A missing enzyme is a property of the genome, but its consequences are a property of the tissue. Two tissues can carry the identical defect and behave completely differently, because the physiological effect depends on two variables that vary from tissue to tissue: whether that tissue expresses the affected enzyme at all, and how much of its function depends on the pathway the enzyme belongs to.
Consider a pathway that converts a fuel substrate into an energy-yielding product. In a tissue that expresses the pathway's enzymes at high levels and uses the product as a major ATP source, blocking one step removes a large fraction of the tissue's energy supply. In a tissue that expresses the same pathway at low levels and relies on a different fuel, the same block removes only a small fraction of energy supply and may be tolerated. The lesion is identical; the dependence is not.
A second variable is the tissue's capacity to use alternative routes. A tissue with a rich set of bypass enzymes and transporters can reroute flux around the block, while a tissue with few alternatives cannot. This is why the same missing enzyme can be silent in one organ and catastrophic in another: the outcome is set by the tissue's enzyme repertoire and its metabolic demand, not by the identity of the missing step alone.