Estimation of minimal disease prevalence from population genomic data: Application to primary familial brain calcification

Biallelic NAA60 variants with impaired n-terminal acetylation capacity cause autosomal recessive primary familial brain calcifications

Primary familial brain calcification (PFBC) is characterized by calcium deposition in the brain, causing progressive movement disorders, psychiatric symptoms, and cognitive decline. PFBC is a heterogeneous disorder currently linked to variants in six different genes, but most patients remain genetically undiagnosed. Here, we identify biallelic NAA60 variants in ten individuals from seven families with autosomal recessive PFBC. The NAA60 variants lead to loss-of-function with lack of protein N-terminal (Nt)-acetylation activity. We show that the phosphate importer SLC20A2 is a substrate of NAA60 in vitro. In cells, loss of NAA60 caused reduced surface levels of SLC20A2 and a reduction in extracellular phosphate uptake. This study establishes NAA60 as a causal gene for PFBC, provides a possible biochemical explanation of its disease-causing mechanisms and underscores NAA60-mediated Nt-acetylation of transmembrane proteins as a fundamental process for healthy neurobiological functioning.

Idiopathic basal ganglia calcification presenting with obsessive-compulsive symptoms: A case report

Background

Idiopathic basal ganglia calcification (IBGC), also known as Farh's disease, is a rare neurodegenerative disorder characterized by calcification of the basal ganglia and other brain regions. This disease usually occurs in middle-aged patients and presents with various neurological and psychiatric symptoms. The exact prevalence is unknown; however, population genomic data analysis suggests a prevalence of at least 4.5/10,000 to 3.3/1000, indicating that the disease is more common than previously thought and remains underdiagnosed.

Case Presentation

We report the case of a middle-aged Japanese man who attempted suicide twice because of obsessive-compulsive ideation caused by trivial triggers. The patient's psychiatric symptoms resolved relatively quickly after hospitalization, and imaging and genetic testing led to a diagnosis of IBGC.

Conclusion

This case report illustrates the importance of including IBGC in the differential diagnosis of psychiatric symptoms that initially develop in middle-aged patients.

The effects of etidronate on brain calcifications in Fahr's disease or syndrome: rationale and design of the randomised, placebo-controlled, double-blind CALCIFADE trial

BACKGROUND

Fahr's disease and syndrome are rare disorders leading to calcification of the small arteries in the basal ganglia of the brain, resulting in a wide range of symptoms comprising cognitive decline, movement disorders and neuropsychiatric symptoms. No disease-modifying therapies are available. Studies have shown the potential of treatment of ectopic vascular calcifications with bisphosphonates. This paper describes the rationale and design of the CALCIFADE trial which evaluates the effects of etidronate in patients with Fahr's disease or syndrome.

METHODS

The CALCIFADE trial is a randomised, placebo-controlled, double-blind trial which evaluates the effects of etidronate 20 mg/kg during 12 months follow-up in patients aged ≥ 18 years with Fahr's disease or syndrome. Etidronate and placebo will be administered in capsules daily for two weeks on followed by ten weeks off. The study will be conducted at the outpatient clinic of the University Medical Center Utrecht, the Netherlands. The primary endpoint is the change in cognitive functioning after 12 months of treatment. Secondary endpoints are the change in mobility, neuropsychiatric symptoms, volume of brain calcifications, dependence in activities of daily living, and quality of life.

RESULTS

Patient recruitment started in April 2023. Results are expected in 2026 and will be disseminated through peer-reviewed journals as well as presentations at national and international conferences.

CONCLUSIONS

Fahr's disease and syndrome are slowly progressive disorders with a negative impact on a variety of health outcomes. Etidronate might be a new promising treatment for patients with Fahr's disease or syndrome.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT05662111. Registered 22 December 2022, https://clinicaltrials.gov/ct2/show/NCT01585402 .

