Glycosidase active site mutations in human alpha-L-iduronidase.

Mucopolysaccharidosis type I (MPS I; McKusick 25280) results from a deficiency in alpha-L-iduronidase activity. Using a bioinformatics approach, we have previously predicted the putative acid/base catalyst and nucleophile residues in the active site of this human lysosomal glycosidase to be Glu182 and Glu299, respectively. To obtain experimental evidence supporting these predictions, wild-type alpha-L-iduronidase and site-directed mutants E182A and E299A were individually expressed in Chinese hamster ovary-K1 cell lines. We have compared the synthesis, processing, and catalytic properties of the two mutant proteins with wild-type human alpha-L-iduronidase. Both E182A and E299A transfected cells produced catalytically inactive human alpha-L-iduronidase protein at levels comparable to the wild-type control. The E182A protein was synthesized, processed, targeted to the lysosome, and secreted in a similar fashion to wild-type alpha-L-iduronidase. The E299A mutant protein was also synthesized and secreted similarly to the wild-type enzyme, but there were alterations in its rate of traffic and proteolytic processing. These data indicate that the enzymatic inactivity of the E182A and E299A mutants is not due to problems of synthesis/folding, but to the removal of key catalytic residues. In addition, we have identified a MPS I patient with an E182K mutant allele. The E182K mutant protein was expressed in CHO-K1 cells and also found to be enzymatically inactive. Together, these results support the predicted role of E182 and E299 in the catalytic mechanism of alpha-L-iduronidase and we propose that the mutation of either of these residues would contribute to a very severe clinical phenotype in a MPS I patient.

Human glucocerebrosidase: heterologous expression of active site mutants in murine null cells.

Using bioinformatics methods, we have previously identified Glu235 and Glu340 as the putative acid/base catalyst and nucleophile, respectively, in the active site of human glucocerebrosidase. Thus, we undertook site-directed mutagenesis studies to obtain experimental evidence supporting these predictions. Recombinant retroviruses were used to express wild-type and E235A and E340A mutant proteins in glucocerebrosidase-deficient murine cells. In contrast to wild-type enzyme, the mutants were found to be catalytically inactive. We also report the results of various studies (Western blotting, glycosylation analysis, subcellular fractionation, and confocal microscopy) indicating that the wild-type and mutant enzymes are identically processed and sorted to the lysosomes. Thus, enzymatic inactivity of the mutant proteins is not the result of incorrect folding/processing. These findings indicate that Glu235 plays a key role in the catalytic machinery of human glucocerebrosidase and may indeed be the acid/base catalyst. As concerns Glu340, the results both support our computer-based predictions and confirm, at the biological level, previous identification of Glu340 as the nucleophile by use of active site labeling techniques. Finally, our findings may help to better understand the molecular basis of Gaucher disease, the human lysosomal disease resulting from deficiency in glucocerebrosidase.

Structural features of normal and mutant human lysosomal glycoside hydrolases deduced from bioinformatics analysis.

Lysosomal storage diseases are due to inherited deficiencies in various enzymes involved in basic metabolic processes. As with other genetic diseases, accurate structure data for these enzymatic proteins should help in better understanding the molecular effects of mutations identified in patients with the corresponding lysosomal diseases; however, no such three-dimensional (3D) structure data are available for many lysosomal enzymes. Thus, we herein intend to illustrate for an audience of molecular geneticists how structure information can nonetheless be obtained via a bioinformatics approach in the case of five human lysosomal glycoside hydrolases. Indeed, using the two-dimensional hydrophobic cluster analysis method to decipher the sequence information available in data banks for the large group of glycoside hydrolases (clan GH-A) to which these human lysosomal enzymes belong, we could deduce structure predictions for their catalytic domains and propose explanations for the molecular effects of mutations described in patients. In addition, in the case of human beta-glucuronidase for which experimental 3D data have been reported, we also show here that bioinformatics methods relying on the available 3D structure information can be used to obtain further insights into the effects of various mutations described in patients with Sly disease. In a broader perspective, our work stresses that, in the context of a rapid increase in protein sequence information through genome sequencing, bioinformatics approaches might be highly useful for generating structure-function predictions based on sequence-structure interrelationships.

Gene delivery systems: Bridging the gap between recombinant viruses and artificial vectors.

