A biallelic mutation links MYORG to autosomal-recessive primary familial brain calcification

Fahr's disease linked to a novel mutation in MYORG variants manifesting as paroxysmal limb stiffness and dysarthria: Case report and literature review

BACKGROUND

Primary familial brain calcification (PFBC) is a rare hereditary neurodegenerative disorder associated with the MYORG gene; however, the clinical and radiological characteristics of MYORG-PFBC remain unclear.

METHODS

We present relevant medical data obtained from a patient affected by PFBC with a novel MYORG variant and conducted a mutational analysis of MYORG in her family members. We reviewed all reported PFBC cases with biallelic MYORG mutations until April 1, 2023, and summarized the associated clinical and radiological features and mutation sites.

RESULTS

The patient (22-year-old woman) exhibited paroxysmal limb stiffness and dysarthria for 3 years. Computed tomography revealed calcifications in the paraventricular white matter, basal ganglia, thalamus, and cerebellum. Whole-exome sequencing revealed a novel homozygous frameshift variant (c.743delG: p.G248Afs*32) in exon 2 of the MYORG gene (NM_020702.5). To date, 62 families and 64 mutation sites have been reported. Among the reported biallelic MYORG mutations, 57% were homozygous and 43% were compound heterozygous. Individuals with biallelic MYORG mutations experience more severe brain calcification with approximately 100% clinical penetrance. Ten single heterozygous mutation sites are associated with significant brain calcifications.

CONCLUSION

All patients with primary brain calcification, particularly younger patients without a family history of the disease, should be screened for MYORG mutations.

The Genetics of Primary Familial Brain Calcification: A Literature Review

Primary familial brain calcification (PFBC), also known as Fahr's disease, is a rare inherited disorder characterized by bilateral calcification in the basal ganglia according to neuroimaging. Other brain regions, such as the thalamus, cerebellum, and subcortical white matter, can also be affected. Among the diverse clinical phenotypes, the most common manifestations are movement disorders, cognitive deficits, and psychiatric disturbances. Although patients with PFBC always exhibit brain calcification, nearly one-third of cases remain clinically asymptomatic. Due to advances in the genetics of PFBC, the diagnostic criteria of PFBC may need to be modified. Hitherto, seven genes have been associated with PFBC, including four dominant inherited genes (SLC20A2, PDGFRB, PDGFB, and XPR1) and three recessive inherited genes (MYORG, JAM2, and CMPK2). Nevertheless, around 50% of patients with PFBC do not have pathogenic variants in these genes, and further PFBC-associated genes are waiting to be identified. The function of currently known genes suggests that PFBC could be caused by the dysfunction of the neurovascular unit, the dysregulation of phosphate homeostasis, or mitochondrial dysfunction. An improved understanding of the underlying pathogenic mechanisms for PFBC may facilitate the development of novel therapies.

A novel MYORG mutation causes primary familial brain calcification with migraine: Case report and literature review

Primary familial brain calcification (PFBC) is a disorder in which pathologic calcification of the basal ganglia, cerebellum, or other brain regions with bilateral symmetry occurs. Common clinical symptoms include dysarthria, cerebellar symptoms, motor deficits, and cognitive impairment. Genetic factors are an important cause of the disease; however autosomal recessive (AR) inheritance is rare. In 2018, the myogenesis-regulated glycosidase (MYORG) gene was the first to be associated with AR-PFBC. The present case is a 24-year-old woman with AR-PFBC that presented with migraine at the age of 16 years. Symmetrical patchy calcifications were seen in the bilateral cerebellopontine nuclei, thalamus, basal ganglia, and radiocoronal area on computed tomography and magnetic resonance imaging. AR-PFBC with migraine as the main clinical symptom is rare. Whole-exome sequencing revealed a compound heterozygous mutation in the MYORG gene, one of which has not been previously reported. Our case highlights the pathogenic profile of the MYORG gene, and demonstrates the need for exclusion of calcium deposits in the brain for migraine patients with AR inheritance.

The primary familial brain calcification-associated protein MYORG is an α-galactosidase with restricted substrate specificity

Primary familial brain calcification (PFBC) is characterised by abnormal deposits of calcium phosphate within various regions of the brain that are associated with severe cognitive impairments, psychiatric conditions, and movement disorders. Recent studies in diverse populations have shown a link between mutations in myogenesis-regulating glycosidase (MYORG) and the development of this disease. MYORG is a member of glycoside hydrolase (GH) family 31 (GH31) and, like the other mammalian GH31 enzyme α-glucosidase II, this enzyme is found in the lumen of the endoplasmic reticulum (ER). Though presumed to act as an α-glucosidase due to its localization and sequence relatedness to α-glucosidase II, MYORG has never been shown to exhibit catalytic activity. Here, we show that MYORG is an α-galactosidase and present the high-resolution crystal structure of MYORG in complex with substrate and inhibitor. Using these structures, we map detrimental mutations that are associated with MYORG-associated brain calcification and define how these mutations may drive disease progression through loss of enzymatic activity. Finally, we also detail the thermal stabilisation of MYORG afforded by a clinically approved small molecule ligand, opening the possibility of using pharmacological chaperones to enhance the activity of mutant forms of MYORG.

