Association between a novel mutation in SLC20A2 and familial idiopathic basal ganglia calcification

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.

Clinical and Functional Assessment of Digenicity in Renal Phosphate Wasting

Apart from increased fluid intake, patients with kidney stone disease (KSD) due to renal phosphate wasting require specific metaphylaxis. NaPi2a, NaPi2c, and NHERF1 regulate plasma phosphate concentration by reabsorbing phosphate in proximal kidney tubules and have been found altered in monogenic hypophosphatemia with a risk of KSD. In this study, we aimed at assessing the combined genetic alterations impacting NaPi2a, NaPi2c, and NHERF1. Therefore, we screened our hereditary KSD registry for cases of oligo- and digenicity, conducted reverse phenotyping, and undertook functional studies. As a result, we identified three patients from two families with digenic alterations in NaPi2a, NaPi2c, and NHERF1. In family 1, the index patient, who presented with severe renal calcifications and a bone mineralization disorder, carried digenic alterations affecting both NaPi transporter 2a and 2c. Functional analysis confirmed an additive genetic effect. In family 2, the index patient presented with kidney function decline, distinct musculature-related symptoms, and intracellular ATP depletion. Genetically, this individual was found to harbor variants in both NaPi2c and NHERF1 pointing towards genetic interaction. In summary, digenicity and gene dosage are likely to impact the severity of renal phosphate wasting and should be taken into account in terms of metaphylaxis through phosphate substitution.

Microglia shield the murine brain from damage mediated by the cytokines IL-6 and IFN-α

Sustained production of elevated levels of the cytokines interleukin (IL)-6 or interferon (IFN)-α in the central nervous system (CNS) is detrimental and directly contributes to the pathogenesis of neurological diseases such as neuromyelitis optica spectrum disorders or cerebral interferonopathies, respectively. Using transgenic mice with CNS-targeted production of IL-6 (GFAP-IL6) or IFN-α (GFAP-IFN), we have recently demonstrated that microglia are prominent target and effector cells and mount stimulus-specific responses to these cytokines. In order to further clarify the phenotype and function of these cells, we treated GFAP-IL6 and GFAP-IFN mice with the CSF1R inhibitor PLX5622 to deplete microglia. We examined their ability to recover from acute microglia depletion, as well as the impact of chronic microglia depletion on the progression of disease. Following acute depletion in the brains of GFAP-IL6 mice, microglia repopulation was enhanced, while in GFAP-IFN mice, microglia did not repopulate the brain. Furthermore, chronic CSF1R inhibition was detrimental to the brain of GFAP-IL6 and GFAP-IFN mice and gave rise to severe CNS calcification which strongly correlated with the absence of microglia. In addition, PLX5622-treated GFAP-IFN mice had markedly reduced survival. Our findings provide evidence for novel microglia functions to protect against IFN-α-mediated neurotoxicity and neuronal dysregulation, as well as restrain calcification as a result of both IL-6- and IFN-α-induced neuroinflammation. Taken together, we demonstrate that CSF1R inhibition may be an undesirable target for therapeutic treatment of neuroinflammatory diseases that are driven by elevated IL-6 and IFN-α production.

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.

Slc20a2-Deficient Mice Exhibit Multisystem Abnormalities and Impaired Spatial Learning Memory and Sensorimotor Gating but Normal Motor Coordination Abilities

BACKGROUND

Primary familial brain calcification (PFBC, OMIM#213600), also known as Fahr's disease, is a rare autosomal dominant or recessive neurodegenerative disorder characterized by bilateral and symmetrical microvascular calcifications affecting multiple brain regions, particularly the basal ganglia (globus pallidus, caudate nucleus, and putamen) and thalamus. The most common clinical manifestations include cognitive impairment, neuropsychiatric signs, and movement disorders. Loss-of-function mutations in SLC20A2 are the major genetic causes of PFBC.

OBJECTIVE

This study aimed to investigate whether Slc20a2 knockout mice could recapitulate the dynamic processes and patterns of brain calcification and neurological symptoms in patients with PFBC. We comprehensively evaluated brain calcifications and PFBC-related behavioral abnormalities in Slc20a2-deficient mice.

METHODS

Brain calcifications were analyzed using classic calcium-phosphate staining methods. The Morris water maze, Y-maze, and fear conditioning paradigms were used to evaluate long-term spatial learning memory, working memory, and episodic memory, respectively. Sensorimotor gating was mainly assessed using the prepulse inhibition of the startle reflex program. Spontaneous locomotor activity and motor coordination abilities were evaluated using the spontaneous activity chamber, cylinder test, accelerating rotor-rod, and narrowing balance beam tests.

