMD onset was 2.8 (range 0-17) years. Mean patient age at last clinical assessment was 13.3 (range 0.5-68) years. Male/female ratio was relatively equal (66:74). A total of 96/140 (68.6%) patients have been previously reported in the literature.

HMD Classification and Body Distribution
The majority of patients (108/140, 77%) had a generalized HMD (Table 1 had significantly later onset of HMD than those with generalized symptoms (4.7 AE 0.7 vs. 2.5 AE 0.3 years, respectively; P = 0.001).

Abnormal Eye Movements
Oculogyric crises were commonly seen in all neurotransmitter defects except autosomal dominant Segawa disease. Eyelid myoclonus and ocular flutter were evident in DTDS (Dopamine Transporter Deficiency Syndrome). Ptosis (TH and AADC (Aromatic L-amino acid decarboxylase) deficiency) and oculomotor dyspraxia (TH deficiency) were also observed. Within the wider cohort, strabismus (FOXG1 and ATP1A3) and nystagmus (MICU1) were also reported. Tonic upgaze associated with severe retrocollis was observed in one patient with KMT2B-dystonia.
Dystonia of the lower limbs was observed in 58 ambulant and 27 nonambulant patients. In the nonambulant group, bilateral dystonic postures included fixed/variable leg flexion/extension, striatal toe, toe clawing, and clenched feet. In the ambulant group, patients had gait dystonia with lower-limb posturing, toe-walking, striatal toe, toe-clawing, and gait disturbance. All patients with KMT2B-HMD had lower-limb symptoms at disease onset, leading to a purely dystonic gait. Lower-limb dystonia was also observed in 21/61 patients with neurotransmitter defects and in 10/30 SGCE patients, the latter associated with myoclonic jerks in three patients. In the remaining patients, gait dystonia was associated with chorea (ATP1A3, GNAO1, and NKX2.1) and ataxia (ATP1A3, NKX2.1, and SLC2A1).
Upper-limb dystonia was observed in 98/140 patients, with fixed/variable dystonic posturing of the arms, hand fisting, superimposed coarse dystonic tremor, and actioninduced writer's cramp.

Tremor
Tremor was observed in 23 patients, most commonly in those with neurotransmitter diseases, but also in KMT2B-, MICU1-, and SLC2A1-related diseases (Fig. 2). Tremor was observed in combination with dystonia (n = 20), parkinsonism (n = 13), ataxia (n = 3), chorea (n = 2), and myoclonus (n = 2). In patients with neurotransmitters defects, a coarse generalized resting tremor was observed in nine infants with parkinsonismdystonia (Video S5). Focal neck and upper limb tremor were observed in seven patients with neurotransmitter defects (DHPR, GCH1, PTPS, SLC6A3, and SPR). Prominent head tremor was also identified in one child with Glut1 deficiency and in a patient with KMT2Bdystonia with retrocollis.

Stereotypies
Fourteen patients with FOXG1-related HMDs displayed complex motor stereotypies of the upper limbs (repetitive mouthing of hands/objects, midline hand wringing, grasping of clothes), lower limbs (pedalling/pulling), body rocking, bruxism, and nail biting. Three children with GNAO1-related HMDs also showed self-injurious stereotypic behavior (lip and nail biting, hair pulling) and repetitive nonpurposeful distal finger movements.

Other Neurological and Systemic Features
Developmental delay was evident in 69/140 (49%) patients, being severe in 36 patients with neurotransmitter defects (SLC6A3, TH, DHPR, DDC) and in GNAO1-, FOXG1-, and ATP1A3-related HMDs. Patients with developmental delay had an earlier age of HMD onset than those with normal neurodevelopment
Patients with genetic HMDs also had difficulties with eating/swallowing (n = 54), as well as DLAs requiring fine motor control (n = 67) ( Table S3 in Appendix S1).

