The Genetics of Primary Familial Brain Calcification: A Literature Review

Molecular and cellular characteristics of cerebrovascular cell types and their contribution to neurodegenerative diseases

Many diseases and disorders of the nervous system suffer from a lack of adequate therapeutics to halt or slow disease progression, and to this day, no cure exists for any of the fatal neurodegenerative diseases. In part this is due to the incredible diversity of cell types that comprise the brain, knowledge gaps in understanding basic mechanisms of disease, as well as a lack of reliable strategies for delivering new therapeutic modalities to affected areas. With the advent of single cell genomics, it is now possible to interrogate the molecular characteristics of diverse cell populations and their alterations in diseased states. More recently, much attention has been devoted to cell populations that have historically been difficult to profile with bulk single cell technologies. In particular, cell types that comprise the cerebrovasculature have become increasingly better characterized in normal and neurodegenerative disease contexts. In this review, we describe the current understanding of cerebrovasculature structure, function, and cell type diversity and its role in the mechanisms underlying various neurodegenerative diseases. We focus on human and mouse cerebrovasculature studies and discuss both origins and consequences of cerebrovascular dysfunction, emphasizing known cell type-specific vulnerabilities in neuronal and cerebrovascular cell populations. Lastly, we highlight how novel insights into cerebrovascular biology have impacted the development of modern therapeutic approaches and discuss outstanding questions in the field.

Astrocytes in Primary Familial Brain Calcification (PFBC): Emphasis on the Importance of Induced Pluripotent Stem Cell-Derived Human Astrocyte Models

Primary familial brain calcification (PFBC) is a rare, progressive central nervous system (CNS) disorder without a cure, and the current treatment methodologies primarily aim to relieve neurological and psychiatric symptoms of the patients. The disease is characterized by abnormal bilateral calcifications in the brain, however, our mechanistic understanding of the biology of the disease is still limited. Determining the roles of the specific cell types and molecular mechanisms involved in the pathophysiological processes of the disease is of great importance for the development of novel and effective treatment methodologies. There is a growing interest in the involvement of astrocytes in PFBC, as recent studies have suggested that astrocytes play a central role in the disease and that functional defects in these cells are critical for the development and progression of the disease. This review aims to discuss recent findings on the roles of astrocytes in PFBC pathophysiology, with a focus on known expression and roles of PFBC genes in astrocytes. Additionally, we discuss the importance of human astrocytes for PFBC disease modeling, and astrocytes as a potential therapeutic target in PFBC. Utilization of species-specific and physiologically relevant PFBC model systems can open new avenues for basic research, drug development, and regenerative medicine.

Early Onset Parkinsonism: Differential diagnosis and what not to miss

Early Onset Parkinsonism (EOP) refers to parkinsonism occurring before the age of 50 years. The causes are diverse and include secondary and genetic causes. Secondary causes related to medications, inflammatory and infective disorders are mostly treatable and well recognized as they usually present with a relatively more rapid clinical course compared to idiopathic Parkinson's disease. Genetic causes of EOP are more challenging to diagnose especially as more of the non-PARK genes are recognized to present with typical and atypical parkinsonism. Some of the genetic disorders such as Spinocerebellar ataxia 2 (SCA2) and Spinocerebellar ataxia 3 (SCA3) may present with levodopa-responsive parkinsonism, indistinguishable from idiopathic Parkinson's disease. Additionally, some of the genetic disorders, including Wilson's disease and cerebrotendinous xanthomatosis (CTX), are potentially treatable and should not be missed. Due to the advent of next generating sequencing techniques, genetic analyses facilitate early identification and proper treatment of diverse causes of EOP. In this review, we outline the clinical approach of EOP highlighting the key clinical features of some of the non-PARK genetic causes of EOP and related investigations, which could assist in clinical diagnosis. This review also encompass genetic diagnostic approaches, emphasizing the significance of pretest counseling and the principles of bioinformatics analysis strategies.

A Homozygous Variant in NAA60 Is Associated with Primary Familial Brain Calcification

BACKGROUND

Primary familial brain calcification (PFBC) is a monogenic disorder characterized by bilateral calcifications in the brain. The genetic basis remains unknown in over half of the PFBC patients, indicating the existence of additional novel causative genes. NAA60 was a recently reported novel causative gene for PFBC.

OBJECTIVE

The aim was to identify the probable novel causative gene in an autosomal recessive inherited PFBC family.

METHODS

We performed a comprehensive genetic study on a consanguineous Chinese family with 3 siblings diagnosed with PFBC. We evaluated the effect of the variant in a probable novel causative gene on the protein level using Western blot, immunofluorescence, and coimmunoprecipitation. Possible downstream pathogenic mechanisms were further explored in gene knockout (KO) cell lines and animal models.

RESULTS

We identified a PFBC co-segregated homozygous variant of c.460_461del (p.D154Lfs*113) in NAA60. Functional assays showed that this variant disrupts NAA60 protein localization to Golgi and accelerated protein degradation. The mutant NAA60 protein alters its interaction with the PFBC-related proteins PiT2 and XPR1, affecting intracellular phosphate homeostasis. Further mass spectrometry analysis in NAA60 KO cell lines revealed decreased expression of multiple brain calcification-associated proteins, including reduced folate carrier (RFC), a folate metabolism-related protein.