RESULTS

Slc20a2 homozygous knockout (Slc20a2-HO) mice showed congenital and global developmental delay, lean body mass, skeletal malformation, and a high proportion of unilateral or bilateral eye defects. Brain calcifications were detected in the hypothalamus, ventral thalamus, and midbrain early at postnatal day 80 in Slc20a2-HO mice, but were seldom found in Slc20a2 heterozygous knockout (Slc20a2-HE) mice, even at extremely old age. Slc20a2-HO mice exhibited spatial learning memory impairments and sensorimotor gating deficits while exhibiting normal working and episodic memories. The general locomotor activity, motor balance, and coordination abilities were not statistically different between Slc20a2-HO and wild-type mice after adjusting for body weight, which was a major confounding factor in our motor function evaluations.

CONCLUSION

The human PFBC-related phenotypes were highly similar to those in Slc20a2-HO mice. Therefore, Slc20a2-HO mice might be suitable for the future evaluation of neuropharmacological intervention strategies targeting cognitive and neuropsychiatric impairments.

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.

A mutation in SLC20A2 (c.C1849T) promotes proliferation while inhibiting hypertrophic differentiation in ATDC5 chondrocytes

AIMS

This study aimed to investigate the effect of solute carrier family 20 member 2 (SLC20A2) gene mutation (identified from a hereditary multiple exostoses family) on chondrocyte proliferation and differentiation.

METHODS

ATDC5 chondrocytes were cultured in insulin-transferrin-selenium medium to induce differentiation. Cells were transfected with pcDNA3.0 plasmids with either a wild-type (WT) or mutated (MUT) SLC20A2 gene. The inorganic phosphate (Pi) concentration in the medium of cells was determined. The expression of markers of chondrocyte proliferation and differentiation, the Indian hedgehog (Ihh), and parathyroid hormone-related protein (PTHrP) pathway were evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting.

RESULTS

The expression of SLC20A2 in MUT group was similar to WT group. The Pi concentration in the medium of cells in MUT group was significantly higher than WT group, which meant the SLC20A2 mutation inhibited Pi uptake in ATDC5 chondrocytes. The proliferation rate of ATDC5 chondrocytes in MUT group was greater than WT group. The expression of aggrecan (Acan), α-1 chain of type II collagen (COL2A1), and SRY-box transcription factor 9 (SOX9) were higher in MUT group than WT group. However, the expression of Runt-related transcription factor 2 (Runx2), α-1 chain of type X collagen (COL10A1), and matrix metallopeptidase 13 (MMP13) was significantly decreased in the MUT group. Similar results were obtained by Alcian blue and Alizarin red staining. The expression of Ihh and PTHrP in MUT group was higher than WT group. An inhibitor (cyclopamine) of Ihh/PTHrP signalling pathway inhibited the proliferation and restored the differentiation of chondrocytes in MUT group.

CONCLUSION

A mutation in SLC20A2 (c.C1948T) decreases Pi uptake in ATDC5 chondrocytes. SLC20A2 mutation promotes chondrocyte proliferation while inhibiting chondrocyte differentiation. The Ihh/PTHrP signalling pathway may play an important role in this process. Cite this article: Bone Joint Res 2020;9(11):751-760.

Cortical myoclonus and epilepsy in a family with a new SLC20A2 mutation

Idiopathic basal ganglia calcification (IBGC) or primary familial brain calcification is a rare genetic condition characterized by an autosomal dominant inheritance pattern and the presence of bilateral calcifications in the basal ganglia, thalami, cerebellum and cerebral subcortical white matter. The syndrome is genetically and phenotypically heterogeneous. Causal mutations have been identified in four genes: SLC20A2, PDGFRB, PDGFB and XPR1. A variety of progressive neurological and psychiatric symptoms have been described, including cognitive impairment, movement disorders, bipolar disorder, chronic headaches and migraine, and epilepsy. Here we describe a family with a novel SLC20A2 mutation mainly presenting with neurological symptoms including cortical myoclonus and epilepsy. While epilepsy, although rare, has been reported in patients with IBGC associated with SLC20A2 mutations, cortical myoclonus seems to be a new manifestation.

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.

A Novel SLC20A2 Mutation Associated with Familial Idiopathic Basal Ganglia Calcification and Analysis of the Genotype-Phenotype Association in Chinese Patients

BACKGROUND

Idiopathic basal ganglia calcification (IBGC) is a genetic disorder characterized by bilateral basal ganglia calcification and neural degeneration. In this study, we reported a new SLC2OA2 mutation of IBGC and reviewed relevant literature to explore the association between phenotypes and genotypes in Chinese IBGC patients.

METHODS

Clinical information of the proband and her relatives were collected comprehensively. Blood samples of both the patient and her father were obtained, and genetic screening related to IBGC was performed using second generation sequencing with their consent. Findings were confirmed by Sanger sequencing. Polyphen-2 was used to predict the potential association between mutations and disease. Then, we retrieved literatures of Chinese IBGC patients and explored the association between phenotype and genotype.

RESULTS

A novel mutation was identified through genetic testing, and it is suggested to be a damage mutation predicted by Polyphen-2. Through literature review, we found that SLC20A2 mutation is the most common cause for IBGC in China. Its hot spot regions are mainly on the 1st and 8th exons; the second common one is PDGFB where the hot spot covered a length of 220-230 bp localized on the 2nd exon; moreover, Chinese IBGC patients featured early-onset, more severe movement disorder and relatively mild cognitive impairment compared with those in other countries.

