Primary brain calcification: an international study reporting novel variants and associated phenotypes

Upstream open reading frame-introducing variants in patients with primary familial brain calcification

More than 50% of patients with primary familial brain calcification (PFBC), a rare neurological disorder, remain genetically unexplained. While some causative genes are yet to be identified, variants in non-coding regions of known genes may represent a source of missed diagnoses. We hypothesized that 5'-Untranslated Region (UTR) variants introducing an AUG codon may initiate mRNA translation and result in a loss of function in some of the PFBC genes. After reannotation of exome sequencing data of 113 unrelated PFBC probands, we identified two upstream AUG-introducing variants in the 5'UTR of PDGFB. One, NM_002608.4:c.-373C>G, segregated with PFBC in the family. It was predicted to create an upstream open reading frame (ORF). The other one, NM_002608.4:c.-318C>T, was found in a simplex case. It was predicted to result in an ORF overlapping the natural ORF with a frameshift. In a GFP reporter assay, both variants were associated with a dramatic decrease in GFP levels, and, after restoring the reading frame with the GFP sequence, the c.-318C>T variant was associated with a strong initiation of translation as measured by western blotting. Overall, we found upstream AUG-introducing variants in the 5'UTR of PDGFB in 2/113 (1.7%) undiagnosed PFBC cases. Such variants thus represent a source of putative pathogenic variants.

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.

Frequency of Hereditary and GBA1-Related Parkinsonism in Latin America: A Systematic Review and Meta-Analysis

BACKGROUND

Identifying hereditary parkinsonism is valuable for diagnosis, genetic counseling, patient prioritization in trials, and studying the disease for personalized therapies. However, most studies were conducted in Europeans, and limited data exist on admixed populations like those from Latin America.

OBJECTIVES

This study aims to assess the frequency and distribution of genetic parkinsonism in Latin America.

METHODS

We conducted a systematic review and meta-analysis of the frequency of parkinsonian syndromes associated with genetic pathogenic variants in Latin America. We defined hereditary parkinsonism as those caused by the genes outlined by the MDS Nomenclature of Genetic Movement Disorders and heterozygous carriers of GBA1 pathogenic variants. A systematic search was conducted in PubMed, Web of Science, Embase, and LILACS in August 2022. Researchers reviewed titles and abstracts, and disagreements were resolved by a third researcher. After this screening, five researchers reanalyzed the selection criteria and extracted information based on the full paper. The frequency for each parkinsonism-related gene was determined by the presence of pathogenic/likely pathogenic variants among screened patients. Cochran's Q and I2 tests were used to quantify heterogeneity. Meta-regression, publication bias tests, and sensitivity analysis regarding study quality were also used for LRRK2-, PRKN-, and GBA1-related papers.

RESULTS

We included 73 studies involving 3014 screened studies from 16 countries. Among 7668 Latin American patients, pathogenic variants were found in 19 different genes. The frequency of the pathogenic variants in LRRK2 was 1.38% (95% confidence interval [CI]: 0.52-2.57), PRKN was 1.16% (95% CI: 0.08-3.05), and GBA1 was 4.17% (95% CI: 2.57-6.08). For all meta-analysis, heterogeneity was high and publication bias tests were negative, except for PRKN, which was contradictory. Information on the number of pathogenic variants in the other genes is further presented in the text.

CONCLUSIONS

This study provides insights into hereditary and GBA1-related parkinsonism in Latin America. Lower GBA1 frequencies compared to European/North American cohorts may result from limited access to gene sequencing. Further research is vital for regional prevalence understanding, enabling personalized care and therapies. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Homozygous SLC20A2 mutations cause congenital CMV infection-like phenotype

BACKGROUND

Idiopathic basal ganglia calcification, also known as Fahr's disease, it is a neurological disease characterized by intracranial calcification caused by heterozygous SLC20A2 mutations. Patients with calcifications can either be asymptomatic or show a wide spectrum of neuropsychiatric symptoms, including parkinsonism, tremor, dystonia, ataxia, and seizures.

OBJECTIVES

The aim of this study was to investigating the clinical implications of the SLCA20A2 gene and identifying a new phenotype through a family.

METHODS

Two siblings with growth retardation, bilateral cataracts, microcephaly, and convulsion were included in the study. The MRI showed cerebral atrophy, corpus callosum hypoplasia, microcalcifications. Chromosomal microarray analysis was performed to identify the existence of copy number variation. The whole exome sequencing analysis of the individual IV-I was performed, and Sanger sequencing was performed for segregation.