Basal Ganglia Calcification: A Case Report of Two Siblings With Fahr's Disease

Fahr's disease is a rare neurodegenerative disorder caused by bilateral and usually symmetrical intracranial calcifications. In most cases, it exhibits an autosomal dominant pattern of inheritance and genetic heterogeneity. Patients may present with movement disorders, cognitive impairment, and psychiatric disorders. Currently, there are no disease-modifying drugs, so the management is based on the treatment of the symptoms. We present two cases involving male siblings, both with psychiatric symptoms as the initial presentation of the disease. Brain computed tomography revealed bilateral calcifications in the basal ganglia for which no underlying cause was found. In both cases, remission of behavioural changes and psychiatric symptoms was achieved with psychotropic drugs.

The Association between Intracranial Calcifications and Symptoms in Patients with Primary Familial Brain Calcification

(1) Background: Primary Familial Brain Calcification (PFBC) is a neurodegenerative disease characterized by bilateral calcifications of the basal ganglia and other intracranial areas. Many patients experience symptoms of motor dysfunction and cognitive disorders. The aim of this study was to investigate the association between the amount and location of intracranial calcifications with these symptoms. (2) Methods: Patients with suspected PFBC referred to our outpatient clinic underwent a clinical work-up. Intracranial calcifications were visualized on Computed Tomography (CT), and a Total Calcification Score (TCS) was constructed. Logistic and linear regression models were performed. (3) Results: Fifty patients with PFBC were included in this study (median age 64.0 years, 50% women). Of the forty-one symptomatic patients (82.0%), 78.8% showed motor dysfunction, and 70.7% showed cognitive disorders. In multivariate analysis, the TCS was associated with bradykinesia/hypokinesia (OR 1.07, 95%-CI 1.02-1.12, p < 0.01), gait ataxia (OR 1.06, 95%-CI 1.00-1.12, p = 0.04), increased fall risk (OR 1.04, 95%-CI 1.00-1.08, p = 0.03), and attention/processing speed disorders (OR 1.06, 95%-CI 1.01-1.12, p = 0.02). Calcifications of the lentiform nucleus and subcortical white matter were associated with motor and cognitive disorders. (4) Conclusions: cognitive and motor symptoms are common among patients with PFBC, and there is an association between intracranial calcifications and these symptoms.

One Train May Hide Another: Two Cases of Co-Occurring Primary Familial Brain Calcification and Alzheimer's Disease

Primary familial brain calcification (PFBC) is a rare disorder that can manifest with a wide spectrum of motor, cognitive, and psychiatric symptoms or even remain asymptomatic. Alzheimer disease (AD) is a common condition that typically starts as a progressive amnestic disorder and progresses to major cognitive impairment. Accurately attributing an etiology to cognitive impairment can sometimes be challenging, especially when multiple pathologies with potentially overlapping symptomatology contribute to the clinical phenotype. Here, we present the case of two patients with autosomal dominant PFBC and non-monogenic AD. Cerebrospinal fluid (CSF) biomarker analysis combined with genetic testing permitted the dual diagnosis. We emphasize the importance of thoroughly characterizing the patient's phenotype at onset and during the follow-up. Particular attention is placed on psychiatric symptoms given that both patients had a history of mood disorder, a frequent condition in the general population and in neurological diseases. We also discuss and challenge the paradigm of seeking a single diagnosis explaining all symptoms, remembering the possibility of a rare disease co-occurring with a common one.

PSEN1/SLC20A2 double mutation causes early-onset Alzheimer's disease and primary familial brain calcification co-morbidity

Primary familial brain calcification (PFBC; formerly Fahr's disease) and early-onset Alzheimer's disease (EOAD) may share partially overlapping pathogenic principles. Although the heterozygous loss-of-function mutation c.1523 + 1G > T in the PFBC-linked gene SLC20A2 was detected in a patient with asymmetric tremor, early-onset dementia, and brain calcifications, CSF β-amyloid parameters and FBB-PET suggested cortical β-amyloid pathology. Genetic re-analysis of exome sequences revealed the probably pathogenic missense mutation c.235G > A/p.A79T in PSEN1. The SLC20A2 mutation segregated with mild calcifications in two children younger than 30 years. We thus describe the stochastically extremely unlikely co-morbidity of genetic PFBC and genetic EOAD. The clinical syndromes pointed to additive rather than synergistic effects of the two mutations. MRI data revealed the formation of PFBC calcifications decades before the probable onset of the disease. Our report furthermore exemplifies the value of neuropsychology and amyloid PET for differential diagnosis.