CONCLUSIONS

There is significant heterogeneity both in phenotype and genotype in Chinese IBGC patients. Further research of pathogenic mechanism of IBGC is required to eventually develop precise treatment for individuals who suffered this disease.

Brain Calcification and Movement Disorders

Brain calcifications may be an incidental finding on neuroimaging in normal, particularly older individuals, but can also indicate numerous hereditary and nonhereditary syndromes, and metabolic, environmental, infectious, autoimmune, mitochondrial, traumatic, or toxic disorders. Bilateral calcifications most commonly affecting the basal ganglia may often be found in idiopathic cases, and a new term, primary familial brain calcification (PFBC), has been proposed that recognizes the genetic causes of the disorder and that calcifications occurred well beyond the basal ganglia. PFBC, usually inherited in an autosomal dominant fashion, is both an intrafamilial and an interfamilial heterogeneous disorder, clinically characterized by an insidious and progressive development of movement disorders, cognitive decline, and psychiatric symptoms, but also cerebellar ataxia, pyramidal signs, and sometimes isolated seizures and headaches/migraines. Heterozygous mutations in four genes (SLC20A2, PDGFRB, PDGFB, XPR1) have recently proved to be the causes of the autosomal dominant forms of PFBC, also suggesting disrupted phosphate homeostasis as "an underlying and converging" pathophysiological mechanism. However, to date, it is not possible to anticipate with acceptable certainty any of known genetic causes of PFBC on the basis of the type, severity, pattern of distribution, or combination of movement disorders (mainly parkinsonism, with or without tremor, but also dystonia, chorea, paroxysmal kinesigenic dyskinesia, orofacial dyskinesia, and gait and speech disorders).

Neuroprotective effect of 5-aminolevulinic acid against low inorganic phosphate in neuroblastoma SH-SY5Y cells

PiT-1 (encoded by SLC20A1) and PiT-2 (encoded by SLC20A2) are type-III sodium-dependent phosphate cotransporters (NaPiTs). Recently, SLC20A2 mutations have been found in patients with idiopathic basal ganglia calcification (IBGC), and were predicted to bring about an inability to transport Pi from the extracellular environment. Here we investigated the effect of low Pi loading on the human neuroblastoma SH-SY5Y and the human glioblastoma A172 cell lines. The results show a different sensitivity to low Pi loading and differential regulation of type-III NaPiTs in these cells. We also examined whether 5-aminolevulinic acid (5-ALA) inhibited low Pi loading-induced neurotoxicity in SH-SY5Y cells. Concomitant application of 5-ALA with low Pi loading markedly attenuated low Pi-induced cell death and mitochondrial dysfunction via the induction of HO-1 by p38 MAPK. The findings provide us with novel viewpoints to understand the pathophysiology of IBGC, and give a new insight into the clinical prevention and treatment of IBGC.

Primary familial brain calcification linked to deletion of 5' noncoding region of SLC20A2

OBJECTIVES

Primary familial brain calcification (PFBC) is a rare neurological disease often inherited as a dominant trait. Mutations in four genes (SLC20A2, PDGFB, PDGFRB, and XPR1) have been reported in patients with PFBC. Of these, point mutations or small deletions in SLC20A2 are most common. Thus far, only one large deletion covering entire SLC20A2 and several smaller, exonic deletions of SLC20A2 have been reported. The aim of this study was to identify the causative gene defect in a Finnish PFBC family with three affected patients.

MATERIALS AND METHODS

A Finnish family with three PFBC patients and five unaffected subjects was studied. Sanger sequencing was used to exclude mutations in the coding and splice site regions of SLC20A2, PDGFRB, and PDGFB. Whole-exome (WES) and whole-genome sequencing (WGS) were performed to identify the causative mutation. A SNP array was used in segregation analysis.

RESULTS

Copy number analysis of the WGS data revealed a heterozygous deletion of ~578 kb on chromosome 8. The deletion removes the 5' UTR region, the noncoding exon 1 and the putative promoter region of SLC20A2 as well as the coding regions of six other genes.

CONCLUSIONS

Our results support haploinsufficiency of SLC20A2 as a pathogenetic mechanism in PFBC. Analysis of copy number variations (CNVs) is emerging as a crucial step in the molecular genetic diagnostics of PFBC, and it should not be limited to coding regions, as causative variants may reside in the noncoding parts of known disease-associated genes.

Deconstructing Fahr's disease/syndrome of brain calcification in the era of new genes

INTRODUCTION

There are now a number genes, known to be associated with familial primary brain calcification (PFBC), causing the so called 'Fahr's' disease or syndrome. These are SCL20A2, PDGFB, PDGFRB and XPR1. In this systematic review, we analyse the clinical and radiological features reported in genetically confirmed cases with PFBC. We have additionally reviewed pseudohypoparathyroidism which is a close differential diagnosis of PFBC in clinical presentation and is also genetically determined.