Although most research in the field of somatic gene therapy has investigated the use of recombinant viruses for transferring genes into somatic target cells, various methods for nonviral gene delivery have also been proposed. Both types of gene delivery systems have advantages and drawbacks. Schematically, viral vectors are particularly efficient for gene delivery, whereas nonviral systems are free of the difficulties associated with the use of recombinant viruses but need to be further optimized to reach their full potential. In order to bridge the gap between viral vectors and synthetic reagents, we discuss here some specific features of the viral vector systems of today that could advantageously be taken into account for the design of improved nonviral gene delivery systems. Indeed, although nonviral systems differ fundamentally from viral systems, one possible approach towards enhanced artificial reagents aims at developing 'artificial viruses' that mimic the highly efficient processes of viral infection.

Active-site motifs of lysosomal acid hydrolases: invariant features of clan GH-A glycosyl hydrolases deduced from hydrophobic cluster analysis.

The clan GH-A is a group of more than 200 proteins representing nine established families of glycosyl hydrolases that act on a large variety of substrates. This clan includes five enzymes implicated in lysosomal storage diseases: beta-glucuronidase (Sly disease), beta-glucocerebrosidase (Gaucher disease), beta-galactosidase (Landing disease and Morquito type B disease), beta-mannosidase (mannosidosis) and alpha-L-iduronidase (Hurler-Scheie disease). Examination of known 3D structures from some families of the clan allowed us to deduce structural and functional features shared by these proteins. We then used the hydrophobic cluster analysis method to study the protein sequences of the entire clan. Our results reveal that, despite low levels of sequence identity, all the proteins of the clan (including the aforementioned lysosomal enzymes) likely share a similar catalytic domain consisting of an (alpha/beta)8 barrel with conserved functional amino acids located at the C-terminal ends of six of the eight strands constituting the beta-barrel. Interestingly, several mutations reported to be responsible for lysosomal storage diseases are located within these conserved regions of the lysosomal enzyme catalytic domains.

Conserved catalytic machinery and the prediction of a common fold for several families of glycosyl hydrolases.

The regions surrounding the catalytic amino acids previously identified in a few "retaining" O-glycosyl hydrolases (EC 3.2.1) have been analyzed by hydrophobic cluster analysis and have been used to define sequence motifs. These motifs have been found in more than 150 glycosyl hydrolase sequences representing at least eight established protein families that act on a large variety of substrates. This allows the localization and the precise role of the catalytic residues (nucleophile and acid catalyst) to be predicted for each of these enzymes, including several lysosomal glycosidases. An identical arrangement of the catalytic nucleophile was also found for S-glycosyl hydrolases (myrosinases; EC 3.2.3.1) for which the acid catalyst is lacking. A (beta/alpha)8 barrel structure has been reported for two of the eight families of proteins that have been grouped. It is suggested that the six other families also share this fold at their catalytic domain. These enzymes illustrate how evolutionary events led to a wide diversification of substrate specificity with a similar disposition of identical catalytic residues onto the same ancestral (beta/alpha)8 barrel structure.

Interferon gamma is effective for BM purging in a patient with CML.

We report a case of Ph-positive CML where the BM was incubated for 24 h with 10(3) IU/ml IFN gamma and then cultured in liquid media for 4 weeks. After 24 h incubation, there was no differential sensitivity of CML CFU-GM to IFN gamma compared with untreated BM. Subsequent long-term culture (LTC) of the IFN gamma treated CML BM, however, demonstrated a 75% inhibition of production of CFU-GM from the second week onwards. Using PCR, we were able to demonstrate two types of BCR-ABL transcript in the diagnostic BM. After 4 weeks of LTC, the J(bcr b3/ABL II) RNA transcript persisted in the untreated BM, whereas neither BCR/ABL RNA transcripts were detected in the culture established with IFN gamma-treated CML BM. This study has two points of interest with the demonstration of (1) a possible antileukaemic effect of IFN gamma on the progenitors generated in the LTC system, and (2) the use of highly sensitive PCR technology to evaluate the effectiveness of IFN gamma to purge CML BM of Ph-positive cells.

Polymerase chain reaction: a method for monitoring tumor cell purge by long-term culture in BCR/ABL positive acute lymphoblastic leukemia.