RESULTS

Whole exome sequencing revealed a homozygous NM_006749.5:c.1794 + 1G > A of the SLC20A2 gene. The Sanger sequencing confirmed the affected siblings were homozygous and the parents were heterozygous.

CONCLUSIONS

SLC20A2 gene heterozygous mutations were associated with the adult-onset phenotype, while homozygous SLC20A2 mutations in the two affected siblings we reported in our study resulted in a severe clinic including growth retardation, bilateral cataracts, microcephaly, and convulsion. We showed that biallelic mutations in the SLC20A2 gene that cause the Fahr's disease lead to more severe phenotypes contrary to what is known. The two siblings, showing similar phonotypic and genotypic characteristics, would be the youngest cases in the pediatric age group published in the literature.

A case report of a patient with primary familial brain calcification with a PDGFRB genetic variant

Fahr's disease, or primary familial brain calcification (PFBC), is a rare genetic neurologic disease characterized by abnormal calcification of the basal ganglia, subcortical white matter and cerebellum. Common clinical features include parkinsonism, neuropsychiatric symptoms, and cognitive decline. Genes implicated in Fahr's disease include PDGFB, PDGFRB, SLC20A2, XPR1, MYORG, and JAM2. We present the case of a 51-year-old woman who developed subacute cognitive and behavioral changes primarily affecting frontal-subcortical pathways and parkinsonism in association with extensive bilateral calcifications within the basal ganglia, subcortical white matter, and cerebellum on neuroimaging. Relevant family history included a paternal aunt with parkinsonism at age 50. Normal parathyroid hormone and calcium levels in the patient's serum ruled out hypoparathyroidism or pseudohypoparathyroidism as causes for the intracranial calcifications. Genetic panel sequencing revealed a variant of unknown significance in the PDGFRB gene resulting in a p.Arg919Gln substitution in the tyrosine kinase domain of PDGFRB protein. To our knowledge this is the first report of a p.Arg919Gln variant in the PDGFRB gene associated with PFBC. Although co-segregation studies were not possible in this family, the location of the variant is within the tyrosine kinase domain of PDGFRB and pathogenicity calculators predict it is likely to be pathogenic. This report adds to the list of genetic variants that warrant functional analysis and could underlie the development of PFBC, which may help to further our understanding of its pathogenesis and the development of targeted therapies for this disorder.

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.

Brain Calcifications: Genetic, Molecular, and Clinical Aspects

Many conditions can present with accumulation of calcium in the brain and manifest with a variety of neurological symptoms. Brain calcifications can be primary (idiopathic or genetic) or secondary to various pathological conditions (e.g., calcium-phosphate metabolism derangement, autoimmune disorders and infections, among others). A set of causative genes associated with primary familial brain calcification (PFBC) has now been identified, and include genes such as SLC20A2, PDGFB, PDGFRB, XPR1, MYORG, and JAM2. However, many more genes are known to be linked with complex syndromes characterized by brain calcifications and additional neurologic and systemic manifestations. Of note, many of these genes encode for proteins involved in cerebrovascular and blood-brain barrier functions, which both represent key anatomical structures related to these pathological phenomena. As a growing number of genes associated with brain calcifications is identified, pathways involved in these conditions are beginning to be understood. Our comprehensive review of the genetic, molecular, and clinical aspects of brain calcifications offers a framework for clinicians and researchers in the field.

Physiological regulation of phosphate homeostasis

Phosphate homeostasis is dependent on the interaction and coordination of four main organ systems: thyroid/parathyroids, gastrointestinal tract, bone and kidneys, and three key hormonal regulators, 1,25-hydroxyvitamin D3, parathyroid hormone and FGF23 with its co- factor klotho. Phosphorus is a critical nutritional element for normal cellular function, but in excess can be toxic to tissues, particularly the vasculature. As phosphate, it also has an important interaction and inter-dependence with calcium and calcium homeostasis sharing some of the same controlling hormones, although this is not covered in our article. We have chosen to provide a current overview of phosphate homeostasis only, focusing on the role of two major organ systems, the gastrointestinal tract and kidneys, and their contribution to the control of phosphate balance. We describe in some detail the mechanisms of intestinal and renal phosphate transport, and compare and contrast their regulation. We also consider a significant example of phosphate imbalance, with phosphate retention, which is chronic kidney disease; why consequent hyperphosphatemia is important, and some of the newer means of managing it.

