Background: The Boston Early-Onset COPD study showed that current or ex-smoking first degree relatives of severe early onset COPD probands have significantly lower forced expiratory volume in 1 second (FEV(1)) and FEV(1)/forced vital capacity (FVC) values than current or ex-smoking control subjects, which suggests the existence of genetic risk factors for the development of COPD in response to cigarette smoking. We hypothesised that first degree relatives of early onset COPD probands may also have lower values of spirometric parameters such as forced expiratory flow at the mid-portion of forced vital capacity (FEF(25-75)) and FEF(25-75)/FVC.
Methods: Using generalised estimating equations, FEF(25-75) and FEF(25-75)/FVC were analysed in 333 first degree relatives of probands with severe early onset COPD and 83 population based controls; analyses were also performed on data stratified by smoking status. Narrow sense heritability estimates were calculated using a variance component approach.
Results: Significantly lower FEF(25-75) and FEF(25-75)/FVC were observed in smoking (FEF(25-75): beta -0.788 l/s (95% CI -1.118 to -0.457), FEF(25-75)/FVC: beta -20.4% (95% CI -29.3 to -11.6, p<0.0001 for both phenotypes) and non-smoking (FEF(25-75): beta -0.357 l/s (95% CI -0.673 to -0.041, p = 0.0271), FEF(25-75)/FVC: beta -9.5% (95% CI -17.1 to -1.9, p = 0.0145)) first degree relatives of early onset COPD probands. Narrow sense heritability estimates for FEF(25-75) (h(2) = 0.38) and FEF(25-75)/FVC (h(2) = 0.45) were similar to those for FEV(1) and FEV(1)/FVC.
Conclusion: Lower values of FEF(25-75) and FEF(25-75)/FVC in non-smoking first degree relatives of early onset COPD probands than in controls suggest a genetic susceptibility to develop obstructive lung disease, independent of smoking, which is magnified by exposure to deleterious environments as suggested by the further decrements in FEF(25-75) and FEF(25-75)/FVC seen in smoking first degree relatives. FEF(25-75) and FEF(25-75)/FVC have high heritability and are important intermediate phenotypes for inclusion in genetic epidemiological studies of COPD.