Choroid Plexus Calcification Correlates with Cortical Microglial Activation in Humans: A Multimodal PET, CT, MRI Study

BACKGROUND AND PURPOSE

The choroid plexus (CP) within the brain ventricles is well-known to produce cerebrospinal fluid (CSF). Recently, the CP has been recognized as critical in modulating inflammation. MRI-measured CP enlargement has been reported in neuroinflammatory disorders like MS as well as with aging and neurodegeneration. The basis of MRI-measured CP enlargement is unknown. On the basis of tissue studies demonstrating CP calcification as a common pathology associated with aging and disease, we hypothesized that previously unmeasured CP calcification contributes to MRI-measured CP volume and may be more specifically associated with neuroinflammation.

MATERIALS AND METHODS

We analyzed 60 subjects (43 healthy controls and 17 subjects with Parkinson's disease) who underwent PET/CT using 11C-PK11195, a radiotracer sensitive to the translocator protein expressed by activated microglia. Cortical inflammation was quantified as nondisplaceable binding potential. Choroid plexus calcium was measured via manual tracing on low-dose CT acquired with PET and automatically using a new CT/MRI method. Linear regression assessed the contribution of choroid plexus calcium, age, diagnosis, sex, overall volume of the choroid plexus, and ventricle volume to cortical inflammation.

RESULTS

Fully automated choroid plexus calcium quantification was accurate (intraclass correlation coefficient with manual tracing = .98). Subject age and choroid plexus calcium were the only significant predictors of neuroinflammation.

CONCLUSIONS

Choroid plexus calcification can be accurately and automatically quantified using low-dose CT and MRI. Choroid plexus calcification-but not choroid plexus volume-predicted cortical inflammation. Previously unmeasured choroid plexus calcium may explain recent reports of choroid plexus enlargement in human inflammatory and other diseases. Choroid plexus calcification may be a specific and relatively easily acquired biomarker for neuroinflammation and choroid plexus pathology in humans.

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.

The clinical and genetic spectrum of primary familial brain calcification

Primary familial brain calcification (PFBC), formerly known as Fahr's disease, is a rare neurodegenerative disease characterized by bilateral progressive calcification of the microvessels of the basal ganglia and other cerebral and cerebellar structures. PFBC is thought to be due to an altered function of the Neurovascular Unit (NVU), where abnormal calcium-phosphorus metabolism, functional and microanatomical alterations of pericytes and mitochondrial alterations cause a dysfunction of the blood-brain barrier (BBB) and the generation of an osteogenic environment with surrounding astrocyte activation and progressive neurodegeneration. Seven causative genes have been discovered so far, of which four with dominant (SLC20A2, PDGFB, PDGFRB, XPR1) and three with recessive inheritance (MYORG, JAM2, CMPK2). Clinical presentation ranges from asymptomatic subjects to movement disorders, cognitive decline and psychiatric disturbances alone or in various combinations. Radiological patterns of calcium deposition are similar in all known genetic forms, but central pontine calcification and cerebellar atrophy are highly suggestive of MYORG mutations and extensive cortical calcification has been associated with JAM2 mutations. Currently, no disease-modifying drugs or calcium-chelating agents are available and only symptomatic treatments can be offered.

