Beta-hexosaminidase isozymes and replacement therapy in Gm2 gangliosidosis.

The problem of cell targeting of lysosomal enzymes is a critical one in the development of strategies for therapeutic enzyme replacement in lysosomal storage diseases. In principle, posttranscriptional isozymes with different carbohydrate-chain composition may be helpful in exploiting existing glycosyl-specific receptors on target cells, if the receptor specificities are known and match the glycosyl composition of available isozymes. In practice, however, the choice is limited to isozymes that can be obtained from tissues available in abundance, such as placenta or blood plasma. Our early experiments show that one can interfere with the interaction between hepatic (RES) receptor and enzyme glycosyl chain, to obtain extrahepatic targeting of beta-hexosaminidase, with catabolic effects. This approach, of course, does not have an immediate therapeutic application, as it involves injection of large amounts of foreign material in order to inhibit hepatic uptake. Modification of the glycosyl chain may be the method of choice in selected instances [Furbish et al. 1981], but is applicability again depends on the knowledge of receptor specificity on target cells and on composition of the glycosyl chain of the enzyme in question. Our recent experiments are a first step toward obtaining enzyme forms that can be endocytosed efficiently by mechanisms that are independent of glycosyl-specific receptors. Charge-mediated, absorptive endocytosis can be obtained by covalent coupling of cationic PLL to beta-hexosaminidase. Given the abundance of negative surface charges on most cell types [Weiss, 1969], this approach may be applicable to different target cells and organs, and possibly also to lysosomal enzymes other than beta-hexosaminidase. The existence of glycosyl recognition signals on beta-hexosaminidase can be obviated by simple chemical manipulations, such as Na-metaperiodate oxidation, which effectively prevents hepatic RES uptake [Rattazzi et al, 1982]. In combination with PLL conjugation, this may ultimately result in an enzyme form that escapes the undesired, preferential RES uptake and is efficiently endocytosed by most cells. It will remain to be seen if this artificially created isozyme (for which we propose the name "ersatzyme") is catabolically effective. This can easily be verified in our animal model, along the lines followed to demonstrate the catabolic effects of native Hex A. Finally, the recent developments in molecular genetics, which allows production of human proteins in bacterial systems by recombinant DNA techniques, make it very likely that abundant beta-hexosaminidase may be similarly obtained for therapeutic applications.(ABSTRACT TRUNCATED AT 400 WORDS)