METHODS

We performed a Medline search, from 1st Jan 2012 through to 7th November 2016, for publications with confirmed mutations of SCL20A2, PDGFB, PDGFRB, and XPR1 and found twenty papers with 137 eligible cases. A second search was done for publications of cases with Pseudohypoparathyroidism or pseudopseudohypoparathyroidism, and found 18 publications with 20 eligible cases.

RESULTS

SLC20A2 was the most common gene involved with 75 out of 137 cases included with PFBC (55%) followed by PDGFB (31%) and PDGFRB (11%). Statistically significant correlation was found between the presence of parkinsonism with SLC20A2 mutations, headache in PDGFB and generalised tonic-clonic seizures in patients with pseudohypoparathyroidism.

CONCLUSION

We combine statistical analysis and clinical inference to suggest a diagnostic algorithm based on the observations in this study to help with investigation of a patient with neurological features and brain calcification.

SLC20A2 Deficiency in Mice Leads to Elevated Phosphate Levels in Cerbrospinal Fluid and Glymphatic Pathway-Associated Arteriolar Calcification, and Recapitulates Human Idiopathic Basal Ganglia Calcification

Idiopathic basal ganglia calcification is a brain calcification disorder that has been genetically linked to autosomal dominant mutations in the sodium-dependent phosphate co-transporter, SLC20A2. The mechanisms whereby deficiency of Slc20a2 leads to basal ganglion calcification are unknown. In the mouse brain, we found that Slc20a2 was expressed in tissues that produce and/or regulate cerebrospinal fluid, including choroid plexus, ependyma and arteriolar smooth muscle cells. Haploinsufficient Slc20a2 +/- mice developed age-dependent basal ganglia calcification that formed in glymphatic pathway-associated arterioles. Slc20a2 deficiency uncovered phosphate homeostasis dysregulation characterized by abnormally high cerebrospinal fluid phosphate levels and hydrocephalus, in addition to basal ganglia calcification. Slc20a2 siRNA knockdown in smooth muscle cells revealed increased susceptibility to high phosphate-induced calcification. These data suggested that loss of Slc20a2 led to dysregulated phosphate homeostasis and enhanced susceptibility of arteriolar smooth muscle cells to elevated phosphate-induced calcification. Together, dysregulated cerebrospinal fluid phosphate and enhanced smooth muscle cell susceptibility may predispose to glymphatic pathway-associated arteriolar calcification.

Slc20a2 is critical for maintaining a physiologic inorganic phosphate level in cerebrospinal fluid

Mutations in the SLC20A2-gene encoding the inorganic phosphate (Pi) transporter PiT2 can explain approximately 40 % of the familial cases of the rare neurodegenerative disorder primary familial brain calcification (Fahr’s disease). The disease characteristic, cerebrovascular-associated calcifications, is also present in Slc20a2-knockout (KO) mice. Little is known about the specific role(s) of PiT2 in the brain. Recent in vitro studies, however, suggest a role in regulation of the [Pi] in cerebrospinal fluid (CSF). We here show that Slc20a2-KO mice indeed have a high CSF [Pi] in agreement with a role of PiT2 in Pi export from the CSF. The implications in relation to disease mechanism are discussed.

PDGF, pericytes and the pathogenesis of idiopathic basal ganglia calcification (IBGC)

Platelet-derived growth factors (PDGFs) are important mitogens for various types of mesenchymal cells, and as such, they exert critical functions during organogenesis in mammalian embryonic and early postnatal development. Increased or ectopic PDGF activity may also cause or contribute to diseases such as cancer and tissue fibrosis. Until recently, no loss-of-function (LOF) mutations in PDGF or PDGF receptor genes were reported as causally linked to a human disease. This changed in 2013 when reports appeared on presumed LOF mutations in the genes encoding PDGF-B and its receptor PDGF receptor-beta (PDGF-Rβ) in familial idiopathic basal ganglia calcification (IBGC), a brain disease characterized by anatomically localized calcifications in or near the blood microvessels. Here, we review PDGF-B and PDGF-Rβ biology with special reference to their functions in brain-blood vessel development, pericyte recruitment and the regulation of the blood-brain barrier. We also discuss various scenarios for IBGC pathogenesis suggested by observations in patients and genetically engineered animal models of the disease.

Genetics and molecular biology of brain calcification

Brain calcification is a common neuroimaging finding in patients with neurological, metabolic, or developmental disorders, mitochondrial diseases, infectious diseases, traumatic or toxic history, as well as in otherwise normal older people. Patients with brain calcification may exhibit movement disorders, seizures, cognitive impairment, and a variety of other neurologic and psychiatric symptoms. Brain calcification may also present as a single, isolated neuroimaging finding. When no specific cause is evident, a genetic etiology should be considered. The aim of the review is to highlight clinical disorders associated with brain calcification and provide summary of current knowledge of diagnosis, genetics, and pathogenesis of brain calcification.