The Pathology of Primary Familial Brain Calcification: Implications for Treatment

Primary familial brain calcification (PFBC) is an inherited neurodegenerative disorder mainly characterized by progressive calcium deposition bilaterally in the brain, accompanied by various symptoms, such as dystonia, ataxia, parkinsonism, dementia, depression, headaches, and epilepsy. Currently, the etiology of PFBC is largely unknown, and no specific prevention or treatment is available. During the past 10 years, six causative genes (SLC20A2, PDGFRB, PDGFB, XPR1, MYORG, and JAM2) have been identified in PFBC. In this review, considering mechanistic studies of these genes at the cellular level and in animals, we summarize the pathogenesis and potential preventive and therapeutic strategies for PFBC patients. Our systematic analysis suggests a classification for PFBC genetic etiology based on several characteristics, provides a summary of the known composition of brain calcification, and identifies some potential therapeutic targets for PFBC.

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.

Microglia control small vessel calcification via TREM2

Microglia participate in central nervous system (CNS) development and homeostasis and are often implicated in modulating disease processes. However, less is known about the role of microglia in the biology of the neurovascular unit (NVU). In particular, data are scant on whether microglia are involved in CNS vascular pathology. In this study, we use a mouse model of primary familial brain calcification, Pdgfbret/ret , to investigate the role of microglia in calcification of the NVU. We report that microglia enclosing vessel calcifications, coined calcification-associated microglia, display a distinct activation phenotype. Pharmacological ablation of microglia with the CSF1R inhibitor PLX5622 leads to aggravated vessel calcification. Mechanistically, we show that microglia require functional TREM2 for controlling vascular calcification. Our results demonstrate that microglial activity in the setting of pathological vascular calcification is beneficial. In addition, we identify a previously unrecognized function of microglia in halting the expansion of vascular calcification.

Biallelic XPR1 mutation associated with primary familial brain calcification presenting as paroxysmal kinesigenic dyskinesia with infantile convulsions

BACKGROUND

Mutations in the XPR1 gene are associated with primary familial brain calcifications (PFBC). All reported mutations are missense and inherited as an autosomal dominant trait. PFBC patients exhibited movement disorders, neuropsychiatric symptoms, and other associated symptoms with diverse severity, even within the same family.

MATERIALS AND METHODS

We identified and enrolled a patient with PFBC. Clinical data were comprehensively collected, including the age of onset, seizure types and frequency, trigger factors of paroxysmal dyskinesia, response to drugs, and general and neurological examination results. Whole-exome sequencing (WES) was performed to detect pathogenic variants. We further systematically reviewed the phenotypic and genetic features of patients with XPR1 mutations.

RESULTS

The patient showed bilateral calcification involving basal ganglia and cerebellar dentate. Clinically, he presented as paroxysmal kinesigenic dyskinesia with infantile convulsions (PKD/IC) with favorable outcome. We identified a compound heterozygous XPR1 mutation (c.786_789delTAGA/p.D262Efs*6, c.1342C>T/p.R448W), which were inherited from unaffected parents respectively. Further literature review shows a wide range of clinical manifestations of patients with XPR1 mutations, with movement disorders being the most common.

CONCLUSIONS

This is the first report of biallelic mutations in XPR1. The findings suggest for the first time a possible link between PKD/IC and XPR1 mutations.

Intracranial Arterial Calcification: Prevalence, Risk Factors, and Consequences: JACC Review Topic of the Week

Intracranial large and small arterial calcifications are a common incidental finding on computed tomography imaging in the general population. Here we provide an overview of the published reports on prevalence of intracranial arterial calcifications on computed tomography imaging and histopathology in relation to risk factors and clinical outcomes. We performed a systematic search in Medline, with a search filter using synonyms for computed tomography scanning, (histo)pathology, different intracranial arterial beds, and calcification. We found that intracranial calcifications are a frequent finding in all arterial beds with the highest prevalence in the intracranial internal carotid artery. In general, prevalence increases with age. Longitudinal studies on calcification progression and intervention studies are warranted to investigate the possible causal role of calcification on clinical outcomes. This might open up new therapeutic directions in stroke and dementia prevention and the maintenance of the healthy brain.