Paroxysmal Fluctuation
Oculogyric crises, triggered by hunger, illness, and fatigue, were recorded in 25/46 (54%) children with DDC-, DHPR-, PTPS-, SLC6A3-, SPR-, and TH-related HMDs. These children also had periodic dystonic attacks that were often associated with orolingual dyskinesia. Diurnal fluctuation of motor symptoms (associated with sleep benefit) was also described in this group. Gait difficulties, poor balance, and frequent falls triggered by fatigue and prolonged exercise (with partial relief of symptoms after sleep) were seen in five of eight patients with GCH1 defects (Table S4 in Appendix S1).
In ATP1A3 patients, paroxysmal paralysis, dystonia, ataxia, and dysphagia were commonly reported. Identified triggers were extremes of temperature, bathing, emotional stress, or fatigue. Paroxysmal exacerbation of baseline choreoathetosis was reported in two children with ADCY5 variants, triggered by drowsiness and sleep in one. Episodes of acute ataxia, quadriplegia, and dystonic posturing were described in all four children with Glut1 deficiency after exercise, fatigue, prolonged fasting, and febrile illness. Patients with GNAO1 mutations showed exacerbation of hyperkinetic movements with intercurrent infection, heightened emotion, and purposeful movements. Periods of severe exacerbation were characterized by relentless hyperkinesia, often necessitating admission to the intensive care unit. Dystonic crises were also reported in patients with KMT2B-related disease (n = 2) and SLC6A3-related disorders (n = 4), most commonly triggered by intercurrent illness.