CONCLUSIONS

Our study replicated the identification of NAA60 as a novel causative gene for autosomal recessive PFBC, demonstrating our causative variant leads to NAA60 loss of function. The NAA60 loss of function disrupts not only PFBC-related proteins (eg, PiT2 and XPR1) but also a wide range of other brain calcification-associated membrane protein substrates (eg, RFC), and provided a novel probable pathogenic mechanism for PFBC. © 2024 International Parkinson and Movement Disorder Society.

JAM2 variants can be more common in primary familial brain calcification (PFBC) cases than those appear; may be due to a founder mutation

INTRODUCTION

Mutations in JAM2 have been linked to ~ 2% of primary familial brain calcification (PFBC) cases. PFBC is a rare neurological disorder characterized by excessive calcium deposition in the brain. It causes movement disorders and psychiatric problems. Six other genes were identified as causing PFBC. However, the genetic basis of ~ 50% of PFBC cases remains unknown. This study presented the results of a comprehensive analysis of five unrelated Iranian PFBC families.

METHODS

Clinical and paraclinical features of all patients were recorded. Whole-exome sequencing (WES) was done on the DNAs of probands. Data was analyzed, and haplotypes were determined.

RESULTS

WES identified two homozygous variants in JAM2 across four families: a novel variant, c.426dup:p.Ser143Leufs*23, in one family and a known mutation, c.685C > T:p.Arg229*, in the remaining three families. Haplotype analysis using six intragenic single-nucleotide polymorphisms (SNPs) in JAM2 revealed an identical haplotype in probands who carried the same mutation, whereas two other probands presented diverse haplotypes.

CONCLUSION

Based on our results, p.Arg229* may be a founder mutation in the Iranian population. The variant has been detected in two out of seven other reported JAM2-related families who may originate from the Middle East and exhibit an identical haplotype. Even though this particular mutation may not be classified as a founder mutation, it does appear to be a hotspot, given that it has been observed in 45% of the 11 JAM2-associated families. Our study expanded the clinical features and mutation spectrum of JAM2 and revealed that mutations in JAM2 may be more common than previously reported.

Non-Motor Symptoms in Primary Familial Brain Calcification

Background/Objectives: Primary Familial Brain Calcification is a rare neurodegenerative disorder of adulthood characterized by calcium deposition in the basal ganglia and other brain areas; the main clinical manifestations include movement disorders, mainly parkinsonism. Non-motor symptoms are not well defined in PFBC. This work aims at defining the burden of non-motor symptoms in PFBC. Methods: A clinical, genetic and neuropsychological evaluation of a cohort of PFBC patients, COMPASS-31 scale administration. Results: A total of 50 PFBC patients were recruited; in 25, the genetic test was negative; 10 carried mutations in SLC20A2 gene, 8 in MYORG, 3 in PDGFB, 1 in PDGFRB, 2 in JAM2 (single mutations), and one test is still ongoing. The main motor manifestation was parkinsonism. Headache was reported in 26% of subjects (especially in PDGFB mutation carriers), anxiety or depression in 62%, psychosis or hallucinations in 10-12%, sleep disturbances in 34%; 14% of patients reported hyposmia, 32% constipation, and 34% urinary disturbances. A neuropsychological assessment revealed cognitive involvement in 56% (sparing memory functions, to some extent). The COMPASS-31 mean score was 20.6, with higher sub-scores in orthostatic intolerance and gastrointestinal problems. MYORG patients and subjects with cognitive decline tended to have higher scores and bladder involvement compared to other groups. Conclusions: The presence of non-motor symptoms is frequent in PFBC and should be systematically assessed to better meet patients' needs.

Case report: Primary familial brain calcification associated with a rare PDGFRB variant, coexisting with nontraumatic osteonecrosis of the femoral head

Primary familial brain calcification (PFBC) is a rare genetic neurodegenerative disorder characterized by bilateral calcifications in the brain. PFBC may manifest with a broad spectrum of motor, cognitive, and neuropsychiatric symptoms. Several causal genes have been identified in PFBC, which are inherited as both autosomal dominant and autosomal recessive traits. Herein, we present the case of a Chinese family diagnosed with PFBC. The family members carry a rare heterozygous variant (p. R334Q) in exon 7 of platelet-derived growth factor receptor β (PDGFRB) gene. The platelet-derived growth factor-B/PDGF receptor β (PDGF-B/PDGFRβ) signaling pathway plays a crucial role in pericyte development in various organs and tissues. Notably, this variant uniquely coexists with nontraumatic osteonecrosis of the femoral head. Additionally, we reviewed previous studies on PFBC-causing variants in PDGFRB.

