Enzymes as agents for the treatment of disease

Engineering thermal stability of L-asparaginase by in vitro directed evolution

L-asparaginase (EC 3.5.1.1, L-ASNase) catalyses the hydrolysis of l-Asn, producing L-Asp and ammonia. This enzyme is an anti-neoplastic agent; it is used extensively in the chemotherapy of acute lymphoblastic leukaemia. In this study, we describe the use of in vitro directed evolution to create a new enzyme variant with improved thermal stability. A library of enzyme variants was created by a staggered extension process using the genes that code for the L-ASNases from Erwinia chrysanthemi and Erwinia carotovora. The amino acid sequences of the parental L-ASNases show 77% identity, but their half-inactivation temperature (T(m)) differs by 10 degrees C. A thermostable variant of the E. chrysamthemi enzyme was identified that contained a single point mutation (Asp133Val). The T(m) of this variant was 55.8 degrees C, whereas the wild-type enzyme has a T(m) of 46.4 degrees C. At 50 degrees C, the half-life values for the wild-type and mutant enzymes were 2.7 and 159.7 h, respectively. Analysis of the electrostatic potential of the wild-type enzyme showed that Asp133 is located at a neutral region on the enzyme surface and makes a significant and unfavourable electrostatic contribution to overall stability. Site-saturation mutagenesis at position 133 was used to further analyse the contribution of this position on thermostability. Screening of a library of random Asp133 mutants confirmed that this position is indeed involved in thermostability and showed that the Asp133Leu mutation confers optimal thermostability.

L-asparaginase-provoked seizures as singular expression of central nervous toxicity

Patients treated with L-asparaginase may present with hemorrhagic and thrombotic cerebrovascular events. This syndrome generally occurs after a few weeks of therapy and may occur after L-asparaginase therapy is completed. Complications appear to result from depletion of plasma proteins involved in coagulation and fibrinolysis. Seizures are uncommon symptoms, and are always caused by cerebrovascular events. We report a case of seizure associated with L-asparaginase therapy but no evidence of hemorrhagic or thrombotic cerebrovascular events.

Proteases in the evaluation of pancreatic function and pancreatic disease

This paper reviews the role of pancreatic proteases (focusing upon trypsin, chymotrypsin and elastase) in the diagnosis and management of chronic pancreatic insufficiency (CPI), emphasizing advances over the last 5 years. Some important novel aspects of these enzymes in acute pancreatitis are also described, including their role in diagnosis and their interaction with cholecystokinin in the pathogenesis of the disease. The recent interest in these enzymes as agents promoting the spread of cancer in animals and human subjects is also described. A hierarchical approach has been taken to explore the advantages and limitations of tests in different source materials: serum, feces, duodenal aspirate, and non-invasive pancreatic function tests. The practical usefulness of fecal elastase-1 and of fecal chymotrypsin concentrations in diagnosis and management of CPI, respectively, is one of the major lessons to be learned from analysis of the recent literature, and forms the principal message of this review.

Keratinocyte gene therapy for adenosine deaminase deficiency: a model approach for inherited metabolic disorders

Disorders in which there is toxic buildup of circulating substrate may be treated by furnishing an enzyme reservoir capable of metabolically processing the excess substrate. The epidermal keratinocyte is a potential site for such a reservoir. In this study, we explore the capacity of genetically modified keratinocytes to metabolize extracellular substrate in a culture model that resembles in vivo epidermal architecture. Keratinocytes from adenosine deaminase (ADA)-deficient patients were transduced with a retroviral vector encoding the human ADA gene and the capacity of this tissue to deaminate deoxyadenosine (dAdo) in vitro was measured. The results show that at a substrate concentration of 10 microM, ADA-corrected keratinocytes deaminate dAdo at a rate of 0.38 nmol/min.10(6) cells. These results indicate that keratinocytes process extracellular substrate at rates that suggest complete substrate conversion in a single pass. This study provides a strong indication that the epidermis, the largest and most accessible tissue of the body, is a valuable site for designing clinically relevant gene therapies.

Stereological and morphometric analysis of dermal fibroblasts before and after bone marrow transplantation in a case of mucopolysaccharidosis I Scheie phenotype

Bone marrow transplantation (BMT) has been used therapeutically in several types of mucopolysaccharidoses (MPS) and other inherited metabolic disorders. Fibroblasts are severely affected in MPS due to the intralysosomal accumulation of glycosaminoglycans. We report a stereological and morphometric study at light and electron microscopy levels of dermal fibroblasts before and 21 months after BMT in a young girl with MPS I Scheie phenotype (MPS I-S). Dermal fibroblasts showed remarkable morphological changes although their density, expressed as number of fibroblasts per unit volume of dermis (number density), was not modified in the post-BMT samples as compared to pre-BMT ones. Stereological and morphometric parameters referring to cell characteristics of post-BMT fibroblasts (nuclear and cell surface densities, and both nucleus/cell and cell/nucleus volume densities) showed significant differences when compared with pre-BMT fibroblasts, and non-significant differences regarding control cells. On the other hand, quantitative parameters of the lysosomal compartment from post-BMT fibroblasts showed intermediate values between pre-BMT and control fibroblasts. These results, at cellular level, are in agreement with previous biochemical and clinical results, and clearly showed a progressive course to a non-pathological state. All parameters estimated may be considered useful tools in evaluating the success of BMT. These parameters provide quantitative data which can be statistically compared, showing the changes due to the reduction of storage material after BMT. Cell/nucleus volume density is especially interesting since not only is it easy to estimate, even by automatic procedures, but it could also constitute a numerical expression of skin anatomopathological analyses performed post-BMT.