Distinct functional classes of PDGFRB pathogenic variants in primary familial brain calcification

Platelet-derived growth factor receptor beta (PDGFRB) is one of the genes associated with primary familial brain calcification (PFBC), an inherited neurological disease (OMIM:173410). Genetic analysis of patients and families revealed at least 13 PDGFRB heterozygous missense variants, including two novel ones described in the present report. Limited experimental data published on five of these variants had suggested that they decrease the receptor activity. No functional information was available on the impact of variants located within the receptor extracellular domains. Here, we performed a comprehensive molecular analysis of PDGFRB variants linked to PFBC. Mutated receptors were transfected in various cell lines to monitor receptor expression, signaling, mitogenic activity, and ligand binding. Four mutants caused a complete loss of tyrosine kinase activity in multiple assays. One of the novel variants, p.Pro154Ser, decreased the receptor expression and abolished binding of platelet-derived growth factor (PDGF-BB). Others showed a partial loss of function related to reduced expression or signaling. Combining clinical, genetic and molecular data, we consider nine variants as pathogenic or likely pathogenic, three as benign or likely benign and one as a variant of unknown significance. We discuss the possible relationship between the variant residual activity, incomplete penetrance, brain calcification and neurological symptoms. In conclusion, we identified distinct molecular mechanisms whereby PDGFRB variants may result in a receptor loss of function. This work will facilitate genetic counselling in PFBC.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

MiR-9-5p Regulates Genes Linked to Cerebral Calcification in the Osteogenic Differentiation Model and Induces Generalized Alteration in the Ion Channels

MicroRNA-9 (miR-9) modulates gene expression and demonstrates high structural conservation and wide expression in the central nervous system. Bioinformatics analysis predicts almost 100 ion channels, membrane transporters and receptors, including genes linked to primary familial brain calcification (PFBC), as possible miR-9-5p targets. PFBC is a neurodegenerative disorder, characterized by bilateral and symmetrical calcifications in the brain, associated with motor and behavioral disturbances. In this work, we seek to study the influence of miR-9-5p in regulating genes involved in PFBC, in an osteogenic differentiation model with SaOs-2 cells. During the induced calcification process, solute carrier family 20 member 2 (SLC20A2) and platelet-derived growth factor receptor beta (PDGFRB) were downregulated, while platelet-derived growth factor beta (PDGFB) showed no significant changes. Significantly decreased levels of SLC20A2 and PDGFRB were caused by the presence of miR-9-5p, while PDGFB showed no regulation. We confirmed the findings using an miR-9-5p inhibitor and also probed the cells in electrophysiological analysis to assess whether such microRNA might affect a broader range of ion channels, membrane transporters and receptors. Our electrophysiological data show that an increase of the miR-9-5p in SaOs-2 cells decreased the density and amplitude of the output ionic currents, indicating that it may influence the activity, and perhaps the expression, of some ionic channels. Additional investigations should determine whether such an effect is specific to miR-9-5p, and whether it could be used, together with the miR-9-5p inhibitor, as a therapeutic or diagnostic tool.

Case Report: Two Novel Frameshift Mutations in SLC20A2 and One Novel Splice Donor Mutation in PDGFB Associated With Primary Familial Brain Calcification

Primary familial brain calcification (PFBC, OMIM#213600), also known as Fahr's disease, is characterized by bilateral and symmetric brain calcification in the basal ganglia (globus pallidus, caudate nucleus, and putamen), thalamus, subcortical white matter, and cerebellum. PFBC can be caused by loss-of-function mutations in any of the six known causative genes. The most common clinical manifestations include movement disorders, cognitive impairment, and neuropsychiatric signs that gradually emerge in middle-aged patients. To broaden the PFBC mutation spectrum, we examined nine members of a family with PFBC and two sporadic cases from clinical departments, and sequenced all PFBC-causative genes in the index case. Two novel frameshift mutations in SLC20A2 [NM_001257180.2; c.806delC, p.(Pro269Glnfs^*49) and c.1154delG, p.(Ser385Ilefs^*70)] and one novel splice donor site mutation (NM_002608.4, c.456+1G>C, r.436_456del) in PDGFB were identified in the patient cohort. c.806delC co-segregated with brain calcification and led to SLC20A2 haploinsufficiency among the affected family members. The c.456+1G>C mutation in PDGFB resulted in aberrant mRNA splicing, thereby forming mature transcripts containing an in-frame 21 base pair (bp) deletion, which might create a stably truncated protein [p.(Val146_Gln152del)] and exert a dominant negative effect on wild-type PDGFB. All three mutations were located in highly conserved regions among multiple species and predicted to be pathogenic, as evaluated by at least eight common genetic variation scoring systems. This study identified three novel mutations in SLC20A2 and PDGFB, which broadened and enriched the PFBC mutation spectrum.

