Novel Compound Heterozygous Mutation on MYORG Gene in a Chinese Patient with Primary Familial Brain Calcification

Sir, MYORG mutations are the major causes of autosomal recessive (AR) primary familial brain calcification (PFBC). We described a novel compound heterozygous mutation in MYORG gene in a Chinese PFBC patient [Figure 1a].Figure 1: (a) Pedigree of the family with MYORG gene mutations. The proband is indicated (arrow). (b-e) Brain CT showed symmetric calcifications in the cerebellum, thalamus, basal ganglia, corona radiate and centrum semiovale. (f-i) MRI T1WI showed isointensive, hypointense or hyperintense signal in the calcified areas. (j-m) MRI T2WI showed hypointense or hyperintense signal in the calcified areas. (n-q) MRI FLAIR showed hypointense or hyperintense signal in the calcified areas. (r-u) MRI DWI showed hypointense signal changes in the calcified areas. (v) The insertion mutation, c.337_348dup (p.Leu113_Arg116dup). (w) Wild-type sequence. (x) The missense mutation, c.1754C>T (p.Pro585Leu). (y) Wild-type sequenceA 65-year-old Chinese man (II: 2, the proband) was hospitalized at our institution for unsteady gait. About one year ago, he complained that he was gradually unsteady in walking, which was persistent, but no progressive. No family history of genetic diseases was identified, and his parents were non-consanguineous. Neurological physical examination revealed the finger-to-nose test and heel-knee-tibia test were mild unstable and mild inaccurate. His gait was mild and wide-based, and the Romberg sign was negative. The serum calcium, phosphate and parathyroid hormone levels of this patient were normal, and secondary brain calcifications were excluded. His brain computed tomography (CT) scan showed symmetrical bilateral calcifications in cerebellum, thalamus, basal ganglia, corona radiate and centrum semiovale [Figure 1b-e]. Brain cranial magnetic resonance imaging (MRI) revealed symmetrical punctate and patchy lesions which appeared to be hyperintense, hypointense or isointensive on T1, T2, fluidattenuated inversion recovery (FLAIR) and diffusion-weighted images (DWI) in the above corresponding calcified areas on brain CT [Figure 1f-u]. Informed consent was obtained, and genomic DNA of blood cells was extracted from all consenting participants using a Genomic-tip kit (QIAGEN, Hilden, Germany). Whole exome sequencing of the proband was performed by Axeq Technologies (RayLee Biotech, Shanghai, China) using Agilent SureSelect v6 reagents for capturing exons and Illumina HiSeq X Ten platform for sequencing. The specific mutation in MYORG was confirmed by Sanger sequencing both the forward and reverse strands in all participants. We analysed whole exome sequencing data regarding the MYORG gene. By whole exome sequencing, a specific MYORG gene compound heterozygous mutation, c. 337_348dup (p.Leu113_Arg116dup) and c.1754C>T (p.Pro585Leu) was identified in the patient [Figure 1v and 1x]. His 70-year-old older brother (II: 1) and his 36-year-old daughter (III: 1) carried the c.1754C>T (p.Pro585Leu) mutation, while his 33-year-old son (III: 2) carried the c.337_348dup (p.Leu113_Arg116dup) mutation. Notably, all of them were asymptomatic. PFBC is a rare, neurological disorder characterized by bilateral symmetrical calcification in the basal ganglia with high clinical and genetic heterogeneity.[1] MYORG gene mutations account for 13% of PFBC cases.[2] Speech disturbance was the most common motor symptom in MYORG gene mutation carriers.[2] In our study, the patient presented with only gait instability and ataxia, which were relatively uncommon manifestations in PFBC. The brain CT imaging displayed symmetrical bilateral calcifications in cerebellum, thalamus, basal ganglia, corona radiate and centrum semiovale, which are some typical features of PFBC.[3] However, the degree of cerebral calcification was relatively severe, which may be related to ataxia symptoms, and there were no brainstem calcifications. This variant, c.337_348dup (p.Leu113_Arg116dup), led to GCCAGGCGGAAG bases duplication on the c. 337_348 sites [Figure 1v-w], and resulted in the insertion of 4 amino acids (Leu, Ala, Phe and Arg) on the p. 113-116 sites of the MYORG protein, also namely c.348_349ins, while preserved the integrity of the reading frame. In 2018, Yao et al. first described this mutation from a South Chinese woman who showed migraine, and it was suggested to be one of the ‘hot spots’ for Chinese patients.[4] The other variant, c.1754C>T (p.Pro585Leu), caused the proline (P) at amino acid position 585 to be replaced by a leucine (L) [Figure 1x-y]. This compound heterozygous mutation was also not listed in the major databases including the gnomAD, Clinvar, dbSNP, HGMD pro and LOVD. According to the American College of Medical Genetics (ACMG) guidelines, the insertional c.337_348dup mutant is considered ‘pathogenic: PM2_Supporting+PM3+PM4+PP1_Strong + PP4’, whereas the missense c.1754C>T mutant is considered ‘likely pathogenic: PM2_Supporting+PM3_Supporting+PP3_Strong+PP4’. Thus, it represented a novel aetiology in a Chinese PFBC family. In 2023, Song et al.[5] reported a novel compound heterozygous mutation in the MYORG gene, a nonsense mutation c.580G>T (p.Q194∗) and an insertional mutation c. 348_c.349insCTGGCCTTCCGC (p.116_117ins LAFR), in a patient with brain calcification and migraine. The MYORG gene encodes KIAA1161 protein, also known as NET37, which likely regulates protein glycosylation in the endoplasmic reticulum (ER) of brain astrocytes.[1,2] Mutations in MYORG damaged astrocytes and impaired the integrity of the BBB and/or blood-CSF barrier (BCB).[3] Meanwhile, KIAA1161 protein belongs to an α-galactosidase and changes in the protein structure may lead to abnormal protein glycosylation and metabolism, producing calcium deposits in the brain.[1] Our study broadens the MYORG gene mutation spectrum, which may help to determine the underlying pathogenic mechanisms of PFBC. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship This study was supported by the Shandong Provincial Natural Science Foundation [ZR2021MH059]. Conflicts of interest There are no conflicts of interest.