Discussion
Through this multicenter study, we have delineated the genetic landscape of complex HMDs, where diagnostic difficulty is frequently encountered, given the paucity of reliable blood/urine biomarkers, often nonspecific neuroimaging, and broad differential diagnosis. Complex HMD disorders are commonly misdiagnosed, for example, as "dyskinetic" or "dystonic" forms of acquired cerebral palsy. 3,4 Accurate clinical diagnosis is essential for prompt instigation of gene-specific treatments, disease prognostication, and genetic counseling.
Despite the individual rarity of these conditions, we were able to characterize a large cohort of patients with HMDs of broad clinical and genetic heterogeneity. As a result, we provide a comprehensive analysis of motor semiology and associated clinical features in pediatric complex HMDs, together with video recorded material, that will be valuable for both diagnostic and educational purposes.
Our study highlights the increasing diagnostic value of multigene panels and whole-exome/genome sequencing, which led to a diagnosis in almost one third of the cohort. The identification of several microdeletions encompassing disease-causing genes (FOXG1, KMT2B, NKX2.1, SGCE) also highlights the diagnostic utility of aCGH for complex HMD. 5,6 Variants in a broad range of genes were identified, with different but interconnected cellular functions (Fig. 1). Genes involved in primary neurotransmitter disorders affect dopamine synthesis (DDC, DHPR, PTPS, SPR, and TH) and dopamine transport (SLC6A3). Mutations in GNAO1 and other G protein subunits (GNAL), adenylyl cyclase (ADCY5), and cyclic nucleotide phosphodiesterase (PDE10A) disrupt the postsynaptic G protein-cAMP pathway axis and may impair neuromodulation or transduction of transmembrane signaling, presynaptic autoinhibitory effects, and altered neuronal excitability. Other HMD genes code for neuronal proteins with key biological roles, including Na + /K + -ATPase transport, osmoregulation, and excitability (ATP1A3), transcriptional repressors involved in the promotion of neurogenesis and cortical neuronal differentiation (FOXG1), transcription factors essential for striatal development (NKX2-1), dystrophin-glycoprotein complex that links the actin cytoskeleton to the extracellular matrix (SGCE), regulation of mitochondrial Ca 2+ uptake and synaptic transmission (MICU1), glucose transport (SLC2A1), and posttranscriptional regulation of gene expression (KMT2B).
From a clinical perspective, pure HMD phenotypes (defined as those with a single manifesting HMD phenotype) were exceptional in our cohort of patients. We observed a pure dystonic motor semiology in patients with KMT2B-and neurotransmitter disorder-related HMDs, leading to gait difficulties in early childhood. 6,7 Also, a minority of patients with NKX2-1 defects showed a pure chorea phenotype. In contrast, most patients had a combination of two or more HMDs, or a complex movement disorder with both hyperkinetic and hypokinetic features, commonly known as dystonia-plus syndromes. In some children with parkinsonism-dystonia caused by biogenic amine defects, hypokinesia and bradykinesia may be severe and predominate over the HMD phenotype. 3,8 Chorea was the second most frequently identified HMD, evident in >50% patients with ADCY5, ATP1A3, FOXG1, NKX2.1, SLC2A1, GNAO1, and PDE10A variants. [9][10][11][12][13][14][15] Chorea was mostly present in combination with dystonia and sometimes myoclonus. Myoclonus was characteristic of SGCE patients, 16 but also observed in ATP1A3 disease, 17 NKX2.1, 13,18 FOXG1, 11 GNAO1, 14 KMT2B, 6 SLC2A1, and some neurotransmitters defects, 19,20 usually in combination with chorea and dystonia. Myoclonus-dystonia has also been reported in children with ADCY5 and GCH 21 defects, but this combination was not observed in our cohort of patients. As previously reported, stereotypies were a key disease feature in FOXG1 and GNAO1 patients, always in combination with other HMDs. 11,14 Abnormal ocular, facial, and oromandibular movements were also important clues for diagnosis: eyelid myoclonus, ocular flutter, and oculogyric crises were exclusively observed in neurotransmitter diseases. 22 Forehead wrinkling, facial grimacing, perioral muscle twitching, jaw dystonia, and tongue dyskinesias were frequently seen in children with generalized choreodystonia phenotypes because of ADCY5, NKX2.1, GNAO1, and ATP1A3 mutations. 9,10,12,14 In our cohort, the majority had a generalized pattern of HMDs. Only a few patients showed a focal or segmental distribution, predominantly affecting the upper limbs and neck, with significantly later onset of the HMD. Genetic defects presenting in the first year of life were more likely to lead to both a generalized HMD pattern and also developmental delay, as observed in patients with neurotransmitter defects (with the exception of dominant GCH1-related disease) and those with ATP1A3, FOXG1, GNAO1, SLC2A1, and PDE10A variants. 10,11,[13][14][15] Furthermore, in NKX2.1-related HMDs, patients had delayed motor development and early hypotonia in infancy. 12,18 In addition to developmental delay, many patients in the cohort had other neurological and systemic features. Epilepsy was present in one quarter of our cohort, especially in patients with FOXG1, GNAO1, ATP1A3, and SLC2A1 variants. The co-occurrence of epilepsy and hyperkinetic phenotypes in genetic syndromes is increasingly recognized. 23 Underlying mechanisms are not entirely clear but may be a consequence of the gene defect, as well as epigenetic phenomena, environmental factors, or the effect of motor evolution in the developing brain. 24 Many complex genetic HMDs are also associated with disease-specific paroxysmal exacerbations. Oculogyric crises appeared to be exclusive to patients with neurotransmitters defects. 3,22 Plegic attacks were identified in patients with ATP1A3 variants and Glut1 deficiency. 13,17 More recently, acute hemiplegia has also been reported in patients with ADCY5 variants. 25 Within our series, status dystonicus was seen in patients with GNAO1, KMT2B, and SLC6A3 mutations. 6,14,26 Some paroxysmal episodes were caused by specific triggers. Exercise-induced HMDs were seen in Segawa disease 27 and Glut1 deficiency syndrome. 28 Diurnal fluctuation, with improvement after sleep, was recorded in all neurotransmitter defects. 3,22 As previously reported, paroxysmal chorea during sleep and on wakening was seen in ADCY5-related disease. 9 We also identified several other triggers in our cohort, including fever, intercurrent illness, fatigue, emotional stress, temperature, purposeful movements, and fasting.
