To assess the genetics of hyperlipidemia in coronary heart disease, family studies were carried out in 2520 relatives and spouses of 176 survivors of myocardial infarction, including 149 hyperlipidemic and 27 normolipidemic individuals. The distribution of fasting plasma cholesterol and triglyceride values in relatives, together with segregation analyses, suggested the presence of five distinct lipid disorders. Three of these—familial hypercholesterolemia, familial hypertriglyceridemia, and familial combined hyperlipidemia—appeared to represent dominant expression of three different autosomal genes, occurring in about 20% of survivors below 60 yr of age and 7% of all older survivors. Two other disorders—polygenic hypercholesterolemia and sporadic hypertriglyceridemia—each affected about 6% of survivors in both age groups.
The most common genetic form of hyperlipidemia identified in this study has hitherto been poorly defined and has been designated as familial combined hyperlipidemia. Affected family members characteristically had elevated levels of both cholesterol and triglyceride. However, increased cholesterol or increased triglyceride levels alone were also frequently observed. The combined disorder was shown to be genetically distinct from familial hypercholesterolemia and familial hypertriglyceridemia for the following reasons: (a) the distribution pattern of cholesterol and triglyceride levels in relatives of probands was unique; (b) children of individuals with combined hyperlipidemia did not express hypercholesterolemia in contrast to the finding of hypercholesterolemic children from families with familial hypercholesterolemia; and (c) analysis of informative matings suggested that the different lipid phenotypes owed their origin to variable expression of a single autosomal dominant gene and not to segregation of two separate genes, such as one elevating the level of cholesterol and the other elevating the level of triglyceride.
Heterozygosity for one of the three lipid-elevating genes identified in this study may have a frequency in the general population of about 1%, constituting a major problem in early diagnosis and preventive therapy.