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.

Expanding the genetic spectrum of primary familial brain calcification due to SLC2OA2 mutations: a case series

Primary familial brain calcification (PFBC) is a neurological condition characterized by the presence of intracranial calcifications, mainly involving basal ganglia, thalamus, and dentate nuclei. So far, six genes have been linked to this condition: SLC20A2, PDGFRB, PDGFB, and XPR1 inherited as autosomal-dominant trait, while MYORG and JAM2 present a recessive pattern of inheritance. Patients mainly present with movement disorders, psychiatric disturbances, and cognitive decline or are completely asymptomatic and calcifications may represent an occasional finding. Here we present three variants in SLC20A2, two exonic and one intronic, which we found in patients with PFBC associated to three different clinical phenotypes. One variant is novel and two were already described as variants of uncertain significance. We confirm the pathogenicity of these three variants and suggest a broadening of the phenotypic spectrum associated with mutations in SLC20A2.

Review of Hereditary and Acquired Rare Choreas

Background:

Movement disorders are often a prominent part of the phenotype of many neurologic rare diseases. In order to promote awareness and diagnosis of these rare diseases, the International Parkinson’s and Movement Disorders Society Rare Movement Disorders Study Group provides updates on rare movement disorders.

Methods:

In this narrative review, we discuss the differential diagnosis of the rare disorders that can cause chorea.

Results:

Although the most common causes of chorea are hereditary, it is critical to identify acquired or symptomatic choreas since these are potentially treatable conditions. Disorders of metabolism and mitochondrial cytopathies can also be associated with chorea.

Discussion:

The present review discusses clues to the diagnosis of chorea of various etiologies. Authors propose algorithms to help the clinician in the diagnosis of these rare disorders.

Interplay between primary familial brain calcification-associated SLC20A2 and XPR1 phosphate transporters requires inositol polyphosphates for control of cellular phosphate homeostasis

Solute carrier family 20 member 2 (SLC20A2) and xenotropic and polytropic retrovirus receptor 1 (XPR1) are transporters with phosphate uptake and efflux functions, respectively. Both are associated with primary familial brain calcification (PFBC), a genetic disease characterized by cerebral calcium-phosphate deposition and associated with neuropsychiatric symptoms. The association of the two transporters with the same disease suggests that they jointly regulate phosphate fluxes and cellular homeostasis, but direct evidence is missing. Here, we found that cross-talk between SLC20A2 and XPR1 regulates phosphate homeostasis, and we identified XPR1 as a key inositol polyphosphate (IP)-dependent regulator of this process. We found that overexpression of WT SLC20A2 increased phosphate uptake, as expected, but also unexpectedly increased phosphate efflux, whereas PFBC-associated SLC20A2 variants did not. Conversely, SLC20A2 depletion decreased phosphate uptake only slightly, most likely compensated for by the related SLC20A1 transporter, but strongly decreased XPR1-mediated phosphate efflux. The SLC20A2-XPR1 axis maintained constant intracellular phosphate and ATP levels, which both increased in XPR1 KO cells. Elevated ATP levels are a hallmark of altered inositol pyrophosphate (PP-IP) synthesis, and basal ATP levels were restored after phosphate efflux rescue with WT XPR1 but not with XPR1 harboring a mutated PP-IP-binding pocket. Accordingly, inositol hexakisphosphate kinase 1-2 (IP6K1-2) gene inactivation or IP6K inhibitor treatment abolished XPR1-mediated phosphate efflux regulation and homeostasis. Our findings unveil an SLC20A2-XPR1 interplay that depends on IPs such as PP-IPs and controls cellular phosphate homeostasis via the efflux route, and alteration of this interplay likely contributes to PFBC.

