Significance of heritability in primary and secondary pediatric hypertension.

BACKGROUND: Patient weight and family history are significant risk factors for the development of hypertension in children. Multiple genetic factors have been identified in primary (essential) hypertension in adults; however, the delineation of genetic factors in the separate populations of children with primary or secondary hypertension are not well understood. Heritability is the proportion of observed variation in a particular trait that can be attributed to an inherited genetic factor in contrast to environmental factors. In the consideration of hypertension, heritability can be assessed in terms of an underlying continuous gradient of the liability for developing hypertension. With this assumption it is possible to compute heritability using hypertension incidence among relatives and described by Falconer. Heritability values range from 0 (no genetic contribution) to 1 (complete genetic contribution). The aim of this study was to determine the genetic contribution to primary and secondary hypertension in a pediatric population through heritability analysis.
METHODS: This was a retrospective case-control analysis of medical records of children (n=276) followed in the Pediatric Nephrology Clinic over a 4-year period from 1999 to 2002. There were 192 children and adolescents with primary hypertension (124 male, 68 female, age 0 to 21 years) and 84 children and adolescents with secondary hypertension (46 male, 38 female, age 0 to 21 years). Each hypertensive group served as the control for the other. Estimates of heritability were made using Falconer's method 2. The model assumes independence between the environment and genetic factors and that the joint distribution of liabilities between parent and child are normally distributed. Problems can arise from computing heritability due to dominance within loci, correlations between nongenetic familial effects, or the presence of a major gene.
RESULTS: Of the children and adolescents with primary hypertension, 49% had parents with primary hypertension; and of the children and adolescents with secondary hypertension, 24% had parents with primary hypertension. Of the children and adolescents with primary hypertension, 10% had parents with secondary hypertension; and of the children and adolescents with secondary hypertension, 46% had parents with secondary hypertension. The estimated heritability for primary hypertension was 0.84 (SE=0.21). The estimated heritability for secondary hypertension was 1.14 (SE=0.21). As the value was >1, this indicates that the fit of the liability model is poor and that a few genes, or even one major gene, were significantly involved in the causes of secondary hypertension in the children and adolescents studied.
CONCLUSIONS: The results suggest that primary and secondary hypertension do not share the same type of genetic profile. Primary hypertension in children and adolescents is likely due to a large number of additive contributions of genes, although a highly correlated environmental component can not be excluded. The continuous liability model is inappropriate for secondary hypertension because the estimate was substantially greater than one. This study supports the model that secondary hypertension in children and adolescents may be related to just a few genes.