Complex genetic HMDs also significantly impact on DLAs. More than half of the cohort showed gait and speech impairment, and a significant proportion also showed difficulties in eating, hygiene, and dressing. It is likely that for some conditions (FOXG1, GNAO1), intellectual disability also contributed to functional impairment. 11,14 Gait impairment was a common finding in our cohort, with half of the patients (GNAO1, DDC, PDE10A, SLC6A3, and FOXG1 defects) either unable to walk at all or requiring assistance for ambulation. In contrast, a mildly impaired but independent gait pattern was commonly observed in patients with drug-responsive neurotransmitters defects (GCH1, DHPR, SPR, TH, and PTPS), nonprogressive chorea (NKX2.1, ADCY5), Glut1 syndrome (SLC2A1), and myoclonus-dystonia (SGCE).
Standard investigations, including blood/urine tests and neuroimaging, are unyielding for the genetic defects included in our study. Rarely, subtle neuroimaging patterns can be present, including corpus callosum abnormalities and brain atrophy in GNAO1 and FOXG1 defects, 11,29 and subtle T2 low signal intensity within the globus pallidus in KMT2B-dystonia. 6 Interestingly, patients with heterozygous mutations in PDE10A show bilateral striatal necrosis, while those included in our study with biallelic variants have normal structural magnetic resonance imaging despite having low levels of PDE10A protein in the striatum. 15 Our study highlights the merit of undertaking cerebrospinal fluid glucose and neurotransmitters, particularly to aid diagnosis of Glut1 deficiency and neurotransmitter defects (Fig. 2). In patients with suspected disorders of monoamine metabolism, urine sepiapterin or blood prolactin levels can also be measured; however, they are neither universally available nor sensitive/specific for diagnostic purposes. Although not disease specific, hypothyroidism was evident in NKX2.1 disease 12 and increased creatine kinase levels in patients with MICU1 defects. 30 Genetic characterization of patients with complex HMDs may facilitate selection of appropriate therapies. Many patients with neurotransmitter defects in our series showed a significant and sustained improvement with dopaminergic agents. 22 However, our study confirms that both AADC deficiency and DTDS remain challenging, where novel treatment strategies, such as gene therapy, may have a role. 31 Ketogenic diet improved motor performance and paroxysmal episodes in four of five patients with Glut1 deficiency. 28 More recently, triheptanoin has been shown to dramatically reduce paroxysmal motor disorders and epileptic discharges in patients with Glut1 deficiency in open-label pilot studies, 32 but these results were not confirmed in a randomized, blinded, placebocontrolled clinical trial, suggesting that triheptanoin is of limited therapeutic use. 33 Recently, the Rare Movement Disorders Study Group of the International Parkinson and Movement Disorder Society designed an online survey to identify worldwide barriers for the genetic diagnosis of movement disorders. They found limited access to genetic testing in all countries compared with Europe and North America. Given these findings, it is important to emphasize that patients with early-onset dystonia of unknown etiology should receive a trial with L-dopa to exclude a possible defect in dopamine metabolism, regardless of the availability of genetic testing. 34 DBS remains important for patients with medically intractable dystonia, and the contribution of genetic testing to outcome from DBS is increasingly recognized. 35 In our cohort, DBS was effective for 13 patients with KMT2B-, SGCE-, and GNAO1-related disease. 6,36,37 DBS reduced the risk for life-threatening hyperkinetic exacerbations in GNAO1 patients. Marked improvement of choreoathetosis (albeit with only mild functional recovery) has also been reported in ADCY5 disease after DBS. 9 Although we studied a broad patient population from multiple centers, there are a number of study limitations. The lack of standardization of video footage may have affected HMD classification, although researchers were stringent in analyzing only videos where the whole body could be assessed. Furthermore, our strict inclusion criteria excluded a number of genetic HMDs, including those with isolated motor semiology (eg, DYT1 dystonia), as well as some metabolic diseases, and disorders where the HMD is a less prominent part of the clinical phenotype (eg, epileptic encephalopathies). This may have led to an overall ascertainment bias when considering complex genetic HMDs, although our deliberate aim was to focus on better delineating complex HMD disorders where the HMD was the main phenotype without highly predictive blood, urine, or radiological biomarkers. Finally, it is important to emphasize that the motor features identified in our cohort are derived from a very small subset of individuals with each genetic defect; therefore, the spectrum of movement disorders analyzed in this study does not fully represent the breath of HMDs that may be associated with a particular gene.
In conclusion, detailed clinical assessment and careful classification of the HMD semiology is key to diagnosing complex genetic HMDs. If neuroimaging, blood, and urine neurometabolic testing are unyielding, cerebrospinal fluid analysis and targeted neurogenetic investigations should be promptly undertaken. In the future, better understanding of the underlying disease mechanisms will no doubt facilitate the development of precision treatment strategies for these disorders.