Bone marrow transplantation in Lesch-Nyhan disease

Enzyme Replacement Therapy for Genetic Disorders Associated with Enzyme Deficiency

Mutations in human genes might lead to loss of functional proteins, causing diseases. Among these genetic disorders, a large class is associated with the deficiency in metabolic enzymes, resulting in both an increase in the concentration of substrates and a loss in the metabolites produced by the catalyzed reactions. The identification of therapeutic actions based on small molecules represents a challenge to medicinal chemists because the target is missing. Alternative approaches are biology-based, ranging from gene and stem cell therapy, CRISPR/Cas9 technology, distinct types of RNAs, and enzyme replacement therapy (ERT). This review will focus on the latter approach that since the 1990s has been successfully applied to cure many rare diseases, most of them being lysosomal storage diseases or metabolic diseases. So far, a dozen enzymes have been approved by FDA/EMA for lysosome storage disorders and only a few for metabolic diseases. Enzymes for replacement therapy are mainly produced in mammalian cells and some in plant cells and yeasts and are further processed to obtain active, highly bioavailable, less degradable products. Issues still under investigation for the increase in ERT efficacy are the optimization of enzymes interaction with cell membrane and internalization, the reduction in immunogenicity, and the overcoming of blood-brain barrier limitations when neuronal cells need to be targeted. Overall, ERT has demonstrated its efficacy and safety in the treatment of many genetic rare diseases, both saving newborn lives and improving patients' life quality, and represents a very successful example of targeted biologics.

Successful unrelated umbilical cord blood transplantation in Lesch-Nyhan syndrome

Lesch-Nyhan syndrome (LNS) is a chronic, progressive neurodevelopmental disorder causing motor and behavioral dysfunction due to decreased synthesis of the enzyme hypoxantine-guanine phosphoribosyltransferase (HPRT). Affected boys have mental retardation, delayed development, extrapyramidal motor disturbances and self-injuring behavior. As hematopoietic stem cell transplantation (HSCT) has been shown to be effective in several neurodevelopmental inborn errors, we hypothesized that it could be favorable in LNS as well. Following a myeloablative conditioning regimen (busulphan 3.2 mg/kg/day for 4 days, cyclophosphamide 60 mg/kg/day for 2 days with ATG Thymoglobin 2.5 mg/kg/day for 4 days) an unrelated umbilical cord blood unit was transfused at the age of 2 years. The graft was a 6/6 HLA-matched at HLA-A, B loci by antigen level, and at DRB1 by allelic level typing. Infused total nucleated cell dose was 3.6 × 10e7 per kilogram body weight. Serum HPRT levels reached normal values by the end of the sixth month post transplant. Slow neurodevelopmental improvement seen during the three-year follow-up and the missing self-injuring behavior can be considered as a proof for the presence of enzyme-competent cells behind the blood-brain barrier.

Lesch-Nyhan disease in a female with a clinically normal monozygotic twin

Lesch-Nyhan disease (LND) is an inborn error of purine metabolism caused by defective activity of the enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT, EC 2.4.2.8), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classic phenotype occurs almost exclusively in males and is characterized by hyperuricemia, mental retardation, severe dystonia, and self-injurious behavior. Heterozygous carrier females are usually clinically normal. However, a small number of clinically affected females have been described. In all previous cases there was a mutation in one HPRT allele and non-random inactivation of the X chromosome carrying the normal HPRT gene. We have analyzed a female MZ twin pair discordant for Lesch-Nyhan disease. The mother and both twins are heterozygous carriers of a HPRT splicing mutation (IVS8 + 4A > G; c.609 + 4A > G) and all three express the mutant allele at similar frequencies in peripheral blood T cells. The mother and one sister are clinically normal. In the affected twin, the clinical phenotype is classical for Lesch-Nyhan disease, despite the fact that HPRT activity in the blood was also normal. X inactivation analysis showed a skewed pattern in the fibroblasts of the affected twin sister, with the X chromosome carrying the normal HPRT allele preferentially inactivated. As in many other reported cases of X-linked diseases, the discordant phenotype of the two monozygous twin sisters suggests that the process responsible for monozygotic twinning can trigger skewed X inactivation.

Adenoviruses encoding HPRT correct biochemical abnormalities of HPRT-deficient cells and allow their survival in negative selection medium

The Lesch-Nyhan syndrome is an X-linked disorder caused by a virtually complete absence of the key enzyme of purine recycling, hypoxanthine-guanine phosphoribosyltransferase (HPRT). It is characterized by uric acid overproduction and severe neurological dysfunction. No treatment is yet available for the latter symptoms. A possible long-term solution is gene therapy, and recombinant adenoviruses have been proposed as vectors for gene transfer into postmitotic neuronal cells. We have constructed an adenoviral vector expressing the human HPRT cDNA under the transcriptional control of a short human cytomegalovirus major immediate early promoter (RAd-HPRT). Here we show that infection of human 1306, HPRT-negative cells with RAd-HPRT, expressed high enough levels of HPRT enzyme activity, as to reverse their abnormal biochemical phenotype, thus enhancing hypoxanthine incorporation and restoring purine recycling, increasing GTP levels, decreasing adenine incorporation, and allowing cell survival in HAT medium in which only cells expressing high levels of HPRT can survive. Infection of murine STO cells, increased hypoxanthine incorporation and restored purine recycling, thus allowing cell survival in HAT medium, and reduced de novo purine synthesis. Although both cells were able to survive in HAT medium post infection with RAd-HPRT, some of the biochemical consequences differed. In summary, even though adenoviral vectors do not integrate into the genome of target HPRT-deficient human or murine cells, RAd-HPRT mediated enzyme replacement corrects abnormal purine metabolism, increases intracellular GTP levels, and allows cells to survive in a negative selection medium.

