Low HDL cholesterol is a common finding in patients with premature CHD and usually precedes clinically apparent CHD (Goldbourt et al., 1997). Low HDL concentrations are most frequently associated with the components of the metabolic syndrome X and are rarely due to single-gene disorders (Durrington, 2003). The rare Tangier disease, characterized by very low or even immeasurable high density lipoprotein (HDL) cholesterol concentrations and reduced LDL cholesteryl ester, is due to mutations in the ATP-binding cassette (ABCA1) transporter gene encoding cholesterol-efflux regulatory protein (Oram, 2002) (see Figure 24.1). Patients with familial HDL deficiency, who are heterozygotes for ABCA1 mutations, have cholesterol concentrations between 0.4 and 0.9 mmol/l and are at increased risk of premature CHD (Marcil et al., 1999). Mutations in the ApoAI gene, leading to under-expression of the apoprotein cause low HDL and premature CHD (Tall, 1998).
Conversely, cholesterol ester transfer protein deficiency, which is a common cause for high HDL concentrations (>2.5 mmol/l) in Japan is thought to be a state of impaired reverse cholesterol transport, which may lead to premature CHD (Nagano et al., 2002). Thus, HDL cholesterol levels, efflux of cellular lipids as well as the kinetics of HDL metabolism are important determinants of the antiatherogenic effect of HDL (Tall, 1998; von Eckardstein, 2001).
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