Haploinsufficiency of the Primary Familial Brain Calcification Gene SLC20A2 Mediated by Disruption of a Regulatory Element

OBJECTIVE

Primary familial brain calcification (PFBC) is a rare cerebral microvascular calcifying disorder with diverse neuropsychiatric expression. Five genes were reported as PFBC causative when carrying pathogenic variants. Haploinsufficiency of SLC20A2, which encodes an inorganic phosphate importer, is a major cause of autosomal-dominant PFBC. However, PFBC remains genetically unexplained in a proportion of patients, suggesting the existence of additional genes or cryptic mutations. We analyzed exome sequencing data of 71 unrelated, genetically unexplained PFBC patients with the aim to detect copy number variations that may disrupt the expression of core PFBC-causing genes.

METHODS

After the identification of a deletion upstream of SLC20A2, we assessed its consequences on gene function by reverse transcriptase droplet digital polymerase chain reaction (RT-ddPCR), an ex vivo inorganic phosphate uptake assay, and introduced the deletion of a putative SLC20A2 enhancer mapping to this region in human embryonic kidney 293 (HEK293) cells by clustered regularly interspaced short palindromic repeats (CRISPR) - CRISPR-associated protein 9 (Cas9).

RESULTS

The 8p11.21 deletion, segregating with PFBC in a family, mapped 35 kb upstream of SLC20A2. The deletion carriers/normal controls ratio of relative SLC20A2 mRNA levels was 60.2% (P < 0.001). This was comparable with that of patients carrying an SLC20A2 premature stop codon (63.4%; P < 0.001). The proband exhibited a 39.3% decrease of inorganic phosphate uptake in blood (P = 0.015). In HEK293 cells, we observed a 39.8% decrease in relative SLC20A2 mRNA levels after normalization on DNA copy number (P < 0.001).

DISCUSSION

We identified a deletion of an enhancer of SLC20A2 expression, with carriers showing haploinsufficiency in similar ranges to loss-of-function alleles, and we observed reduced mRNA levels after deleting this element in a cellular model. We propose a 3-step strategy to identify and easily assess the effect of such events. © 2020 International Parkinson and Movement Disorder Society.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Primary familial brain calcifications: genetic and clinical update

PURPOSE OF REVIEW

In the last 7 years, changes in five genes [SLC20A2, PDGFRB, PDGFB, XPR1, and MYORG] have been implicated in the pathogenesis of primary familial brain calcification (PFBC), allowing for genetic delineation of this phenotypically complex neurodegenerative disorder. This review explores how the ensuing plethora of reported PFBC patients and their disease-causing variants improved our understanding of disease, pathogenesis, clinical manifestation, and penetrance.

RECENT FINDINGS

In PFBC patients, pathogenic changes have been most frequently described in SLC20A2, accounting for approximately the same number of patients as the variants in the other four PFBC genes combined. There is no appreciable relationship between any combination of the following three variables: the type of disease-causing change, the pattern or extent of calcifications, and the presence or nature of clinical manifestation in PFBC patients. Nevertheless, elucidation of underlying genetic factors provided important recent insights into the pathogenic mechanisms of PFBC, which collectively point toward a compromised neurovascular unit.

SUMMARY

The ongoing clinical and molecular research increases our understanding of PFBC facilitating diagnosis and identifying potential therapeutic targets for this multifaceted and likely underdiagnosed condition.

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.

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.

Clinical and neuroimaging phenotypes of genetic parkinsonism from infancy to adolescence