Age-dependent selection against hypoxanthine phosphoribosyl transferase-deficient cells in mouse haematopoiesis

The basis of a previously observed difference in the level of contribution of hypoxanthine phosphoribosyltransferase-deficient cells between the haematopoietic and non-haematopoietic tissues of chimaeric and heterozygous mice has been clarified by studying two populations of female mice that differ only in that one is heterozygous for a null allele at the hprt locus and the other is wild type at this locus. Both populations are heterozygous for an electrophoretic variant allele at the X-linked Pgk-1 locus, so that X-chromosome inactivation generates cells expressing different isozymes of phosphoglycerate kinase which can be assayed to monitor cell selection. The results show that hypoxanthine phosphoribosyltransferase deficiency itself, rather than an effect of another X-linked gene, causes a reduced level of contribution to haematopoietic tissues. Further, the extent of the depletion increases significantly with age, and this effect is due to a progressive reduction in the level of contribution to haematopoietic tissues rather than to an increase in the level of contribution to non-haematopoietic tissues.

Clinical application of somatic gene therapy in inborn errors of metabolism

Rapid advances in recombinant DNA and gene transfer technologies provide the potential for somatic gene therapy of inborn errors of metabolism in which the genetically defective function will be restored by transfer of a normal gene into somatic cells. The therapeutic potential and safety of gene therapy has been explored in cultured cells and experimental animals, but therapeutic clinical trials have not yet been proposed or performed. The technologies which may make somatic gene replacement therapy feasible need to be considered and criticised from a clinical perspective. Clinical trials will be necessary to determine the efficacy of somatic gene therapy and address concerns about safety.

Expression of human HPRT mRNA in brains of mice infected with a recombinant herpes simplex virus-1 vector

Complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, the Lesch-Nyhan syndrome. This disorder has been identified as a candidate for initial attempts at somatic cell gene therapy. We have previously reported the construction of a recombinant herpes simplex virus type 1 (HSV-1) vector containing human hprt cDNA sequences under the regulatory control of the viral thymidine kinase gene (tk) [Palella et al., Mol. Cell. Biol. 8 (1988) 457-460]. Infection of HPRT- cultured rat neuronal cells with these vectors resulted in transient expression of human hprt. In this paper, we report the expression of human hprt mRNA transcripts in the brains of mice infected in vivo with this vector by direct intracranial inoculation. Human hprt transcripts were distinguished from endogenous mouse transcripts by RNase A mapping using riboprobes transcribed from human hprt cDNA. These initial studies demonstrate the transfer and transcription of a human gene in brain cells by direct in vivo infection with recombinant HSV-1 vectors.

Herpes simplex virus-mediated human hypoxanthine-guanine phosphoribosyltransferase gene transfer into neuronal cells

The virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, Lesch-Nyhan syndrome. Transfer of the HPRT gene into fibroblasts and lymphoblasts in vitro and into hematopoietic cells in vivo has been accomplished by other groups with retroviral-derived vectors. It appears to be necessary, however, to transfer the HPRT gene into neuronal cells to correct the neurological dysfunction of this disorder. The neurotropic virus herpes simplex virus type 1 has features that make it suitable for use as a vector to transfer the HPRT gene into neuronal tissue. This report describes the isolation of an HPRT-deficient rat neuroma cell line, designated B103-4C, and the construction of a recombinant herpes simplex virus type 1 that contained human HPRT cDNA. These recombinant viruses were used to infect B103-4C cells. Infected cells expressed HPRT activity which was human in origin.

Herpes simplex virus-mediated human hypoxanthine-guanine phosphoribosyltransferase gene transfer into neuronal cells

The virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, Lesch-Nyhan syndrome. Transfer of the HPRT gene into fibroblasts and lymphoblasts in vitro and into hematopoietic cells in vivo has been accomplished by other groups with retroviral-derived vectors. It appears to be necessary, however, to transfer the HPRT gene into neuronal cells to correct the neurological dysfunction of this disorder. The neurotropic virus herpes simplex virus type 1 has features that make it suitable for use as a vector to transfer the HPRT gene into neuronal tissue. This report describes the isolation of an HPRT-deficient rat neuroma cell line, designated B103-4C, and the construction of a recombinant herpes simplex virus type 1 that contained human HPRT cDNA. These recombinant viruses were used to infect B103-4C cells. Infected cells expressed HPRT activity which was human in origin.