Partial deletion of the gene encoding sterol 27-hydroxylasein a subject with cerebrotendinous xanthomatosis

An Italian subject with cerebrotendinous xanthomatosis (CTX) was found to have a partial deletion of the gene encoding the enzyme sterol 27-hydroxylase (CYP27 gene). Southern blot analysis revealed that this deletion (approximately 2 kb) spans from intron 6 to the 3' flanking (3'FLK) region, eliminating exons 7-9, the last three exons of CYP27 gene. No sterol 27-hydroxylase mRNA was detected in proband cells, either by Northern blot analysis or by reverse transcription polymerase chain reaction (PCR). This suggests that the mutant mRNA devoid of the exon encoding the whole untranslated sequence (exon 9) might be rapidly degraded in the cytoplasm. We used inverse PCR to obtain a partial sequence of the 3'FLK region of the normal CYP27 gene; this allowed us to define the mechanism underlying the deletion. The established sequence was used to design suitable primers to perform step-wise sequences of a 1.7 kb segment of the 3'FLK region of the normal gene and of the deletion joint in the CTX patient. The analysis of the sequence data indicate that the deletion might result from a complex mechanism involving two intragenic recombinations between a) two 14 nucleotide complementary sequences, one in intron 6 and the other in the 3'FLK region: and b) AT-rich complementary sequences of the 3'FLK region, and a slipped mispairing between two 6 nucleotide direct repeats, one in intron 6 and the other in the 3'FLK region. Such repeats are brought close to each other by the formation of the stem-loops induced by the two intragenic recombinations. This is the first example of CTX caused by a rearrangement of CYP27 gene.