We report the successful purging of leukemia cells bearing the Philadelphia chromosome and BCR/ABL transcripts by long-term marrow culture (LTC), and subsequent grafting of the purged marrow in a case of refractory acute lymphoblastic leukemia. The efficiency of the purge was evaluated by polymerase chain reaction (PCR) for BCR/ABL transcripts. In two LTCs initiated in the blastic stage, we demonstrated the selective effect of three culture media (serum dependent, serum-free (SF) supplemented or not with IL3 and GM-CSF) on the proliferative potential of normal hematopoietic (CFU-GM/BFU-E) and leukemic progenitors (CFU-ALL). BCR/ABL positive cells disappeared after 3 to 4 weeks of culture. The addition of IL3 and GM-CSF to the SF medium enhanced the growth of CFU-GM/BFU-E and shortened the purging period. We therefore carried out a LTC in the presence of IL3 and GM-CSF with marrow harvested in morphological remission. BCR/ABL positivity was detected at the outset, although no leukemia cells could be identified. The BCR/ABL was no longer found by PCR in the 7 and 14 day LTCs. The patient, consolidated by high dose polychemotherapy and total body irradiation, was infused with the 14 day LTC. This study indicates that PCR is a useful and sensitive technique for monitoring tumor cell reduction after LTC prior to autografting.

Detection of BCR/ABL translocation by polymerase chain reaction in leukemic progenitor cells (ALL-CFU) from patients with acute lymphoblastic leukemia (ALL).

In about 30% of cases, BCR/ABL transcripts are present in freshly isolated mononuclear cells from the bone marrow of patients with acute lymphoblastic leukemia (ALL) using the polymerase chain reaction (PCR) technique. We applied PCR to investigate the leukemic nature of ALL colony-forming unit (ALL-CFU) colonies grown in a clonogenic assay using a double feeder system. The high sensitivity of PCR enables us to detect 1 leukemic cell among 10(5) normal cells. Several controls were taken to assess contamination. In this report we studied three patients with ALL. In all of them, BCR/ABL transcripts were detected at initial diagnosis. We showed that the PCR technique could identify the leukemic nature of ALL-CFU progenitors. In addition, we demonstrated the nonleukemic nature of granulocyte-macrophage colony-forming unit (CFU-GM) and erythroid burst-forming unit (BFU-E) colonies using the same analysis. We conclude that the PCR technique is of great value in the identification of leukemic clones whenever specific molecular markers are present.

Serum-free liquid marrow culture in patients with acute lymphoblastic leukaemia: a potential application to purge marrow for autologous transplantation.

We have previously established a serum-free (SF) culture medium, which supports normal haemopoietic progenitor cell growth for at least 4 weeks as does conventional serum dependent (SD) medium. In the present study, we investigated the efficacy of such a defined SF liquid medium which sustained in vitro residual normal haemopoietic proliferation of marrow derived from ALL patients and which was detrimental for the leukaemic population. Evidence for a potential selective effect of SF culture was obtained by a leukaemic progenitor cell assay (ALL-CFU) and the detection of the bcr/abl translocation by polymerase chain reaction (PCR). In 13 experiments including 12 patients, morphological blast cells and ALL-CFU were dramatically reduced within 3 weeks of incubation in both SF and SD cultures. Likewise, in 5/5 experiments in SD and 2/5 experiments in SF conditions, leukaemic cells expressing the bcr/abl fusion gene disappeared within 3-4 weeks. In contrast, the absolute numbers of supernatant cells harvested weekly from SF and SD cultures were similar. No difference in CFU-GM production was detected for the two culture systems. Erythropoiesis in SF medium exhibited a slower decline than that found in SD. These results indicate that liquid marrow culture may selectively deplete leukaemic lymphoblastic cells and enable repopulation by residual normal haemopoietic cells. This technique may be useful to purge leukaemic cells for clinical autologous bone marrow transplantation in patients with ALL.

Application of serum-free liquid bone marrow cultures to bone marrow purging for autologous bone marrow transplantation in acute lymphoblastic leukemia.

We have previously established a serum-free (SF) culture medium which allows normal haematopoietic progenitor cells to be maintained for at least 4 weeks as in the conventional serum dependent SD) medium. In the present study we investigated the efficiency of the SF liquid system to sustain normal residual haemopoiesis to the detriment of the leukemic population in patients with ALL. Probes for a potential selective effect were brought through leukemic progenitor cell assay (CFU-ALL) and the polymerase chain reaction (PCR) study of the bcr/abl translocation. In 13 experiments done in 12 patients, morphological blast cells and the ALL-CFU were dramatically reduced within 3 weeks of incubation in both SF and SD cultures. In 5/5 experiments in SD conditions and 2/5 experiments in SF conditions, leukemic cells expressing the bcr/abl fusion gene also disappeared within the same period. There was no difference in the CFU-GM production between SF and SD mediums. Erythropoiesis exhibited a slower decline in conditions SF, compared to the conditions SD. These results indicate that the liquid marrow culture may selectively deplete the leukemic lymphoblastic cells and enable repopulation by residual normal hemopoietic cells. It may be useful to purge leukemic cells for clinical autologous bone marrow transplantation in patients with ALL.