A new SLC20A2 mutation identified in southern Italy family with primary familial brain calcification

BACKGROUND

Primary familial brain calcification (PFBC) is a rare neurodegenerative disease characterized by bilateral calcifications mostly located in the basal ganglia and in the thalami, cerebellum and subcortical white matter. Clinical manifestations of this disease include a large spectrum of movement disorders and neuropsychiatric disturbances. PFBC is genetically heterogeneous and typically transmitted in an autosomal dominant fashion. Three causative genes have been reported: SLC20A2, PDGFRB and PDGFB.

OBJECTIVE

We screened three PFBC Italian families for mutations in the SLC20A2, PDGFRB and PDGFB genes.

METHODS

Phenotypic data were obtained by neurologic examination, CT scan and magnetic resonance imaging. Mutation screening of SLC20A2, PDGFRB and PDGFB was performed by sequencing.

RESULTS

We identified a new heterozygous deletion c.21_21delG (p.L7Ffs*10) in SLC20A2 gene in one of these families. No mutations were detected in the other two families.

CONCLUSIONS

Our data confirm that mutations in SLC20A2 are a major cause of familial idiopathic basal ganglia calcification.

The genetics of primary familial brain calcifications

Bilateral accumulation of calcium in the brain, most commonly in the basal ganglia, but also in the cerebellum, thalamus, and brainstem can be inherited in an autosomal dominant fashion and is then referred to as primary familial brain calcifications (PFBC). Clinical manifestations include a spectrum of movement disorders and neuropsychiatric abnormalities. In the past 2 years, 3 genes have been identified to cause PFBC, (ie, SLC20A2, PDGFRB, and PDGFB). SCL20A2 encodes the Type III sodium-dependent inorganic phosphate (Pi) transporter 2 (PiT2) and, when mutated, uptake of Pi is severely impaired likely causing buildup of calcium phosphate. The second identified cause of PFBC is mutations in PDGFRB, which codes for platelet-derived growth factor receptor β (PDGF-Rβ). Interestingly, the third PFBC gene is PDGFB that encodes the ligand of PDGF-Rβ, which is secreted during angiogenesis to recruit pericytes, thereby implying impairment of the blood-brain barrier as a disease mechanism of PFBC.

Update and Mutational Analysis of SLC20A2: A Major Cause of Primary Familial Brain Calcification

Primary familial brain calcification (PFBC) is a heterogeneous neuropsychiatric disorder, with affected individuals presenting a wide variety of motor and cognitive impairments, such as migraine, parkinsonism, psychosis, dementia, and mood swings. Calcifications are usually symmetrical, bilateral, and found predominantly in the basal ganglia, thalamus, and cerebellum. So far, variants in three genes have been linked to PFBC: SLC20A2, PDGFRB, and PDGFB. Variants in SLC20A2 are responsible for most cases identified so far and, therefore, the present review is a comprehensive worldwide summary of all reported variants to date. SLC20A2 encodes an inorganic phosphate transporter, PiT-2, widely expressed in various tissues, including brain, and is part of a major family of solute carrier membrane transporters. Fifty variants reported in 55 unrelated patients so far have been identified in families of diverse ethnicities and only few are recurrent. Various types of variants were detected (missense, nonsense, frameshift) including full or partial SLC20A2 deletions. The recently reported SLC20A2 knockout mouse will enhance our understanding of disease mechanism and allow for screening of therapeutic compounds. In the present review, we also discuss the implications of these recent exciting findings and consider the possibility of treatments based on manipulation of inorganic phosphate homeostasis.

Primary familial brain calcification: update on molecular genetics

Primary familial brain calcification is a neuropsychiatric disorder with calcium deposits in the brain, especially in basal ganglia, cerebellum and subcortical white matter. The disease is characterized by a clinical heterogeneity, with a various combination of symptoms that include movement disorders and psychiatric disturbances; asymptomatic patients have been also reported. To date, three causative genes have been found: SLC20A2, PDGFRB and PDGFB. SLC20A2 gene codes for the 'sodium-dependent phosphate transporter 2' (PiT-2), a cell membrane transporters of inorganic phosphate, involved in Pi uptake by cells and maintenance of Pi body levels. Over 40 pathogenic variants of SLC20A2 have been reported, affecting the regulation of Pi homeostasis. It was hypothesized that SLC20A2 mutations cause brain calcification most likely through haploinsufficiency. PDGFRB encodes for the platelet-derived growth factor receptor-β (PDGFRβ), a cell-surface tyrosine-kinase (RTK) receptor that regulates cell proliferation, migration, survival and differentiation. PDGFB encodes for the 'platelet-derived growth factor beta' (PDGFβ), the ligand of PDGFRβ. The loss of function of PDGFRβ and PDGFβ could lead to the impairment of the pericytes function and blood brain barrier integrity, causing vascular and perivascular calcium accumulation. SLC20A2 accounts for about 40 % of familial form and 14 % of sporadic cases, while PDGFRB and PDGFB mutations are likely rare. However, approximately 50 % of patients are not genetically defined and there should be at least another causative gene.

