Segregation analysis of bronchial hyperresponsiveness in a general population in north Italy.

Bronchial hyperresponsiveness (BHR) is an intermediate phenotype of asthma, with a heritability component of 30-67% and possible linkage to regions on chromosome arms 5q, 11q, and 20p. Familial correlation analysis and segregation analysis for BHR, using the FCOR and REGC programs of the S.A.G.E package, were performed to examine inheritance patterns of BHR in a general population of 1,167 subjects in 550 families from the Po River Delta. BHR was assessed using the log10 of the slope of the methacholine dose-response curve (log slope) for each subject who met inclusion criteria. Using multiple linear regression analysis, the log slope values were adjusted for age, age2, sex, and height, and used in the familial correlation and segregation analyses. Father-offspring correlations are statistically significant, due specifically to high father-son correlations (r = 0.296, P < 0.001, adjusted values). Segregation analysis of BHR in the overall population, with and without a smoking covariate (number of packyears smoked), indicates an apparent absence of genetic transmission within families. However, in a segregation analysis of BHR in smoking families only, after adjusting for number of packyears smoked, the Mendelian transmission models could not be rejected. This may be evidence of a gene by smoking effect, and suggests that in families of smokers, a single locus gene may in part explain the inheritance of a compound phenotype (BHR × packyears).