Unmasking the genetic heterogeneity of cystic fibrosis in the Torbat Jam region: From common variants to large deletions and the challenge of missing heritability.

BACKGROUND: Cystic Fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. The genetic spectrum of CF in Iran is highly heterogeneous due to ethnic diversity and consanguinity, particularly in the Khorasan province. Data regarding the Torbat Jam region is scarce.

METHODS: We analyzed 7 unrelated patients with a clinical diagnosis of CF (sweat chloride >60 mmol/L). Genomic DNA was subjected to Sanger sequencing of all 27 CFTR exons. In cases with no identifiable mutations, Multiplex Ligation-dependent Probe Amplification (MLPA) was performed to detect Copy Number Variations (CNVs). Whole Exome Sequencing (WES) was utilized for patients who remained negative after Sanger and MLPA. This work has been approved with the Approval number: IR.ZUMS.BLC.1404-051.

RESULTS: Mutations were identified in 4 out of 7 patients (57.1%). We detected the hallmark c.1521_1523delCTT (p.Phe508del) in a homozygous state in one patient and heterozygous in another. We identified the c.1210-11T > G (5T; TG12 variant) splice-site mutation in two compound heterozygous patients. Additionally, a missense mutation c.547C > A (p.Leu183Ile) was found. Notably, one patient with no sequence variants harbored a homozygous deletion of the entire Exon 2, detected via MLPA. Three patients (42.9%) remained genetically unresolved despite WES analysis.

CONCLUSION: This study highlights a distinct mutational profile in Torbat Jam, characterized by severe alleles and large deletions. The high rate of unresolved cases despite WES suggests the involvement of deep intronic variants or extra-genic modifiers, necessitating Whole Genome Sequencing (WGS) for comprehensive diagnosis in this population.