Population-based study of cystic fibrosis disease severity and haemochromatosis gene mutations

Gene modifiers of cystic fibrosis lung disease: A systematic review

BACKGROUND

Lung disease is the major source of morbidity and mortality in cystic fibrosis (CF), with large variability in severity between patients. Although accurate prediction of lung disease severity would be extremely useful, no robust methods exist. Twin and sibling studies have highlighted the importance of non-cystic fibrosis transmembrane conductance regulator (CFTR) genes in determining lung disease severity but how these impact on the severity in CF remains unclear.

METHODS

A systematic review was undertaken to answer the question "In patients with CF which non-CFTR genes modify the severity of lung disease?" The method for this systematic review was based upon the "Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)" statement, with a narrative synthesis of results planned.

RESULTS

A total of 1168 articles were screened for inclusion, with 275 articles undergoing detailed assessment for inclusion. One hundred and forty articles were included. Early studies focused on candidate genes, whereas more recent studies utilized genome-wide approaches and also examined epigenetic mechanisms, gene expression, and therapeutic response.

DISCUSSION

A large body of evidence regarding non-CFTR gene modifiers of lung disease severity has been generated, examining a wide array of genes. Limitations to existing studies include heterogeneity in outcome measures used, limited replication, and relative lack of clinical impact. Future work examining non-CFTR gene modifiers will have to overcome these limitations if gene modifiers are to have a meaningful role in the care of patients with CF.

ERAD defects and the HFE-H63D variant are associated with increased risk of liver damages in Alpha 1-Antitrypsin Deficiency

BACKGROUND

The most common and severe disease causing allele of Alpha 1-Antitrypsin Deficiency (1ATD) is Z-1AT. This protein aggregates in the endoplasmic reticulum, which is the main cause of liver disease in childhood. Based on recent evidences and on the frequency of liver disease occurrence in Z-1AT patients, it seems that liver disease progression is linked to still unknown genetic factors.

METHODS

We used an innovative approach combining yeast genetic screens with next generation exome sequencing to identify and functionally characterize the genes involved in 1ATD associated liver disease.

RESULTS

Using yeast genetic screens, we identified HRD1, an Endoplasmic Reticulum Associated Degradation (ERAD) associated protein, as an inducer of Z-mediated toxicity. Whole exome sequencing of 1ATD patients resulted in the identification of two variants associated with liver damages in Z-1AT homozygous cases: HFE H63D and HERPUD1 R50H. Functional characterization in Z-1AT model cell lines demonstrated that impairment of the ERAD machinery combined with the HFE H63D variant expression decreased both cell proliferation and cell viability, while Unfolded Protein Response (UPR)-mediated cell death was hyperstimulated.

CONCLUSION

This powerful experimental pipeline allowed us to identify and functionally validate two genes involved in Z-1AT-mediated severe liver toxicity. This pilot study moves forward our understanding on genetic modifiers involved in 1ATD and highlights the UPR pathway as a target for the treatment of liver diseases associated with 1ATD. Finally, these findings support a larger scale screening for HERPUD1 R50H and HFE H63D variants in the sub-group of 1ATD patients developing significant chronic hepatic injuries (hepatomegaly, chronic cholestasis, elevated liver enzymes) and at risk developing liver cirrhosis.

Genetic influences on cystic fibrosis lung disease severity

Understanding the causes of variation in clinical manifestations of disease should allow for design of new or improved therapeutic strategies to treat the disease. If variation is caused by genetic differences between individuals, identifying the genes involved should present therapeutic targets, either in the proteins encoded by those genes or the pathways in which they function. The technology to identify and genotype the millions of variants present in the human genome has evolved rapidly over the past two decades. Originally only a small number of polymorphisms in a small number of subjects could be studied realistically, but speed and scope have increased nearly as dramatically as cost has decreased, making it feasible to determine genotypes of hundreds of thousands of polymorphisms in thousands of subjects. The use of such genetic technology has been applied to cystic fibrosis (CF) to identify genetic variation that alters the outcome of this single gene disorder. Candidate gene strategies to identify these variants, referred to as “modifier genes,” has yielded several genes that act in pathways known to be important in CF and for these the clinical implications are relatively clear. More recently, whole-genome surveys that probe hundreds of thousands of variants have been carried out and have identified genes and chromosomal regions for which a role in CF is not at all clear. Identification of these genes is exciting, as it provides the possibility for new areas of therapeutic development.