Genetic early-onset parkinsonism presenting from infancy to adolescence (≤21 years old) is a clinically diverse syndrome often combined with other hyperkinetic movement disorders, neurological and imaging abnormalities. The syndrome is genetically heterogeneous, with many causative genes already known. With the increased use of next-generation sequencing in clinical practice, there have been novel and unexpected insights into phenotype-genotype correlations and the discovery of new disease-causing genes. It is now recognized that mutations in a single gene can give rise to a broad phenotypic spectrum and that, conversely different genetic disorders can manifest with a similar phenotype. Accurate phenotypic characterization remains an essential step in interpreting genetic findings in undiagnosed patients. However, in the past decade, there has been a marked expansion in knowledge about the number of both disease-causing genes and phenotypic spectrum of early-onset cases. Detailed knowledge of genetic disorders and their clinical expression is required for rational planning of genetic and molecular testing, as well as correct interpretation of next-generation sequencing results. In this review we examine the relevant literature of genetic parkinsonism with ≤21 years onset, extracting data on associated movement disorders as well as other neurological and imaging features, to delineate syndromic patterns associated with early-onset parkinsonism. Excluding PRKN (parkin) mutations, &gt;90% of the presenting phenotypes have a complex or atypical presentation, with dystonia, abnormal cognition, pyramidal signs, neuropsychiatric disorders, abnormal imaging and abnormal eye movements being the most common features. Furthermore, several imaging features and extraneurological manifestations are relatively specific for certain disorders and are important diagnostic clues. From the currently available literature, the most commonly implicated causes of early-onset parkinsonism have been elucidated but diagnosis is still challenging in many cases. Mutations in ∼70 different genes have been associated with early-onset parkinsonism or may feature parkinsonism as part of their phenotypic spectrum. Most of the cases are caused by recessively inherited mutations, followed by dominant and X-linked mutations, and rarely by mitochondrially inherited mutations. In infantile-onset parkinsonism, the phenotype of hypokinetic-rigid syndrome is most commonly caused by disorders of monoamine synthesis. In childhood and juvenile-onset cases, common genotypes include PRKN, HTT, ATP13A2, ATP1A3, FBX07, PINK1 and PLA2G6 mutations. Moreover, Wilson’s disease and mutations in the manganese transporter are potentially treatable conditions and should always be considered in the differential diagnosis in any patient with early-onset parkinsonism.

Partial reduced Pi transport function of PiT-2 might not be sufficient to induce brain calcification of idiopathic basal ganglia calcification

Idiopathic basal ganglia calcification (IBGC) is a rare intractable disease characterized by abnormal mineral deposits, including mostly calcium in the basal ganglia, thalamus, and cerebellum. SLC20A2 is encoding the phosphate transporter PiT-2 and was identified in 2012 as the causative gene of familial IBGC. In this study, we investigated functionally two novel SLC20A2 variants (c.680C > T, c.1487G > A) and two SLC20A2 variants (c.82G > A, c.358G > C) previously reported from patients with IBGC. We evaluated the function of variant PiT-2 using stable cell lines. While inorganic phosphate (Pi) transport activity was abolished in the cells with c.82G > A, c.358G > C, and c.1487G > A variants, activity was maintained at 27.8% of the reference level in cells with the c.680C > T variant. Surprisingly, the c.680C > T variant had been discovered by chance in healthy members of an IBGC family, suggesting that partial preservation of Pi transport activity may avoid the onset of IBGC. In addition, we confirmed that PiT-2 variants could be translocated into the cell membrane to the same extent as PiT-2 wild type. In conclusion, we investigated the PiT-2 dysfunction of four SLC20A2 variants and suggested that a partial reduced Pi transport function of PiT-2 might not be sufficient to induce brain calcification of IBGC.

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.

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.

Characterization of XPR1/SLC53A1 variants located outside of the SPX domain in patients with primary familial brain calcification

Primary familial brain calcification (PFBC) is a rare neurological disease characterized by deposits of calcium phosphate in the basal ganglia and other regions of the brain. Pathogenic variants in the XPR1/SLC53A1 gene, which encodes the only known inorganic phosphate exporter, cause an autosomal dominant form of PFBC. These variants are typically located in the SPX N-terminal domain of the protein. Here, we characterize three XPR1 variants outside of SPX in three PFBC patients with an apparently sporadic presentation: c.1375C > T p.(R459C), c.1855A > G p.(N619D) and c.1886T > G p.(I629S), with the latter identified as the first XPR1/SLC53A1 de novo mutation to occur in a PFBC proband. When tested in an in vitro physiological complementation assay, the three XPR1 variants were impaired in phosphate export function, although they were normally expressed at the cell surface and could serve as functional receptors for retrovirus entry. Moreover, peripheral blood cells from the p.N619D patient could be assayed ex vivo and displayed significantly impaired phosphate export. Our results establish for the first time the clinical and molecular characteristics of XPR1 variants located outside the SPX domain and assert a direct link between these variants, deficient phosphate export, and PFBC. Moreover, we unveiled new structural features in XPR1 C-terminal domain that play a role in phosphate export and disease.

