MYORG is associated with recessive primary familial brain calcification

Abstract

Objective: To investigate the genetic basis of the recessive form of primary familial brain calcification and study pathways linking a novel gene with known dominant genes that cause the disease.

Methods: Whole exome sequencing and Sanger-based segregation analysis were used to identify possible disease causing mutations. Mutation pathogenicity was validated by structural protein modeling. Functional associations between the candidate gene, MYORG, and genes previously implicated in the disease were examined through phylogenetic profiling.

Results: We studied nine affected individuals from two unrelated families of Middle Eastern origin. The median age of symptom onset was 29.5 years (range 21-57 years) and dysarthria was the most common presenting symptom. We identified in the MYORG gene, a homozygous c.1233delC mutation in one family and c.1060_1062delGAC mutation in another. The first mutation results in protein truncation and the second in deletion of a highly conserved aspartic acid that is likely to disrupt binding of the protein with its substrate. Phylogenetic profiling analysis of the MYORG protein sequence suggests co-evolution with a number of calcium channels as well as other proteins related to regulation of anion transmembrane transport (False Discovery Rate, FDR < 10^-8) and with PDCD6IP, a protein interacting with PDGFR β which is known to be involved in the disease.

Interpretation: MYORG mutations are linked to a recessive form of primary familial brain calcification. This association was recently described in patients of Chinese ancestry. We suggest the possibility that MYORG mutations lead to calcification in a PDGFR β-related pathway.

Figure 1

Pedigrees, genotypes, and brain imaging findings of the studied families. (A). Pedigrees of the two studied families. Individuals with extensive brain calcification are marked with black symbols, these with punctuate globus pallidus (GP) calcification are marked with white dotted symbols and these without calcification are marked with white symbols. Gray symbols represent individuals with no CT available; all of them are clinically asymptomatic. Genetic status of MYORG mutation is depicted for homozygotes (Hom), heterozygotes (Het), wild type (WT), and untested (NT) individuals. (B). Representative CT scans. All individuals homozygous for MYORG mutations had extensive calcification of the basal ganglia and thalamus (e.g., F1/II‐4, F1/II‐6, F2/II‐4), cerebellar lobes, and vermis (e.g., F1/II‐2, F2/II‐5). In addition to these involved brain areas, some individuals also presented calcification of the cortex (e.g., F1/II‐4), sub‐cortex (e.g., F1/II‐6), and brainstem (e.g., F1/II‐2, F1/II‐10, F2/II‐5). (C). In three (out of five) individuals heterozygotes for MYORG mutation, punctuate calcification limited to the globus pallidus were seen (e.g., F2/II‐1, F2/II‐2).

Figure 2

Multiple sequence alignment (MSA), modeled protein conformation and possible link of MYORG to other PFBC related proteins. (A) MSA of the MYORG sequence around the D354 deletion, along with other members of the Glycosyl hydrolase GH31 family. Background indicates the level of conservation of each position in the MSA ranging from white (unconserved) to purple (fully conserved). (B). Modeling of MYORG predicts a conformational change around residue D353 that disrupts substrate binding. Deletion of D354 causes: (1) loop shortening in the mutant protein (magenta sticks) relative to the wild type (blue sticks), (2), deviation of approximately 1 Å of the mutant C‐alpha atoms position (magenta versus blue sphere) and, (3) change in the orientation of the D353 terminal oxygens that prevents binding of substrate (red and white spheres). (C). Proteins that co‐evolved with MYORG and interactions with the proteins they encode suggest a common pathway with PDGFR β.