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.