But the complex story doesn’t end there. In the new study, the researchers revealed that the extra chromosome present in individuals with Down’s syndrome, chromosome 21, results in increased levels of a small RNA molecule called miRNA-155. This molecule doesn’t contain the instructions for the production of a protein like many stretches of RNA, but instead decreases the production of SNX27.
Alongside regulating receptor expression, SNX27 was found to be critical for the generation of a protein called β-amyloid. It carries out this important function by preventing the activity of an enzyme called gamma-secretase. This particular enzyme chops up the precursor of β-amyloid to produce β-amyloid proteins. When SNX27 levels are reduced, there is more active gamma-secretase hanging around in cells, and hence β-amyloid levels are elevated because more precursor proteins can be chopped up.