Primary familial brain calcification: Genetic analysis and clinical spectrum

BACKGROUND

Primary familial brain calcification (PFBC) is a rare autosomal dominant disorder with bilateral calcification of basal ganglia and other cerebral regions, movement disorders, and neuropsychiatric disturbances. So far, three causative genes have been discovered: SLC20A2, PDGFRB and PDGFB, accounting for approximately 50% of cases.

METHODS

Seven unrelated families with primary brain calcification were recruited to undergo clinical and genetic analysis, including Sanger sequencing of SLC20A2, PDGFRB, and PDGFB, and copy number analysis of SLC20A2.

RESULTS

Mutations in SLC20A2 have been detected in three families: p.Glu368Glyfs*46, p.Ser434Trp, and p.Thr595Met. Intrafamilial phenotype variability has been observed. In spite of this, we found similar neuroimaging pattern among members of the same family.

CONCLUSIONS

This molecular analysis expands the mutational spectrum of SLC20A2, which remains the major causative gene of primary familial brain calcification, and suggests the existence of disease-causing mutations in at least another, still unknown gene.

Loss of PiT-1 results in abnormal endocytosis in the yolk sac visceral endoderm

PiT-1 protein is a transmembrane sodium-dependent phosphate (Pi) transporter. PiT-1 knock out (KO) embryos die from largely unknown causes by embryonic day (E) 12.5. We tested the hypothesis that PiT-1 is required for endocytosis in the embryonic yolk sac (YS) visceral endoderm (VE). Here we present data supporting that PiT-1 KO results in a YS remodeling defect and decreased endocytosis in the YS VE. The remodeling defect is not due to an upstream cardiomyocyte requirement for PiT-1, as SM22αCre-specific KO of PiT-1 in the developing heart and the YS mesodermal layer (ME) does not recapitulate the PiT-1 global KO phenotype. Furthermore, we find that high levels of PiT-1 protein localize to the YS VE apical membrane. Together these data support that PiT-1 is likely required in YS VE. During normal development maternal immunoglobulin (IgG) is endocytosed into YS VE and accumulates in the apical side of the VE in a specialized lysosome termed the apical vacuole (AV). We have identified a reduction in PiT-1 KO VE cell height and a striking loss of IgG accumulation in the PiT-1 KO VE. The endocytosis genes Tfeb, Lamtor2 and Snx2 are increased at the RNA level. Lysotracker Red staining reveals a loss of distinct AVs, and yolk sacs incubated ex vivo with phRODO Green Dextran for Endocytosis demonstrate a functional loss of endocytosis. As yolk sac endocytosis is controlled in part by microautophagy, but expression of LC3 had not been examined, we investigated LC3 expression during yolk sac development and found stage-specific LC3 RNA expression that is predominantly from the YS VE layer at E9.5. Normalized LC3-II protein levels are decreased in the PiT-1 KO YS, supporting a requirement for PiT-1 in autophagy in the YS. Therefore, we propose the novel idea that PiT-1 is central to the regulation of endocytosis and autophagy in the YS VE.

Genetic screening and functional characterization of PDGFRB mutations associated with basal ganglia calcification of unknown etiology

Three causal genes for idiopathic basal ganglia calcification (IBGC) have been identified. Most recently, mutations in PDGFRB, encoding a member of the platelet-derived growth factor receptor family type β, and PDGFB, encoding PDGF-B, the specific ligand of PDGFRβ, were found implicating the PDGF-B/PDGFRβ pathway in abnormal brain calcification. In this study, we aimed to identify and study mutations in PDGFRB and PDGFB in a series of 26 patients from the Mayo Clinic Florida Brain Bank with moderate to severe basal ganglia calcification (BCG) of unknown etiology. No mutations in PDGFB were found. However, we identified one mutation in PDGFRB, p.R695C located in the tyrosine kinase domain, in one BGC patient. We further studied the function of p.R695C mutant PDGFRβ and two previously reported mutants, p.L658P and p.R987W PDGFRβ in cell culture. We show that, in response to PDGF-BB stimulation, the p.L658P mutation completely suppresses PDGFRβ autophosphorylation, whereas the p.R695C mutation results in partial loss of autophosphorylation. For the p.R987W mutation, our data suggest a different mechanism involving reduced protein levels. These genetic and functional studies provide the first insight into the pathogenic mechanisms associated with PDGFRB mutations and provide further support for a pathogenic role of PDGFRB mutations in BGC.

Single nucleotide polymorphisms for feed efficiency and performance in crossbred beef cattle

Background

This study was conducted to: (1) identify new SNPs for residual feed intake (RFI) and performance traits within candidate genes identified in a genome wide association study (GWAS); (2) estimate the proportion of variation in RFI explained by the detected SNPs; (3) estimate the effects of detected SNPs on carcass traits to avoid undesirable correlated effects on these economically important traits when selecting for feed efficiency; and (4) map the genes to biological mechanisms and pathways. A total number of 339 SNPs corresponding to 180 genes were tested for association with phenotypes using a single locus regression (SLRM) and genotypic model on 726 and 990 crossbred animals for feed efficiency and carcass traits, respectively.

