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

Current status of drugs targeting PDGF/PDGFR

As an important proangiogenic factor, platelet-derived growth factor (PDGF) and its receptor PDGFR are highly expressed in a variety of tumors, fibrosis, cardiovascular and neurodegenerative diseases. Targeting the PDGF/PDGFR pathway is therefore a promising therapeutic strategy. At present, a variety of PDGF/PDGFR targeted drugs with potential therapeutic effects have been developed, mainly including PDGF agonists, inhibitors targeting PDGFR and proteolysis targeting chimera (PROTACs). This review clarifies the structure, biological function and disease correlation of PDGF and PDGFR, and it discusses the current status of PDGFR-targeted drugs, so as to provide a reference for subsequent research.

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.

Living with primary brain calcification with PDGFB variants: A qualitative study

Introduction

Primary brain calcification (PBC) is a rare and intractable neurodegenerative disease. SLC20A2 and PDGFB are two major causative genes. As there is no effective treatment to avoid further progression or to prevent the onset of the disease, the patients may experience psychological distress. There is a qualitative study on the experiences of patients with primary brain calcification with SLC20A2 variants. However, the experiences of patients with PDGFB variants of the disease have not been explored. The purpose of this study is to identify the experiences of patients with PDGFB variants after diagnosis.

Materials and methods

Semi-structured interviews were conducted once or twice a year for three years with five patients over the age of 21. The data were analyzed using inductive qualitative methods.

Results

Seven categories, 15 subcategories, and 129 codes were extracted. The seven categories are as follows: [Shock at hearing the term ‘brain calcification’ for the first time], [Anxiety regarding the risk of heredity], [Anxiety, along with severe headaches, and various other symptoms], [Gratitude for the family members who care], [Accepting the disease as a non-life-threatening illness], [Feeling alienated due to the rare intractable disease], and [Modifying lifestyle due to the illness].

Discussion

The most stressful aspect of the disease was the headache that persisted even with the use of analgesics, which was different from patients with the SLC20A2 variants. In addition, we found unique concepts such as anxiety regarding the risk of heredity and a feeling of alienation due to the rare and intractable disease.

A heterozygous deletion of PDGFB gene causes paroxysmal kinesigenic dyskinesia with primary familial brain calcification

BACKGROUND

Primary familial brain calcification (PFBC) is a neurodegenerative disease characterized with calcium deposition in multiple brain regions. Mutations in PDGFB have been discovered in sporadic and familial PFBC cases. While several known variants displayed loss-of function, no complete deletion of platelet-derived growth factor B (PDGFB) has been reported.

METHODS

For the diagnostic purpose, brain computerized tomography or magnetic resonance imaging scanning and whole-genome sequencing were performed on the proband and family members in the pedigree.

RESULTS

We identified a heterozygous PDGFB complete deletion in a Chinese pedigree. The proband presented with paroxysmal kinesigenic dyskinesia (PKD), a rare symptom in PFBC. The proband's mother carrying the same mutation was asymptomatic.

CONCLUSIONS

For the first time, we reported a PFBC with a heterozygous deletion of PDGFB, and provided evidence of haploinsufficiency in the pathogenesis of PFBC.

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.

Hereditary Disorders of Cardiovascular Calcification

Arterial calcification is a common phenomenon in the elderly, in patients with atherosclerosis or renal failure and in diabetes. However, when present in very young individuals, it is likely to be associated with an underlying hereditary disorder of arterial calcification. Here, we present an overview of the few monogenic disorders presenting with early-onset cardiovascular calcification. These disorders can be classified according to the function of the respective disease gene into (1) disorders caused by an altered purine and phosphate/pyrophosphate metabolism, (2) interferonopathies, and (3) Gaucher disease. The finding of arterial calcification in early life should alert the clinician and prompt further genetic work-up to define the underlying genetic defect, to establish the correct diagnosis, and to enable appropriate therapy.

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.