Twin Studies in Altitude and Hypoxia Research

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Humans exhibit high individual variation in response to acute hypoxia exposure. A number of published studies have used a classic 'twin study' model, comparing responses within pairs of monozygotic and dizygotic twins, to separate genetic from environmental contributions to the variation in altitude acclimatization. Available data suggest that some aspects of acclimatization have a heritable component. Most prominent is the hypoxic ventilatory response (HVR), which was repeatedly shown to be heritable in a number of age groups spanning infancy to adulthood (F-ratio range: 2.03 to 5.26). The ventilatory response to hypercapnia appears to only be heritable when tested in hypoxic conditions, providing additional evidence for a genetic component to the HVR (F-ratio range: 0.31, 6.92). A number of studies reported an estimate of heritability for more general hypoxic responses, such as heart rate, blood pressure, and blood gases; however, many of these studies relied on relatively small sample sizes and used inaccurate estimates of heritability and thus provided inconclusive evidence to elucidate the source of variation. Future genetic inquiries into the basis of variation in altitude acclimatization might benefit from further use of the classic twin study model: these experiments could identify the specific endophenotypes of altitude acclimatization that are heritable and therefore promising candidates for subsequent molecular studies, such as candidate-gene or genome-wide association studies.