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.

Biallelic MYORG mutation carriers exhibit primary brain calcification with a distinct phenotype

Primary familial brain calcification (PFBC) is a rare neurogenetic disorder with diverse neuropsychiatric expression. Mutations in four genes cause autosomal dominant PFBC: SLC20A2, XPR1, PDGFB and PDGFRB. Recently, biallelic mutations in the MYORG gene have been reported to cause PFBC with an autosomal recessive pattern of inheritance. We screened MYORG in 29 unrelated probands negatively screened for the autosomal dominant PFBC genes and identified 11 families with a biallelic rare or novel predicted damaging variant. We studied the clinical and radiological features of 16 patients of these 11 families and compared them to that of 102 autosomal dominant PFBC patients carrying a mutation in one of the four known autosomal dominant PFBC genes. We found that MYORG patients exhibited a high clinical penetrance with a median age of onset of 52 years (range: 21-62) with motor impairment at the forefront. In particular, dysarthria was the presenting sign in 11/16 patients. In contrast to patients with autosomal dominant PFBC, 12/15 (80%) symptomatic patients eventually presented at least four of the following five symptoms: dysarthria, cerebellar syndrome, gait disorder of any origin, akinetic-hypertonic syndrome and pyramidal signs. In addition to the most severe clinical pattern, MYORG patients exhibited the most severe pattern of calcifications as compared to the patients from the four autosomal dominant PFBC gene categories. Strikingly, 12/15 presented with brainstem calcifications in addition to extensive calcifications in other brain areas (lenticular nuclei, thalamus, cerebellar hemispheres, vermis, ±cortex). Among them, eight patients exhibited pontine calcifications, which were observed in none of the autosomal dominant PFBC patients and hence appeared to be highly specific. Finally, all patients exhibited cerebellar atrophy with diverse degrees of severity on CT scans. We confirmed the existence of cerebellar atrophy by performing MRI voxel-based morphometry analyses of MYORG patients with autosomal dominant PFBC mutation carriers as a comparison group. Of note, in three families, the father carried small pallido-dentate calcifications while carrying the mutation at the heterozygous state, suggesting a putative phenotypic expression in some heterozygous carriers. In conclusion, we confirm that MYORG is a novel major PFBC causative gene and that the phenotype associated with such mutations may be recognized based on pedigree, clinical and radiological features.

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.

Mechanisms of calcification in Fahr disease and exposure of potential therapeutic targets

Purpose of review

There is growing interest in disorders involved in ectopic mineralization. Fahr disease or idiopathic basal ganglia calcification can serve as a model for ectopic mineralization in the basal ganglia, which is fairly common in the general population. In this review, we will focus on causative gene mutations and corresponding pathophysiologic pathways in Fahr disease.

Recent findings

Patients with Fahr disease have a variability of symptoms, such as movement disorders, psychiatric signs, and cognitive impairment, but can also be asymptomatic. Fahr disease is mostly autosomal dominant inherited, and there are mutations found in 4 causative genes. Mutations in SLC20A2 and XPR1 lead to a disrupted phosphate metabolism involving brain-specific inorganic phosphate transporters. Mutations in PDGFB and PDGFRB are associated with disrupted blood-brain barrier integrity and dysfunctional pericyte maintenance. In addition, the MYORG gene has recently been discovered to be involved in the autosomal recessive inheritance of Fahr.

Summary

Knowledge about the mutations and corresponding pathways may expose therapeutic opportunities for patients with Fahr disease and vascular calcifications in the brain in general.