Results

Strong evidence of associations for RFI were located on chromosomes 8, 15, 16, 18, 19, 21, and 28. The strongest association with RFI (P = 0.0017) was found with a newly discovered SNP located on BTA 8 within the ELP3 gene. SNPs rs41820824 and rs41821600 on BTA 16 within the gene HMCN1 were strongly associated with RFI (P = 0.0064 and P = 0.0033, respectively). A SNP located on BTA 18 within the ZNF423 gene provided strong evidence for association with RFI (P = 0.0028). Genomic estimated breeding values (GEBV) from 98 significant SNPs were moderately correlated (0.47) to the estimated breeding values (EBVs) from a mixed animal model. The significant (P < 0.05) SNPs (98) explained 26% of the genetic variance for RFI. In silico functional analysis for the genes suggested 35 and 39 biological processes and pathways, respectively for feed efficiency traits.

Conclusions

This study identified several positional and functional candidate genes involved in important biological mechanisms associated with feed efficiency and performance. Significant SNPs should be validated in other populations to establish their potential utilization in genetic improvement programs.

SLC20A2 and THAP1 deletion in familial basal ganglia calcification with dystonia

Idiopathic basal ganglia calcification (IBGC) is characterized by bilateral calcification of the basal ganglia associated with a spectrum of neuropsychiatric and motor syndromes. In this study, we set out to determine the frequency of the recently identified IBGC gene SLC20A2 in 27 IBGC cases from the Mayo Clinic Florida Brain Bank using both Sanger sequencing and TaqMan copy number analysis to cover the complete spectrum of possible mutations. We identified SLC20A2 pathogenic mutations in two of the 27 cases of IBGC (7 %). Sequencing analysis identified a p.S113* nonsense mutation in SLC20A2 in one case. TaqMan copy number analysis of SLC20A2 further revealed a genomic deletion in a second case, which was part of a large previously reported Canadian IBGC family with dystonia. Subsequent whole-genome sequencing in this family revealed a 563,256-bp genomic deletion with precise breakpoints on chromosome 8 affecting multiple genes including SLC20A2 and the known dystonia-related gene THAP1. The deletion co-segregated with disease in all family members. The deletion of THAP1 in addition to SLC20A2 in the Canadian IBGC family may contribute to the severe and early onset dystonia in this family. The identification of an SLC20A2 genomic deletion in a familial form of IBGC demonstrates that reduced SLC20A2 in the absence of mutant protein is sufficient to cause neurodegeneration and that previously reported SLC20A2 mutation frequencies may be underestimated.

Fahr's syndrome: literature review of current evidence

Fahr’s disease or Fahr’s syndrome is a rare, neurological disorder characterized by abnormal calcified deposits in basal ganglia and cerebral cortex. Calcified deposits are made up of calcium carbonate and calcium phosphate, and are commonly located in the Basal Ganglia, Thalamus, Hippocampus, Cerebral cortex, Cerebellar Subcortical white matter and Dentate Nucleus. Molecular genetics of this disease haven’t been studied extensively; hence evidence at the molecular and genetic level is limited. Fahr’s disease commonly affects young to middle aged adults. Etiology of this syndrome does not identify a specific agent but associations with a number of conditions have been noted; most common of which are endocrine disorders, mitochondrial myopathies, dermatological abnormalities and infectious diseases. Clinical manifestations of this disease incorporate a wide variety of symptoms, ranging from neurological symptoms of extrapyramidal system to neuropsychiatric abnormalities of memory and concentration to movement disorders including Parkinsonism, chorea and tremors amongst others. Diagnostic criteria for this disease has been formulated after modifications from previous evidence and can be stated briefly, it consist of bilateral calcification of basal ganglia, progressive neurologic dysfunction, absence of biochemical abnormalities, absence of an infectious, traumatic or toxic cause and a significant family history. Imaging modalities for the diagnosis include CT, MRI, and plain radiography of skull. Other investigations include blood and urine testing for hematologic and biochemical indices. Disease is as yet incurable but management and treatment strategies mainly focus on symptomatic relief and eradication of causative factors; however certain evidence is present to suggest that early diagnosis and treatment can reverse the calcification process leading to complete recovery of mental functions. Families with a known history of Fahr’s disease should be counseled prior to conception so that the birth of affected babies can be prevented. This review was written with the aim to remark on the current substantial evidence surrounding this disease.

