Mutation analysis of exon 9 of the LDL receptor gene in Thai subjects with primary hypercholesterolemia.

The low density lipoprotein (LDL) receptor plays an important role in cholesterol homeo- stasis. A mutation in this gene causes an autosomal codominant disorder, namely familial hypercholesterolemia (FH). In this study, single strand conformation polymorphism (SSCP) analy- sis was used to screen for mutations in exon 9 of the LDL receptor gene in a group of 45 Thai patients (11 males and 34 females) with primary hypercholesterolemia. The peptide encoded by exon 9 belongs to the epidermal growth factor (EGF) precursor homology domain which is highly conserved in the LDL receptor protein. An abnormal SSCP pattern was observed in one female patient. The same screening strategy was also used to screen DNA samples from 33 normolipi- demic subjects. All of these samples showed normal SSCP pattern. By direct DNA sequencing, the underlying mutation in the DNA with abnormal SSCP pattern was identified. The index subject was heterozygous for a T to C transition at nucleotide 1235. This transition would cause a noncon- servative substitution of a nonpolar side chain amino acid "methionine" at codon 391, with an uncharged polar side chain amino acid "threonine", note M391T. From multiple amino acid sequence alignment in six species, the amino acid at codon 391 and the others nearby are com- pletely conserved. Such nonconservative substitution of an amino acid residue in a highly con- served region could consequently result in a functional and/or structural defect in the receptor protein. In conclusion, we propose that M391 T is likely to be the cause of hypercholesterolemia in this index subject. Key word : Hypercholesterolemia, LDL Receptor Gene, Mutation