Ossified blood vessels in primary familial brain calcification elicit a neurotoxic astrocyte response

Brain calcifications are commonly detected in aged individuals and accompany numerous brain diseases, but their functional importance is not understood. In cases of primary familial brain calcification, an autosomally inherited neuropsychiatric disorder, the presence of bilateral brain calcifications in the absence of secondary causes of brain calcification is a diagnostic criterion. To date, mutations in five genes including solute carrier 20 member 2 (SLC20A2), xenotropic and polytropic retrovirus receptor 1 (XPR1), myogenesis regulating glycosidase (MYORG), platelet-derived growth factor B (PDGFB) and platelet-derived growth factor receptor β (PDGFRB), are considered causal. Previously, we have reported that mutations in PDGFB in humans are associated with primary familial brain calcification, and mice hypomorphic for PDGFB (Pdgfb^ret/ret) present with brain vessel calcifications in the deep regions of the brain that increase with age, mimicking the pathology observed in human mutation carriers. In this study, we characterize the cellular environment surrounding calcifications in Pdgfb^ret/ret animals and show that cells around vessel-associated calcifications express markers for osteoblasts, osteoclasts and osteocytes, and that bone matrix proteins are present in vessel-associated calcifications. Additionally, we also demonstrate the osteogenic environment around brain calcifications in genetically confirmed primary familial brain calcification cases. We show that calcifications cause oxidative stress in astrocytes and evoke expression of neurotoxic astrocyte markers. Similar to previously reported human primary familial brain calcification cases, we describe high interindividual variation in calcification load in Pdgfb^ret/ret animals, as assessed by ex vivo and in vivo quantification of calcifications. We also report that serum of Pdgfb^ret/ret animals does not differ in calcification propensity from control animals and that vessel calcification occurs only in the brains of Pdgfb^ret/ret animals. Notably, ossification of vessels and astrocytic neurotoxic response is associated with specific behavioural and cognitive alterations, some of which are associated with primary familial brain calcification in a subset of patients.

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.

Basal ganglia calcifications (Fahr's syndrome): related conditions and clinical features

Basal ganglia calcifications could be incidental findings up to 20% of asymptomatic patients undergoing CT or MRI scan. The presence of neuropsychiatric symptoms associated with bilateral basal ganglia calcifications (which could occur in other peculiar brain structures, such as dentate nuclei) identifies a clinical picture defined as Fahr's Disease. This denomination mainly refers to idiopathic forms in which no metabolic or other underlying causes are identified. Recently, mutations in four different genes (SLC20A2, PDGFRB, PDGFB, and XPR1) were identified, together with novel mutations in the Myogenic Regulating Glycosylase gene, causing the occurrence of movement disorders, cognitive decline, and psychiatric symptoms. On the other hand, secondary forms, also identified as Fahr's syndrome, have been associated with different conditions: endocrine abnormalities of PTH, such as hypoparathyroidism, other genetically determined conditions, brain infections, or toxic exposure. The underlying pathophysiology seems to be related to an abnormal calcium/phosphorus homeostasis and transportation and alteration of the blood-brain barrier.

Underestimated disease prevalence and severe phenotypes in patients with biallelic variants: A cohort study of primary familial brain calcification from China

BACKGROUND

Primary familial brain calcification (PFBC) is a rare calcifying disorder of the brain with extensive clinical and genetic heterogeneity. Its prevalence is underestimated due to clinical selection bias (compared with symptomatic PFBC patients, asymptomatic ones are less likely to undergo genetic testing).

METHODS

A total of 273 PFBC probands were enrolled in a multicenter retrospective cohort study by two different approaches. In Group I (nonsystematic approach), 37 probands diagnosed at our clinic were enrolled. In Group II (systematic approach), 236 probands were enrolled by searching the medical imaging databases of 50 other hospitals using specific keywords. Genetic testing of four genes known to be causative of autosomal dominant PFBC was performed in all probands using cDNA. All identified variants were further confirmed using genomic DNA and classified according to ACMG-AMP recommendations.

RESULTS

Thirty-two variants including 22 novel variants were detected in 37 probands. Among these probands, 83.8% (31/37) were asymptomatic. Two probands with homozygous pathogenic SLC20A2 variants presented more severe brain calcification and symptoms. Based on the variant detection rate of probands in Group II, we extrapolated an overall minimal prevalence of PFBC of 6.6 per 1,000, much higher than previously reported (2.1 per 1000).

