Characterization of a population of monozygotic twins with asthma

Asthma is a complex inflammatory lung disease that results from allergic and nonallergic environmental exposures in genetically susceptible individuals. The interrelationship and impact of environmental and genetic determinants on the development of asthma is still unclear. Monozygotic twins (MZT) have been considered a potentially useful population to investigate the role of genetic and environmental factors on asthma because of their identical genetic background, similar childhood environmental exposures, and expected high concordance rate for asthma.

A detailed questionnaire designed to obtain demographic information, medical history/severity of asthma, and environmental exposures was distributed to 43 pairs of MZT pairs where one or both probands had a history of asthma. All twin pairs were clinically evaluated for atopy by skin testing. Asthma was assessed by questionnaire, baseline spirometry, disease severity, airway reactivity, and medication scores.

Based on a history of physician-diagnosed asthma, disease severity, airway reactivity, and medication scores, 22 MZT pairs were identified as concordant (C) for asthma while 21 MZT pairs were discordant (D) for asthma. Those MZT-C for asthma were on average older and more often female. Adjusted odds ratios for age and gender revealed that MZT-C for asthma had a 2.3 greater likelihood of childhood tobacco exposure and a 1.8 greater likelihood of early exposure to a pet compared to MZT-D pairs. In addition, MZT-C for asthma had a 2.4 greater likelihood of being full term vs. premature at birth. None of these odds ratios were statistically significant due to a small sample size. However, a trend for a greater likelihood of earlier environmental exposures was observed for MZT-C asthma pairs. Among MZT-D probands, a history of asthma in one proband was supported by a lower FEV1 and higher airway reactivity, disease severity, asthma symptom, and medication scores compared to the nonasthma proband. There was significant intrapair heterogeneity among MZT-C and MZT-D probands in their positive skin test responses to specific aeroallergens.

Cross-sectional evaluation of larger MZT populations where one or both proband has asthma with a long-term prospective follow-up may be a feasible way for identifying the impact of environmental determinants as independent risk factors for the development of asthma.