Phenotypic spectrum of probable and genetically-confirmed idiopathic basal ganglia calcification

Idiopathic basal ganglia calcification is characterized by mineral deposits in the brain, an autosomal dominant pattern of inheritance in most cases and genetic heterogeneity. The first causal genes, SLC20A2 and PDGFRB, have recently been reported. Diagnosing idiopathic basal ganglia calcification necessitates the exclusion of other causes, including calcification related to normal ageing, for which no normative data exist. Our objectives were to diagnose accurately and then describe the clinical and radiological characteristics of idiopathic basal ganglia calcification. First, calcifications were evaluated using a visual rating scale on the computerized tomography scans of 600 consecutively hospitalized unselected controls. We determined an age-specific threshold in these control computerized tomography scans as the value of the 99th percentile of the total calcification score within three age categories: <40, 40-60, and >60 years. To study the phenotype of the disease, patients with basal ganglia calcification were recruited from several medical centres. Calcifications that rated below the age-specific threshold using the same scale were excluded, as were patients with differential diagnoses of idiopathic basal ganglia calcification, after an extensive aetiological assessment. Sanger sequencing of SLC20A2 and PDGFRB was performed. In total, 72 patients were diagnosed with idiopathic basal ganglia calcification, 25 of whom bore a mutation in either SLC20A2 (two families, four sporadic cases) or PDGFRB (one family, two sporadic cases). Five mutations were novel. Seventy-one per cent of the patients with idiopathic basal ganglia calcification were symptomatic (mean age of clinical onset: 39 ± 20 years; mean age at last evaluation: 55 ± 19 years). Among them, the most frequent signs were: cognitive impairment (58.8%), psychiatric symptoms (56.9%) and movement disorders (54.9%). Few clinical differences appeared between SLC20A2 and PDGFRB mutation carriers. Radiological analysis revealed that the total calcification scores correlated positively with age in controls and patients, but increased more rapidly with age in patients. The expected total calcification score was greater in SLC20A2 than PDGFRB mutation carriers, beyond the effect of the age alone. No patient with a PDGFRB mutation exhibited a cortical or a vermis calcification. The total calcification score was more severe in symptomatic versus asymptomatic individuals. We provide the first phenotypical description of a case series of patients with idiopathic basal ganglia calcification since the identification of the first causative genes. Clinical and radiological diversity is confirmed, whatever the genetic status. Quantification of calcification is correlated with the symptomatic status, but the location and the severity of the calcifications don't reflect the whole clinical diversity. Other biomarkers may be helpful in better predicting clinical expression.

Loss of function of Slc20a2 associated with familial idiopathic Basal Ganglia calcification in humans causes brain calcifications in mice

Familial idiopathic basal ganglia calcification (FIBGC) is a neurodegenerative disorder with neuropsychiatric and motor symptoms. Deleterious mutations in SLC20A2, encoding the type III sodium-dependent phosphate transporter 2 (PiT2), were recently linked to FIBGC in almost 50 % of the families reported worldwide. Here, we show that knockout of Slc20a2 in mice causes calcifications in the thalamus, basal ganglia, and cortex, demonstrating that reduced PiT2 expression alone can cause brain calcifications.

Mutations in the gene encoding PDGF-B cause brain calcifications in humans and mice

Calcifications in the basal ganglia are a common incidental finding and are sometimes inherited as an autosomal dominant trait (idiopathic basal ganglia calcification (IBGC)). Recently, mutations in the PDGFRB gene coding for the platelet-derived growth factor receptor β (PDGF-Rβ) were linked to IBGC. Here we identify six families of different ancestry with nonsense and missense mutations in the gene encoding PDGF-B, the main ligand for PDGF-Rβ. We also show that mice carrying hypomorphic Pdgfb alleles develop brain calcifications that show age-related expansion. The occurrence of these calcium depositions depends on the loss of endothelial PDGF-B and correlates with the degree of pericyte and blood-brain barrier deficiency. Thus, our data present a clear link between Pdgfb mutations and brain calcifications in mice, as well as between PDGFB mutations and IBGC in humans.

Mutation Analysis of SLC20A2 and SPP2 as Candidate Genes for Familial Idiopathic Basal Ganglia Calcification

BACKGROUND

Familial Idiopathic Basal Ganglia Calcification (IBGC) is a rare neurodegenerative disorder which is usually transmitted as an autosomal dominant trait. IBGC is genetically heterogeneous and SLC20A2, on chromosome 8p21.1-8q11.23, is the first gene found in IBGC-affected patients with varied ancestry. On the other hand, several candidate genes for IBGC on chromosome 2q37, including the SPP2 gene, may play a role in inhibiting calcification.

METHODS

Totally, 22 members of a three generational Iranian family affected by IBGC, with an autosomal dominant pattern of inheritance were included in this study. DNA was extracted from the whole blood using standard salting out method. To find a mutation responsible for IBGC, we sequenced the coding region of SLC20A2 as well as promoter and coding region of SPP2 in the index subject of IBGC-affected family.

RESULTS

Pathogenic mutation was found neither in SLC20A2 nor in SPP2.

CONCLUSION

Our results strengthen genetic heterogeneity of this condition. Additional mutation studies are necessary to find a gene or genes responsible for IBGC in this affected family.