MYORG is an enzyme genetically linked to primary familial brain calcification that has historically been presumed to act as an α-glucosidase. This study describes the crystal structure of dimeric MYORG and, surprisingly, reveals it to be an α-galactosidase with restricted specificity.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Mutation Analysis of MYORG in a Chinese Cohort With Primary Familial Brain Calcification

Primary familial brain calcification (PFBC) is a progressive neurological disorder manifesting as bilateral brain calcifications in CT scan with symptoms as parkinsonism, dystonia, ataxia, psychiatric symptoms, etc. Recently, pathogenic variants in MYORG have been linked to autosomal recessive PFBC. This study aims to elucidate the mutational and clinical spectrum of MYORG mutations in a large cohort of Chinese PFBC patients with possible autosomal recessive or absent family history. Mutational analyses of MYORG were performed by Sanger sequencing in a cohort of 245 PFBC patients including 21 subjects from 10 families compatible with a possibly autosomal-recessive trait and 224 apparently sporadic cases. In-depth phenotyping and neuroimaging features were investigated in all patients with novel MYORG variants. Two nonsense variants (c.442C > T, p. Q148*; c.972C > A, p. Y324*) and two missense variants (c.1969G>C, p. G657R; c.2033C > G, p. P678R) of MYORG were identified in four sporadic PFBC patients, respectively. These four novel variants were absent in gnomAD, and their amino acid were highly conserved, suggesting these variants have a pathogenic impact. Patients with MYORG variants tend to display a homogeneous clinical spectrum, showing extensive brain calcification and parkinsonism, dysarthria, ataxia, or vertigo. Our findings supported the pathogenic role of MYORG variants in PFBC and identified two pathogenic variants (c.442C > T, c.972C > A), one likely pathogenic variant (c.2033C > G), and one variant of uncertain significance (c.1969G>C), further expanding the genetic and phenotypic spectrum of PFBC-MYORG.

First pediatric case with primary familial brain calcification due to a novel variant on the MYORG gene and review of the literature

Variants in the myogenesis-regulating glycosidase (MYORG) gene which is known as the first autosomal recessive gene that has been associated with primary familial brain calcification (AR-PFBC). Although adult patients have been reported, no pediatric case has been reported until now. Herein, we review the clinical and radiological features of all AR- PFBC patients with biallelic variants in the MYORG gene who were reported until now, and we report the youngest patient who has a novel homozygous variant. Since the first identification of the MYORG gene in 2018, 74cases of MYORG variants related to AR-PFBC were evaluated. The ages of symptom onset of the patients ranged between 7.5 and 87 years. The most frequent clinical courses were speech impairment, movement disorder and cerebellar signs. All patients showed basal ganglia calcification usually bilaterally with different severities. Conclusion; herein, we reported the first pediatric patient in the literature who had a novel homozygous variant in the MYORG gene with mild clinic findings.

A novel mutation in MYORG leads to primary familial brain calcification and cerebral infarction

Primary familial brain calcification (PFBC) is a rare inherited disorder characterized by bilateral calcification mainly in the basal ganglia, thalamus, and cerebellar nuclei. Recently, the MYORG gene, as the first autosomal recessive causal gene for PFBC, was reported in six unrelated Chinese families. Patients with PFBC rarely present with cerebrovascular disease. Here, we report a young patient with PFBC who carried a novel homozygous mutation in the MYORG gene presenting with cerebral infarction involving the posterior limb of the right internal capsule. Brain computed tomography (CT) demonstrated symmetric calcifications in the basal ganglia, thalamus, midbrain, pons, cerebellum and frontal lobes. We found one homozygous mutation in the MYORG gene (NM_020702.3 exon2: c.830delC; p.P277Qfs*3) in this patient by Sanger sequencing. Currently, the association of PFBC and cerebral infarction, as well as the physiological role of the MYORG gene, is not clear and worth special attention and further investigation.

