Structural basis of the butyrylcholinesterase H-variant segregating in two Danish families.

The rare H-variant of human butyrylcholinesterase is a quantitative variant that reduces serum butyrylcholinesterase activity by about 90%. Individuals who are heterozygous for both the H-variant and the atypical variant are abnormally sensitive to the muscle relaxant succinylcholine. By using standard phenotypic serum assays, the Danish Cholinesterase Research Unit identified four individuals from two unrelated pedigrees who were heterozygous for both the H-variant (H) and the atypical (A) variant. DNA of these A/H individuals was extracted from white blood cells. Using the polymerase chain reaction and subsequent DNA sequencing, a point mutation was found at nucleotide 424 which changed amino acid 142 from valine to methionine. The previously identified atypical mutation, Asp 70 to Gly, was also seen, which segregated apart from the H-variant mutation in family studies. These two mutations were found in all four A/H individuals.