CONCLUSIONS

We identified a higher proportion of genetically confirmed PFBC probands who were asymptomatic. These patients would be overlooked due to clinical selection bias, leading to underestimation of the disease prevalence. Considering that PFBC patients with biallelic variants had more severe phenotypes, this specific condition should be focused on in genetic counseling.

Estimating the prevalence of allelic variants in the transthyretin gene by analysing large-scale sequencing data

Transthyretin amyloidosis (ATTR amyloidosis) is a rare disease characterised by extracellular deposition of amyloid fibrils composed by transthyretin. ATTR amyloidosis can be sub-classified as wild-type ATTR (ATTR-wt) or as hereditary amyloidosis (ATTR-m); the prevalence of both types are likely underestimated. There are tools that can help us to study ATTR-m, as gnomAD database. Our primary aim was to estimate prevalence of variants, especially amyloidogenic variants, in the TTR gene using gnomAD database. We analysed TTR missense variants found in gnomAD. The variables studied were classified according to their clinical significance and according to the different populations. We found 71 missense variants in the TTR gene. Eleven variants were described as affects function variants (prevalence 1:230). The most frequently detected variant were c.424G>A (p.(Val142Ile)) (prevalence 1:332, MAF 0.00151) and c.148G>A (p.(Val50Met)) (prevalence 1:4924, MAF 0.000102), which represented 88% and 5%, respectively, of all affects function variants detected. Seventeen variants were classified as probably affects function, 29 as unknown variants, 4 as probably does not affect function and 10 as does not affect function variants. In terms of different populations, c.424G>A (p.(Val142Ile)) was especially prevalent in African population (MAF 0.01602; prevalence of 1:31) and c.148G>A (p.(Val50Met)) in European population (MAF 0.000179; prevalence of 1:2792). Prevalence of amyloidogenic variants in the general population was higher than prevalence heretofore described. This difference could be explained by incomplete penetrance of the disease, but other factors contributing to this fact, fundamentally the underdiagnosis of the disease.

Pericytes in Primary Familial Brain Calcification

Pericytes are perivascular cells along capillaries that are critical for the development of a functional vascular bed in the central nervous system and other organs. Pericyte functions in the adult brain are less well understood. Pericytes have been suggested to mediate functional hyperemia at the capillary level, regulate the blood-brain barrier and to give rise to scar tissue after spinal cord injury. Furthermore, pericyte loss has been suggested to precede cognitive decline in mouse models of Alzheimer's disease. Despite this observation, there is no convincing causality between pericyte loss and the pathogenesis of Alzheimer's disease. However, recent loss-of-function mutations in PDGFB and PDGFRB genes have implicated pericytes as the principle cell type affected in primary familiar brain calcification (PFBC), a neuropsychiatric disorder with dominant inheritance. Here we review the role of the PDGFB/PDGFRB signaling pathway in pericyte development and briefly discuss homeostatic functions of pericytes in the brain. We provide an overview of recent studies with mouse models of PFBC and discuss suggested pathogenic mechanisms for PFBC with special reference to pericytes.

Fahr's Disease: A Differential to Be Considered for Various Neuropsychiatric Presentations

Fahr's disease, also known as familial idiopathic basal ganglia calcification, is a neurodegenerative disorder affecting cerebral microvessels, mainly the basal ganglia, and presenting with diverse neuropsychiatric manifestations. It is considered to be mainly hereditary, with autosomal dominant inheritance. In light of its various presentations and incomplete penetrance, Fahr's disease is known to be underestimated and underdiagnosed. Here, an early-onset case of Fahr's disease is presented mainly with pure psychiatric symptoms. Given the diversity of the presenting symptoms, and variations in the age of onset, further investigation of organic etiologies in patients presenting with neuropsychiatric symptoms, family members of patients with Fahr's disease, and patients with unexplained cerebral calcification is recommended.