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Background: High TGFβ1-producing variants cause severe clinical disease in F508del homozygous patients. Lately, we showed that a single nucleotide polymorphism (SNP), rs41266431, in the GJA4 gene modifies the disease severity of cystic fibrosis (CF). Our aim was to investigate whether the clinical phenotype associated with GJA4 variants was independent of TGFβ1 variants. Methods: Homozygous F508del patients (n = 115, mean age 27.2 years, m/f (65/50)) were included in this study. A deep sequence analysis was performed for GJA4 and TGBβ1, and disease severity was assessed over 3 years using lung function tests (LFTs), body mass index, diabetes mellitus, colonization with Pseudomonas aeruginosa, survival to end-stage lung disease (ESLD), as well as distinct inflammatory biomarkers. Results: The analyses revealed that one SNP (rs41266431) in GJA4 may be clinically relevant. Carriers homozygous for the G variant (n = 84; 73%) presented with worse LFTs (forced vital capacity (FVC) % predicted: mean 80/86.6, p < 0.035) and a lower survival to ESLD (p < 0.029). For the TGBβ1 variant: 509 carriers of the C variant (CT + CC genotype, n = 105, 91.3%) had better LFTs (Forced expiratory flow at 75% of the FVC (FEF75% predicted: median 40/29.5, p < 0.015), although a similar outcome to ESLD. A gene–gene interaction was not observed between TGBβ1 and GJA4 variants for any clinical measure. Conclusions: GJA4 variants are independent of TGBβ1 variants. Both variants had an impact on the LFTs, although only GJA4 variants were associated with an improved outcome for ESLD. Clinical Trial Registration: The study was registered with ClinicalTrials.gov, number NCT04242420, retrospectively on January 24th, 2020.

Background: Recently, we provided evidence that a single nucleotide polymorphism (SNP), rs41266431, on the gap junction protein alpha 4 (GJA4) gene, acts as a modifier for clinical disease severity in patients with cystic fibrosis (CF). These features are very similar to those of variants of the mannose-binding lectin (MBL). This study aimed to clarify whether the clinical disease phenotype associated with GJA4 variants is independent of MBL variants. Methods: One hundred and twelve patients with homozygous F508del (mean age, 27.6 years; m/f, 61/51) were recruited from the CF centers of Bonn, Frankfurt, and Amsterdam. A sequence analysis was performed for GJA4 and MBL. The clinical phenotype was assessed over three years using pulmonary function tests, body mass index, Pseudomonas aeruginosa colonization, diabetes mellitus, survival to end-stage lung disease, and inflammatory markers. Results: A clinically relevant SNP of GJA4 was identified by sequence analysis. Pulmonary function (FVC% pred, mean 78/85; p < 0.055) and survival to end-stage lung disease were lower (p < 0.043) for this variant (rs41266431) in carriers homozygous for the G variant (n = 82/112; 73%) than in other carriers. Serum MBL (820/372 ng/mL, p < 0.001) was significantly higher in “MBL-sufficient” genotypes (n = 79/112; 71%) than in “MBL-insufficient” genotypes, and a trend for a significant difference in BMI percentiles (35.2/23.8; p < 0.059) was observed. For the MBL-sufficient genotype (median age at death, 38/26 years), there was a trend for better survival (p < 0.076). There was no augmentation by gene-gene interaction between MBL and GJA4 variants for any outcome parameter. Conclusions: The clinical disease phenotype associated with GJA4 variants is independent of MBL variants. MBL-sufficient variants were associated with superior BMI and a trend for better survival than MBL insufficient variants.