A novel PDGFRB sequence variant in a family with a mild form of primary familial brain calcification: a case report and a review of the literature

Background

Primary familial brain calcification is a rare autosomal dominant or recessive neurodegenerative disease, characterized by bilateral brain calcifications in different areas of the brain. It is a clinically heterogeneous disease and patients are reported to exhibit a wide spectrum of neurological and psychiatric symptoms. Mutations in five genes have been identified so far including SLC20A2, PDGFRB, PDGFB, XPR1, and MYORG. PDGFRB encodes the platelet-derived growth factor receptor-beta, and is expressed in neurons, vascular smooth muscle cells and pericytes. Patients with a PDGFRB mutation seem to exhibit a milder phenotype and milder brain calcification on brain imaging than patients with SLC20A2 and PDGFB mutations. However, this is based on a few observations so far.

Case presentation

We present a Danish family with bilateral brain calcifications and mild clinical symptoms of primary familial brain calcification, segregating with a novel PDGFRB sequence variant: c.1834G > A; p.(Gly612Arg), detected by whole exome sequencing. The variant results in physiochemical changes at the amino acid level, and affects a highly conserved nucleotide as well as amino acid. It is located in the tyrosine kinase domain of PDGFRβ. Segregation analysis and in silico analyses predicted the missense variant to be disease causing.

Conclusion

Our study confirms that PDGFRB mutation carriers in general have a mild clinical phenotype, and basal ganglia calcifications can be detected by a CT scan, also in asymptomatic mutation carriers.

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.

Functional evaluation of PDGFB-variants in idiopathic basal ganglia calcification, using patient-derived iPS cells

Causative genes in patients with idiopathic basal ganglia calcification (IBGC) (also called primary familial brain calcification (PFBC)) have been reported in the past several years. In this study, we surveyed the clinical and neuroimaging data of 70 sporadic patients and 16 families (86 unrelated probands in total) in Japan, and studied variants of PDGFB gene in the patients. Variant analyses of PDGFB showed four novel pathogenic variants, namely, two splice site variants (c.160 + 2T > A and c.457−1G > T), one deletion variant (c.33_34delCT), and one insertion variant (c.342_343insG). Moreover, we developed iPS cells (iPSCs) from three patients with PDGFB variants (c.160 + 2T > A, c.457−1G > T, and c.33_34 delCT) and induced endothelial cells. Enzyme-linked immunoassay analysis showed that the levels of PDGF-BB, a homodimer of PDGF-B, in the blood sera of patients with PDGFB variants were significantly decreased to 34.0% of that of the control levels. Those in the culture media of the endothelial cells derived from iPSCs of patients also significantly decreased to 58.6% of the control levels. As the endothelial cells developed from iPSCs of the patients showed a phenotype of the disease, further studies using IBGC-specific iPSCs will give us more information on the pathophysiology and the therapy of IBGC in the future.

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.

Hypophosphatemic Rickets

Hypophosphatemic rickets, mostly of the X-linked dominant form caused by pathogenic variants of the PHEX gene, poses therapeutic challenges with consequences for growth and bone development and portends a high risk of fractions and poor bone healing, dental problems and nephrolithiasis/nephrocalcinosis. Conventional treatment consists of PO4 supplements and calcitriol requiring monitoring for treatment-emergent adverse effects. FGF23 measurement, where available, has implications for the differential diagnosis of hypophosphatemia syndromes and, potentially, treatment monitoring. Newer therapeutic modalities include calcium sensing receptor modulation (cinacalcet) and biological molecules targeting FGF23 or its receptors. Their long-term effects must be compared with those of conventional treatments.

Spectrum of SLC20A2, PDGFRB, PDGFB, and XPR1 mutations in a large cohort of patients with primary familial brain calcification

Primary familial brain calcification (PFBC) is a rare neurodegenerative disorder with four causative genes (SLC20A2, PDGFRB, PDGFB, and XPR1) that have been identified. Here, we aim to describe the mutational spectrum of four causative genes in a series of 226 unrelated Chinese PFBC patients. Mutations in four causative genes were detected in 16.8% (38/226) of PFBC patients. SLC20A2 mutations accounted for 14.2% (32/226) of all patients. Mutations in the other three genes were relatively rare, accounting for 0.9% (2/226) of all patients, respectively. Clinically, 44.8% of genetically confirmed patients (probands and relatives) were considered symptomatic. The most frequent symptoms were chronic headache, followed by movement disorders and vertigo. Moreover, the total calcification score was significantly higher in the symptomatic group compared to the asymptomatic group. Functionally, we observed impaired phosphate transport induced by seven novel missense mutations in SLC20A2 and two novel mutations in XPR1. The mutation p.D164Y in XPR1 might result in low protein expression through an enhanced proteasome pathway. In conclusion, our study further confirms that mutations in SLC20A2 are the major cause of PFBC and provides additional evidence for the crucial roles of phosphate transport impairment in the pathogenies of PFBC.