Biallelic loss-of-function mutations in JAM2 cause primary familial brain calcification

Primary familial brain calcification is a monogenic disease characterized by bilateral calcifications in the basal ganglia and other brain regions, and commonly presents motor, psychiatric, and cognitive symptoms. Currently, four autosomal dominant (SLC20A2, PDGFRB, PDGFB, XPR1) and one autosomal recessive (MYORG) causative genes have been identified. Compared with patients with autosomal dominant primary familial brain calcification, patients with the recessive form of the disease present with more severe clinical and imaging phenotypes, and deserve more clinical and research attention. Biallelic mutations in MYORG cannot explain all autosomal recessive primary familial brain calcification cases, indicating the existence of novel autosomal recessive genes. Using homozygosity mapping and whole genome sequencing, we detected a homozygous frameshift mutation (c.140delT, p.L48*) in the JAM2 gene in a consanguineous family with two affected siblings diagnosed with primary familial brain calcification. Further genetic screening in a cohort of 398 probands detected a homozygous start codon mutation (c.1A>G, p.M1?) and compound heterozygous mutations [c.504G>C, p.W168C and c.(67+1_68-1)_(394+1_395-1), p.Y23_V131delinsL], respectively, in two unrelated families. The clinical phenotypes of the four patients included parkinsonism (3/4), dysarthria (3/4), seizures (1/4), and probable asymptomatic (1/4), with diverse onset ages. All patients presented with severe calcifications in the cortex in addition to extensive calcifications in multiple brain areas (lenticular nuclei, caudate nuclei, thalamus, cerebellar hemispheres, ± brainstem; total calcification scores: 43-77). JAM2 encodes junctional adhesion molecule 2, which is highly expressed in neurovascular unit-related cell types (endothelial cells and astrocytes) and is predominantly localized on the plasma membrane. It may be important in cell-cell adhesion and maintaining homeostasis in the CNS. In Chinese hamster ovary cells, truncated His-tagged JAM2 proteins were detected by western blot following transfection of p.Y23_V131delinsL mutant plasmid, while no protein was detected following transfection of p.L48* or p.1M? mutant plasmids. In immunofluorescence experiments, the p.W168C mutant JAM2 protein failed to translocate to the plasma membrane. We speculated that mutant JAM2 protein resulted in impaired cell-cell adhesion functions and reduced integrity of the neurovascular unit. This is similar to the mechanisms of other causative genes for primary familial brain calcification or brain calcification syndromes (e.g. PDGFRB, PDGFB, MYORG, JAM3, and OCLN), all of which are highly expressed and functionally important in the neurovascular unit. Our study identifies a novel causative gene for primary familial brain calcification, whose vital function and high expression in the neurovascular unit further supports impairment of the neurovascular unit as the root of primary familial brain calcification pathogenesis.

MYORG-related disease is associated with central pontine calcifications and atypical parkinsonism

OBJECTIVE

To identify the phenotypic, neuroimaging, and genotype-phenotype expression of MYORG mutations.

METHODS

Using next-generation sequencing, we screened 86 patients with primary familial brain calcification (PFBC) from 60 families with autosomal recessive or absent family history that were negative for mutations in SLC20A2, PDGFRB, PDGBB, and XPR1. In-depth phenotyping and neuroimaging investigations were performed in all cases reported here.

RESULTS

We identified 12 distinct deleterious MYORG variants in 7 of the 60 families with PFBC. Overall, biallelic MYORG mutations accounted for 11.6% of PFBC families in our cohort. A heterogeneous phenotypic expression was identified within and between families with a median age at onset of 56.4 years, a variable combination of parkinsonism, cerebellar signs, and cognitive decline. Psychiatric disturbances were not a prominent feature. Cognitive assessment showed impaired cognitive function in 62.5% of cases. Parkinsonism associated with vertical nuclear gaze palsy was the initial clinical presentation in 1/3 of cases and was associated with central pontine calcifications. Cerebral cortical atrophy was present in 37% of cases.

CONCLUSIONS

This large, multicentric study shows that biallelic MYORG mutations represent a significant proportion of autosomal recessive PFBC. We recommend screening MYORG mutations in all patients with primary brain calcifications and autosomal recessive or negative family history, especially when presenting clinically as atypical parkinsonism and with pontine calcification on brain CT.

MYORG Mutation Heterozygosity Is Associated With Brain Calcification

BACKGROUND

Biallelic mutations in the MYORG gene were first identified as the cause of recessively inherited primary familial brain calcification. Interestingly, some heterozygous carriers also exhibited brain calcifications.

OBJECTIVES

To further investigate the role of single heterozygous MYORG mutations in the development of brain calcifications.

METHODS

A nation-wide cohort of Chinese primary familial brain calcification probands was enrolled from March 2016 through September 2019. Mutational analysis of MYORG was performed in 435 primary familial brain calcification probands who were negative for mutations in the other four known primary familial brain calcification-causative genes (SLC20A2, PDGFRB, PDGFB, and XPR1).

