Identification of SLC20A2 deletions in patients with primary familial brain calcification

Primary familial brain calcification (PFBC) is a rare neurological disorder. Mutations in five genes (SLC20A2, PDGFRB, PDGFB, XPR1, and MYORG) have been linked to PFBC. Here, we used SYBR green-based real-time quantitative polymerase chain reaction (PCR) assay and denaturing high-performance liquid chromatography analysis to detect copy number variants (CNVs) in 20 unrelated patients with PFBC, negatively sequenced for the five known genes. We identified three deletions in SLC20A2, including a large de novo full gene deletion and two exonic deletions confined to exon 2 and exon 6, respectively. Subsequent linked-read whole-genome sequencing of the patient with the large deletion showed a 1.7 Mb heterozygous deletion which removed the entire coding regions of SLC20A2 as well as 21 other genes. In the family with a deletion of exon 6, a missense variant of uncertain significance (SLC20A2: p.E267Q) also co-segregated with the disease. Functional assay showed the deletion could result in significantly impaired phosphate transport, whereas the p.E267Q variant did not. Our results confirm that deletion in SLC20A2 is a causal mechanism for PFBC and highlight the importance of functional study for classifying a rare missense variant as (likely) pathogenic.