The major aims of this grant are to define the molecular, biochemical, and clinical defects in sitosterolemia. In this recessively inherited disease, homozygotes show accelerated atherosclerosis with aortic stenosis, fatal myocardial infarctions, tendon and tuberous xanthomas, hemolytic episodes with deformed erythrocytes and thrombocytopenia, and attacks of disabling arthritis. Chemically, plant sterols (sitosterol, stigmasterol, campesterol, and avenosterol) and their respective 5alpha-dihydro derivatives (sitostanol and campestanol) and cholestanol accumulate in all tissues except brain because of enhanced intestinal absorption and reduced hepatic removal. In addition, cholesterol biosynthesis is discordantly down-regulated in monocytes with upregulated LDL receptors to produce increased cholesterol and plant sterol deposits such that the cells resemble atherogenic foam cells. Key research objectives are (1) locate and clone the sitosterolemia gene responsible for hyperabsorption that has been mapped to 2p21, sequence mutations, and elucidate the mechanism by with the mutated product permits unrestricted uptake and transport of plant sterols through the enterocyte. Strategies include fine mapping with a dense set of microsatilliate markers to narrow the abnormal gene region to approximately 1cM, construction of YAC contig and BAC contig with critical cDNAs from the suspected and adjoining regions. Sequence candidate genes from the region for possible mutations. DNA from 40 affected homozygotes from 30 sitosterolemic families have been assembled. (2) Investigate cholesterol and plant sterol metabolism in 3 rat models where campesterol and sitosterol constitute approximately 15 percent of the plasma sterols similar to human sitosterolemia. In these models, we propose to measure sitosterol and cholesterol absorption, assess cholesterol biosynthesis (inhibited in human sitosterolemia) and evaluate the effect of long term cholesterol feeding on development of atherosclerosis, plant sterol and cholesterol accumulation and metabolism. (3) Examine the conversion of sitosterol (24-ethyl cholesterol) to cholic acid and chenodeoxycholic acid and ascertain the pathway. Sitosterolemia is a rare disease but understanding the mechanism of enhanced sterol absorption and accumulation will provide key information to formulate better treatment of atherosclerosis in the general population.
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