Identification of novel and known variants in the JAM2 gene in the Iranian families affected to Primary familial brain calcification (PFBC)

Identification of novel and known variants in the JAM2 gene in the Iranian families affected to Primary familial brain calcification (PFBC)
Mana Khojasteh,¹ Aida ghasemi,² Parsa Soleimani,³ Mohammad Rohani,⁴ Afagh Alavi,^5,*
1. Genetics Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
2. Neuromuscular Research Center, Tehran University of Medical Sciences, Tehran, Iran
3. Genetics Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
4. Department of Neurology, The Five Senses Health Institute, Iran University of Medical Sciences, Tehran, Iran
5. Genetics Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
Introduction: Primary familial brain calcification (PFBC) represents a rare neurological condition characterized by the abnormal accumulation of calcium deposits within the brain, resulting in severe manifestations such as movement disorders, psychiatric disturbances, and cognitive impairments. Mutations in the JAM2 gene have been linked to less than 2% of PFBC cases. Additionally, seven other genes have been identified that linked to PFBC (SLC20A2, PDGFRB, PDGFB, and XPR1 with dominant (AD), MYORG, CMPK2, and NAA60 with recessive (AR) patterns of inheritance. Nevertheless, the genetic causes of almost half of the cases with PFBC are still unknown. Until now, no comprehensive genetic analysis has been performed on this disease in Iran. The present study aims to conduct genetic analyses of five families affected by PFBC within Iranian population.
Methods: Five Iranian probands with PFBC were assessed in both clinical and paraclinical contexts. The informed consent form was signed. DNA was extracted from their peripheral blood leukocytes by salting out method. The probands' DNA was subjected to whole-exome sequencing (WES). Splicing and non-synonymous exonic variants were taken to consideration. Subsequently, variants in public genome databases with minor allele frequency (MAF) less than 0.01 were taken into account. Finally, Sanger sequencing was used to confirm the candidate variants in the probands and co-segregate of those in the family members. Additionally, copy number variations (CNVs) in known PFBC genes and other relevant genes were analyzed using WES data, utilizing tools such as GermlineCNVCaller and AnnotSV. Haplotype analysis was conducted in the JAM2 gene, using six intragenic single-nucleotide polymorphisms (SNPs) in JAM2 within the exome data of probands.
Results: Clinical assessments revealed a variety of symptom presentations across the five families studied. Key symptoms included bradykinesia, slurred speech, rigidity, anxiety, obsessive-compulsive disorder (OCD) and headache, with JAM2-related patients displaying an earlier average age of onset (mean onset: 10.1 years) in comparison to other PFBC patients. Family analyses highlighted the considerable variability in clinical manifestations, illustrating the inherent heterogeneity of the disorder even among siblings. Through whole-exome sequencing, pathogenic variants were successfully identified in four of the five probands, with the recurrent c.685C>T:p.Arg229* mutation in the JAM2 gene found in three families and a novel c.426dup:p.Ser143Leufs23 variant identified in another. These findings suggest a notable prevalence of JAM2 mutations in this population. Additionally, haplotypes for all probands were determined, identifying six intragenic SNPs in JAM2 linked to the disease-causing variant and indicate that p.Arg229* may function as a founder mutation or a hotspot codon within Iranian cohorts. It is also noteworthy that one proband lacked any identifiable candidate variant, indicating the possibility of novel genes contributing to the condition. Imaging studies revealed consistent patterns of calcification among JAM2-related patients, predominantly involving the basal ganglia, thalami, and dentate nuclei, while cortical involvement was notably absent, distinguishing them from other PFBC mutation reports. Furthermore, the presence of p.Arg229* was found to correlate with particular brain calcification profiles, supporting the need for further cohort studies to investigate potential phenotype-genotype associations.
Conclusion: The comprehensive examination of JAM2 mutations in Iranian families with PFBC highlights a greater-than-anticipated prevalence and underscores significant clinical and genetic heterogeneity. The identification of p.Arg229* as a likely founder mutation in this demographic expands the spectrum of JAM2-associated disorder, reinforcing the imperative for ongoing genetic research to discover additional variants implicated in PFBC. The results advocate for increased awareness and further inquiry into the pathophysiological ramifications of JAM2 mutations across various populations, as well as the critical importance of gene identification in advancing diagnostic and therapeutic strategies for PFBC. This study emphasizes the genetic complexities of PFBC and suggests that progress in genetic research may provide crucial insights into previously unrecognized mutations and their corresponding phenotypic manifestations.
Keywords: Primary familial brain calcification (PFBC); FAHR disease; JAM2; Whole-exome sequencing (WES); Found