RESULTS

Biallelic MYORG mutations were identified in 14 primary familial brain calcification patients from 10 unrelated families. Interestingly, 12 heterozygous carriers from seven of these families also exhibited mild-to-moderate brain calcifications. Moreover, single heterozygous mutations were detected in an additional 9 probands and in 7 of their family members affected with brain calcifications. In our cohort, clinical and imaging penetrance of individuals with biallelic mutations were 100%, whereas among individuals with heterozygous mutations, penetrance of imaging phenotype was reduced to 73.7% (28 of 38) and clinical penetrance was much lower. Most (34 of 38) remained asymptomatic whereas 4 carriers had symptoms of uncertain clinical significance (nonspecific depression, epilepsy and late-onset parkinsonism). Compared with individuals with biallelic MYORG mutations, individuals with heterozygous mutations had brain calcifications with much lower calcification scores (P < 2e-16).

CONCLUSIONS

Presence of brain calcifications in individuals with heterozygous MYORG mutations suggested a semidominant inheritance pattern with incomplete penetrance. This finding further expanded the genotype-phenotype correlations of MYORG-related primary familial brain calcification. © 2020 International Parkinson and Movement Disorder Society.

Design of a multiplex ligation-dependent probe amplification assay for SLC20A2: identification of two novel deletions in primary familial brain calcification

Primary familial brain calcification (PFBC) is a rare disease characterized by brain calcifications that mainly affect the basal ganglia, thalamus, and cerebellum. Among the four autosomal-dominant genes known to be associated with the disease, SLC20A2 pathogenic variants are the most common, accounting for up to 40% of PFBC dominant cases; variants include both point mutations, small insertions/deletions and intragenic deletions. Over the last 7 years, we have collected a group of 50 clinically diagnosed PFBC patients, who were screened for single nucleotide changes and small insertions/deletions in SLC20A2 by Sanger sequencing. We found seven pathogenic/likely pathogenic variants: four were previously described by our group, and three are reported here (c.303delG, c.21delG, and c.1795-1G>A). We developed and validated a synthetic Multiplex Ligation-dependent Probe Amplification (MLPA) assay for SLC20A2 deletions, covering all ten coding exons and the 5' UTR (SLC20A2-MLPA). Using this method, we screened a group of 43 PFBC-patients negative for point mutations and small insertions/deletions, and identified two novel intragenic deletions encompassing exon 6 NC_000008.10:g.(42297172_42302163)_(423022281_42317413)del, and exons 7-11 including the 3'UTR NC_000008.10:g.(?_42275320)_(42297172_42302163)del. Overall, SLC20A2 deletions may be highly underestimated PFBC cases, and we suggest MLPA should be included in the routine molecular test for PFBC diagnosis.

MYORG Mutations: a Major Cause of Recessive Primary Familial Brain Calcification

PURPOSE OF REVIEW

Until recently, the gene associated with the recessive form of familial brain calcification (PFBC, Fahr disease) was unknown. MYORG, a gene that causes recessive PFBC was only recently discovered and is currently the only gene associated with a recessive form of this disease. Here, we review the radiological and clinical findings in adult MYORG mutation homozygous and heterozygous individuals.

RECENT FINDINGS

MYORG was shown to be the cause of a large fraction of recessive cases of PFBC in patients of different ethnic populations. Pathogenic mutations include inframe insertions and deletions in addition to nonsense and missense mutations that are distributed throughout the entire MYORG coding region. Homozygotes have extensive brain calcification in all known cases, whereas in some carriers of heterozygous mutation, punctuated calcification of the globus pallidus is demonstrated. The clinical spectrum in homozygotes ranges from the lack of neurological symptoms to severe progressive neurological syndrome with bulbar and cerebellar signs, parkinsonism and other movement disorders, and cognitive impairments. Heterozygotes are clinically asymptomatic. MYORG is a transmembrane protein localized to the endoplasmic reticulum and is mainly expressed in astrocytes. While the biochemical pathways of the protein are still unknown, information from its evolution profile across hundreds of species (phylogenetic profiling) suggests a role for MYORG in regulating ion homeostasis via its glycosidase domain. MYORG mutations are a major cause for recessive PFBC in different world populations. Future studies are required in order to reveal the cellular role of the MYORG protein.

Primary familial brain calcification caused by a novel homozygous MYORG mutation in a consanguineous Italian family

Primary familial brain calcification (PFBC) is a rare disorder mostly characterized by calcium deposits in the basal ganglia and a wide spectrum of neurologic and psychiatric symptoms, typically inherited as an autosomal dominant trait. Recently, MYORG was reported as the first autosomal recessive causal gene in PFBC patients of Chinese and Middle Eastern origin. Herein, we describe the first PFBC patient of European descent found to carry a novel homozygous MYORG mutation (p.N511Tfs*243). Interestingly, the patient's father, a heterozygous carrier of the same mutation, showed diffuse bilateral cerebral calcifications with no symptoms other than very mild postural tremor.