Lack of Major Ophthalmic Findings in Patients with Primary Familial Brain Calcification Linked to SLC20A2 and PDGFB

Primary familial brain calcification (PFBC) is a rare neurodegenerative disorder characterized by symmetrical and bilateral brain calcification. It is typically inherited as an autosomal dominant disorder, and de novo variants have also been described. Interestingly, just recent studies have reported the first autosomal recessive PFBC-causative gene. PFBC patients exhibit high clinical heterogeneity including Parkinsonism, dystonia, ataxia, depression, and migraine. Mice studies, an important research tool, have been a breakthrough in increasing the understanding of PFBC's main signs and symptoms, and many findings reported in these mice have been subsequently reported in patients. One phenotype that has been observed in PFBC mice models but not in PFBC patients, however, is the development of ophthalmic abnormalities. This way, this report focused on performing an ophthalmic assessment in six Brazilian patients genetically diagnosed with PFBC. The assessments showed that none of the PFBC individuals included presented any of the ophthalmic abnormalities reported in mice models, such as cataracts, ocular calcification, abnormal iris and lens morphology, and retinal deterioration. Additionally, of the six PFBC patients described, two SLC20A2 mutation carriers showed physiological excavation of the optic nerve head and partial vitreous detachment, while just one individual presented bilateral narrowing of retinal arterioles. In summary, no evidence of similar ophthalmological abnormalities found in mice were found in our patients; nonetheless, further studies in larger sample size are warranted to corroborate with our findings. To our knowledge, this study is the first to focus on investigating, in PFBC patients, the ophthalmological phenotypes described in the PFBC mice models.

Evaluation of MYORG mutations as a novel cause of primary familial brain calcification

BACKGROUND

Very recently, the MYORG gene was identified as a novel causative gene for autosomal-recessive primary familial brain calcification.

OBJECTIVE

To investigate the clinical, genetic, and neuroradiological characteristics of primary familial brain calcification patients with biallelic MYORG mutations in China.

METHODS

We collected clinical and neuroradiological data of 169 Chinese patients with primary familial brain calcification, including 151 sporadic patients and 18 patients from 13 families compatible with an autosomal-recessive mode of inheritance. Mutational analysis of MYORG was performed in the cohort.

RESULTS

We identified four, including three novel, MYORG mutations segregating in four families with 5 patients: one nonsense mutation (c.1431C>A, p.Y477*), one missense mutation (c.687G>T, p.W229C), and two nonframeshift indels (c.348_349insCTGGCCTTCCGC, p.116_117insLAFR; c. 428_442delTGCACTTCTTCATCC, p.143_147delLHFFI). The 12-base-pair insertion, c.348_349insCTGGCCTTCCGC, was found in either homozygous or heterozygous state in 2 probands of our cohort and another Chinese primary familial brain calcification patient previously reported on in the literature. Haplotype analysis of our patients harboring the insertion indicated a founder effect in the ethnic Han Chinese population. To date, biallelic MYORG mutations have been reported in 17 patients (including our cohort). Most patients were symptomatic (13 of 17; 76.5%), and the most recurrent symptoms were movement disorders (10 of 17; 58.8%), cognitive decline (7 of 17; 41.2%), and cerebellar symptoms (6 of 17; 35.3%). All patients had calcifications on comprehensive cranial CT, most frequently located in the basal ganglia (17 of 17; 100%), cerebellum (17 of 17; 100%), subcortical white matter (14 of 17; 82.4%), and thalamus (13 of 17; 76.5%).

CONCLUSIONS

We confirmed MYORG as a novel causative gene for primary familial brain calcification and further expanded the mutational and phenotypic spectrum of MYORG-related primary familial brain calcification. © 2018 International Parkinson and Movement Disorder Society.