Exploring the Biological Overlapping Between Brain Calcifications and Tumorgenesis

This article discusses a rare case of coexistent meningiomas and Primary familial brain calcification (PFBC). PFBC is a neurodegenerative disease characterized by brain calcifications and a variety of neuropsychiatric symptoms and signs, with pathogenic variants in specific genes. The study explores the potential link between PFBC and meningiomas, highlighting shared features like intralesional calcifications and common genes such as MEA6. The article also revisits PFBC patients developing other brain tumors, particularly gliomas, emphasizing the intersection of oncogenes like PDGFB and PDGFRB in both calcifications and tumor progression. In recent investigations, attention has extended beyond brain tumors to breast cancer metastasis, unveiling a noteworthy connection. These findings suggest a broader connection between brain calcifications and tumors, encouraging a reevaluation of therapeutic approaches for PFBC.

XPR1: a regulator of cellular phosphate homeostasis rather than a Pi exporter

Phosphate (Pi) is an essential nutrient, and its plasma levels are under tight hormonal control. Uphill transport of Pi into cells is mediated by the two Na-dependent Pi transporter families SLC34 and SLC20. The molecular identity of a potential Pi export pathway is controversial, though XPR1 has recently been suggested by Giovannini and coworkers to mediate Pi export. We expressed XPR1 in Xenopus oocytes to determine its functional characteristics. Xenopus isoforms of proteins were used to avoid species incompatibility. Protein tagging confirmed the localization of XPR1 at the plasma membrane. Efflux experiments, however, failed to detect translocation of Pi attributable to XPR1. We tested various counter ions and export medium compositions (pH, plasma) as well as potential protein co-factors that could stimulate the activity of XPR1, though without success. Expression of truncated XPR1 constructs and individual domains of XPR1 (SPX, transmembrane core, C-terminus) demonstrated downregulation of the uptake of Pi mediated by the C-terminal domain of XPR1. Tethering the C-terminus to the transmembrane core changed the kinetics of the inhibition and the presence of the SPX domain blunted the inhibitory effect. Our observations suggest a regulatory role of XPR1 in cellular Pi handling rather than a function as Pi exporter. Accordingly, XPR1 senses intracellular Pi levels via its SPX domain and downregulates cellular Pi uptake via the C-terminal domain. The molecular identity of a potential Pi export protein remains therefore elusive.

Infectious Diseases and Basal Ganglia Calcifications: A Cross-Sectional Study in Patients with Fahr's Disease and Systematic Review

Background: It is unclear whether patients with basal ganglia calcifications (BGC) should undergo infectious disease testing as part of their diagnostic work-up. We investigated the occurrence of possibly associated infections in patients with BGC diagnosed with Fahr's disease or syndrome and consecutively performed a systematic review of published infectious diseases associated with BGC. Methods: In a cross-sectional study, we evaluated infections in non-immunocompromised patients aged ≥ 18 years with BGC in the Netherlands, who were diagnosed with Fahr's disease or syndrome after an extensive multidisciplinary diagnostic work-up. Pathogens that were assessed included the following: Brucella sp., cytomegalovirus, human herpesvirus type 6/8, human immunodeficiency virus (HIV), Mycobacterium tuberculosis, rubella virus, and Toxoplasma gondii. Next, a systematic review was performed using MEDLINE and Embase (2002-2023). Results: The cross-sectional study included 54 patients (median age 65 years). We did not observe any possible related infections to the BGC in this population. Prior infection with Toxoplasma gondii occurred in 28%, and in 94%, IgG rubella antibodies were present. The positive tests were considered to be incidental findings by the multidisciplinary team since these infections are only associated with BGC when congenitally contracted and all patients presented with adult-onset symptoms. The systematic search yielded 47 articles, including 24 narrative reviews/textbooks and 23 original studies (11 case series, 6 cross-sectional and 4 cohort studies, and 2 systematic reviews). Most studies reported congenital infections associated with BGC (cytomegalovirus, HIV, rubella virus, Zika virus). Only two studies reported acquired pathogens (chronic active Epstein-Barr virus and Mycobacterium tuberculosis). The quality of evidence was low. Conclusions: In our cross-sectional study and systematic review, we found no convincing evidence that acquired infections are causing BGC in adults. Therefore, we argue against routine testing for infections in non-immunocompromised adults with BGC in Western countries.

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.

Generation and characterization of human induced pluripotent stem cell line METUi002-A from a patient with primary familial brain calcification (PFBC) carrying a heterozygous mutation (c.687dupT (p.Val230CysfsTer28)) in the SLC20A2 gene

Primary familial brain calcification (PFBC) is a rare neurological condition characterized by abnormal calcification commonly in basal ganglia and multiple other brain regions, leading to neuropsychiatric, cognitive, and motor symptoms. SLC20A2, one of the known causative genes for PFBC, contains the highest number of variants directly associated with the disease. Here, we established an iPSC line (METUi002-A) from the peripheral blood mononuclear cells of a clinically diagnosed PFBC patient carrying a SLC20A2 mutation (c.687dupT) using the integration-free Sendai reprogramming. METUi002-A can serve as a valuable tool to generate cellular models to investigate the mechanistic effects of this mutation in PFBC.