Pulmonary hypertension (PH) is significantly associated with an increased risk of death in cystic fibrosis (CF) patients with advanced lung diseases [1]. Most CF patients with nonsense mutations of CF transmembrane conductance regulator (CFTR) were observed to have pulmonary arterial enlargement, and some of them had higher pulmonary vascular resistance and other adverse outcomes [2]. In this study, we aimed to analyze whether increased PH risk was causally associated with CFTR.

The Mendelian randomization (MR) method, a novel approach for the estimation of causality, is generally utilized to eliminate the effect of confounders by using single-nucleotide polymorphisms (SNPs) as instrumental variables [3]. Specifically, we conducted a two-sample MR study to assess whether CFTR was the etiology of PH. Analyses were performed using the package TwoSampleMR (version 0.4.26) in R (version 3.6.3) [3]. The protein expression levels were detected using western blotting. See the Online Supplemental Material for details.

To demonstrate the potential relationship between CFTR dysfunction and PH, we conducted a two-sample MR study using one SNP associated with CFTR from the GTEx resource of a published independent cis-acting expression quantitative trait locus [4]. The beta value of rs35715578, which reflects the effect size on decreased CFTR expression, was −0.5794. We also derived the summary data of PH, which was identified with the secondary diagnosis ICD-10 code I27.0 Primary PH (UKB-b439, 285 cases and 462,725 controls). The Wald ratio method showed that the odds ratio in genetically determined CFTR was 0.9998 (95% confidence interval (CI) 0.9996–0.9999, P = 0.037). This study suggested that decreased CFTR expression was causally associated with a decreased risk of primary PH.

To further illustrate the causality between CFTR dysfunction and PH, we reported a young Chinese male with severe idiopathic pulmonary arterial hypertension (IPAH) carrying a missense mutation in CFTR. Sorting Intolerant From Tolerant (SIFT) software predicted c.650A > G (p. Glu217Gly) in CFTR, which affects a highly conserved amino acid, to be deleterious, with a score of −3.056 (Fig. 1). Western blot analysis indicated the upregulation of CFTR protein in the lung tissue of two rat models with established severe PH: sugen/hypoxia- or monocrotaline-induced PH (Fig. 2).

Fig. 1
figure 1

Characterization of a novel CFTR missense mutation in the Chinese patient with IPAH. A Confirmation of a missense mutation in the CFTR by direct sequencing: a novel adenine-to-guanine substitution occurs at nucleotide 650 in exon 5 (c.650A > G). B SIFT software results showing c.650A > G to be a disease-causing mutation. C The variant in the CFTR affects a highly conserved amino acid.

Full size image
Fig. 2
figure 2

The expression of CFTR protein in lung tissue of experimental pulmonary hypertension rat models. A Representative immunoblots (a) and summarized data (b) showing the protein level of CFTR in lung tissue from normoxia- and sugen/hypoxia-exposed rats. Data were represented as means ± SE; n = 5 in each group. Independent t test was performed, **p < 0.01 vs. Nor. B Representative immunoblots (a) and summarized data (b) showing protein levels of CFTR in the lung tissue from control (Cont)- and monocrotaline (MCT)-treated rats. Data were represented as means ± SE; n = 6 in each group. Independent t test was performed, **p < 0.01 vs. Cont. The asterisk-marked band (*) indicates nonspecific binding by anti-CFTR antibody.

Full size image

In summary, CFTR dysfunction may participate in the development of PH. The MR results suggested that decreased CFTR expression had a protective effect against primary PH. We identified the SNPs influencing some phenotypes as unconfounded indicators that were akin to randomly allocated in individuals such that a randomized controlled trial can be mimicked in MR analysis. Unfortunately, we cannot assess the impact on horizontal pleiotropy due to data limitations. Of note, we identified a novel CFTR missense mutation in a Chinese patient with IPAH, which provided evidence for the relationship between CFTR mutations and PAH. We observed that the expression of the CFTR protein was upregulated in lung tissues of PH rat models. Further studies are needed to explore the mechanisms of CFTR dysfunction in the development of PH.

Interestingly, Tabeling et al. demonstrated that defective CFTR in the lungs partially protected against PH and pulmonary arterial remodeling in chronic hypoxic mice [5]. To our knowledge, dysfunction of the CFTR protein, a cyclic adenosine monophosphate-dependent chloride channel that mediates contraction and relaxation in smooth muscle cells, may play an important role in the pathogenesis of PH. However, the absence of CFTR is generally recognized as the cause of classic CF phenotypes, which are generally accompanied by secondary PH in advanced lung diseases. Taken together, these studies draw attention to the viewpoint of whether CFTR dysfunction contributes to primary PH, but defective CFTR partially offsets the effect on PH secondary to CF, which should be reconsidered and needs verification in further research.