Is there a relationship between 3-hydroxy-3-methylglutaryl coenzyme a reductase activity and forebrain pathology in...

Previous reports have suggested that elevated levels of phenylalanine inhibit cholesterol synthesis. The goals of this study were to investigate if perturbations in cholesterol synthesis exist in the PAH(enu2) genetic mouse model for phenylketonuria (PKU), and if so, initiate studies determining if they might underlie the white matter pathology that exists in PKU forebrain. Gross sections and electron microscopy showed that select tracts were hypomyelinated in adult PKU mouse forebrain but not hindbrain. The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), the rate controlling enzyme in the cholesterol biosynthetic pathway, was examined in isolated microsomes from forebrain, hindbrain, and liver to assess if perturbations in cholesterol biosynthesis were occurring. HMGR activity was normal in unaffected PKU hindbrain and was increased 2-4-fold in PKU liver compared to control. HMGR activity in the forebrain, however, was decreased by 30%. Because normal numbers of MBP-expressing glia (oligodendrocytes) were present, but the number of glia expressing HMGR was reduced by 40% in the hypomyelinated tracts, the decreased HMGR activity seemed to result from a down-regulation of HMGR expression in affected oligodendrocytes. Exposure of an oligodendrocyte-like glioma cell line to physiologically relevant elevated levels of Phe resulted in a 30% decrease in cholesterol synthesis, a 28% decrease in microsomal HMGR activity, and a 28% decrease in HMGR protein levels. Measurement of HMGR activity after addition of exogenous Phe to control brain microsomes revealed that Phe is a noncompetitive inhibitor of HMGR; physiologically relevant elevated levels of exogenous Phe inhibited HMGR activity by 30%. Taken together, these data suggest that HMGR is moderately inhibited in the PKU mouse. Unlike other cell types in the body, a subset of oligodendrocytes in the forebrain seems to be unable to overcome this inhibition. We speculate that this may be the cause of the observed pathology in PKU brain.