The quality of medical science

Is the quality of science aimed at understanding and treating disease inferior to more basic investigations of basic biology, or does the different quality of medical science reflect a distinct nature? This duality in the meaning of quality is central to any critique of the quality of medical science. The nature of medical science, which deals with the dysfunction of integrated genetic, epigenetic, environmental, or stochastic phenomenon, is distinct from that of basic science which seeks to describe discrete biological processes. In physical sciences it is generally accepted that there are practical boundaries between disciplines such as quantum physics, thermodynamics, and chemistry even though, in theory, such investigations are related. In meteorology, the development of chaos theory establishes that there are even theoretical limits to the ability to predict large scale processes from their essential, constituent processes. So too, the quality of medical science may be distinct from that of basic biology.

Prospects for academically trained pediatricians in academic medicine

There is widespread concern about the commitment and ability of physicians to enter careers in academic medicine. We studied MEDLINE citations by 150 academically trained pediatricians to determine whether they remained in academic medicine and to assess the nature of their contribution. We identified 2098 citations by 122 of the 150 individuals which reflected substantive involvement in clinical and basic research. Most individuals published their first papers within three years after residency and thereafter exhibited a stable level of productivity. Individuals who published prior to completing residency were most likely to enter academic medicine and achieve a high level of productivity. These reassuring data suggest that individuals who pursue an academic track may assume the traditional role of the physician scientist, and suggest measures which may be taken to attract more physicians to academics.

The challenge of follow-up for clinical trials of somatic gene therapy

Although extensive efforts have been made to optimize the safety of vectors for somatic gene transfer and somatic gene therapy, the safety of these agents must ultimately be assessed in clinical trials. A statistically significant assessment of safety will be complicated by the relatively small number of patients who will be enrolled in initial clinical trials, the need for long-term longitudinal follow-up of patients and perhaps their progeny, and the traditionally poor participation in long-term follow-up by many patients. This article reviews the potential risks of retroviral-mediated gene transfer, the statistical power required to assess the true incidence of potential complications, the number of patients who may participate in clinical trials involving retroviral vectors, and the factors that make thorough follow-up and uniform data ascertainment difficult. The role of the FDA in assessing the safety of retroviral vectors and the potential role of registries for patient tracking and data ascertainment are discussed.

Correction of methylmalonyl-CoA mutase deficiency inMut o fibroblasts and constitution of gene expression in primary human hepatocytes by retroviral-mediated gene transfer

Methylmalonic acidemia is an often fatal inborn error of organic acid metabolism due to deficiency of methylmalonyl-CoA mutase. The cloning of genes encoding this enzyme and the advent of technologies for gene transfer have introduced the possibility of somatic gene therapy for this disorder. Gene therapy may require replacement of the defective enzyme in hepatocytes, which have a greater capacity for propionate metabolism than other somatic cells and represent the principle physiological site of propionate metabolism. We describe construction of an amphotropic retroviral vector containing the human methylmalonyl-CoA mutase cDNA. This vector is shown to transduce primary MCM-deficient fibroblasts and restore levels of [14C]propionate metabolism by cultures of nonselected cells to normal. This vector will transduce primary human hepatocytes and direct transcription of recombinant human MCM from the integrated provirus. This work demonstrates the feasibility of retroviral-mediated gene transfer of methylmalonyl-CoA mutase into primary human cells, including hepatocytes which represent a difficult, but potentially necessary, target for gene therapy of methylmalonic acidemia.

Transduction of primary human hepatocytes with amphotropic and xenotropic retroviral vectors

Experiments in animal models suggest that it is feasible to consider hepatic gene therapy using a strategy in which hepatocytes would be isolated by partial hepatectomy, transduced with recombinant retroviral vectors containing genes of therapeutic importance, and then transplanted back into the patient by autologous hepatocellular transplantation. The application of this strategy in clinical trials will require adapting these methods to human cells. We describe the transduction of primary human hepatocytes with two forms of retroviral vectors: amphotropic vectors, which have been used previously in clinical trials, and xenotropic vectors, which have a different host range. Human hepatocytes were harvested from organs preserved in Belzer's solution and were cultivated in a serum-free, tyrosine-free, hormonally defined medium. These cells proliferated for 3-5 days in culture, exhibited characteristic hepatocyte morphology, and expressed liver-specific functions, including phenylalanine hydroxylase, alpha 1-antitrypsin, and glutamine synthase. Transduction with an amphotropic LNL6 retroviral vector resulted in stable incorporation of the provirus into 1% of the cells as estimated by semiquantitative PCR. Consistently higher transduction efficiencies (as much as 10% of the cells) were observed with a xenotropic N2 vector. These data support the feasibility of using LNL6 as a marker gene in clinical trials of hepatocellular transplantation. These data also suggest that the efficiency of transducing hepatocytes with amphotropic vectors in animal models may not accurately reflect the utility of these vectors for human applications. Consideration should be given to the use of xenotropic vectors for optimizing the efficiency of transduction for human applications.

The use of DiI-marked hepatocytes to demonstrate orthotopic, intrahepatic engraftment following hepatocellular transplantation

A novel method is described for marking primary hepatocytes with the fluorescent dye DiI prior to hepatocellular transplantation and identifying these cells within the hepatic parenchyma of recipient animals by fluorescence microscopy and flow cytometry. Optimal conditions are described for marking cells with DiI in suspension or in monolayer cultures prior to transplantation. DiI is shown to be nontoxic to hepatocytes and not to be exchanged between adjacent cells in vitro. Histological analysis of transplanted tissues shows DiI staining of engrafted hepatocytes and phagocytotic cells (Kupffer cells). This analysis shows that hepatocytes engraft within the hepatic parenchyma and exhibit a histological appearance indistinguishable from normal by conventional hematoxylin and eosin staining. Many previous reports of hepatocellular transplantation have been limited by their inability to unequivocally identify transplanted cells within the liver. These data illustrate the importance of having specific markers for transplanted cells that engraft in an orthotopic location and assume a normal morphological appearance.

DiI as a marker for cellular transplantation into solid organs

Research in cellular transplantation is frequently compromised by an inability to identify transplanted cells engrafting into orthotopic sites if they exhibit normal morphology and no unique antigenic markers. A method is described for using the fluorescent dye DiI as a marker for cell transplantation studies. This dye is not metabolized or exchanged between cells in vitro or in vivo and enables identification of engrafted cells by fluorescence microscopy, flow cytometry or fluorescence-activated cell sorting. Applications are described in autologous hepatocellular and thyroid follicular cell transplantation.

Phenotype of disease in three patients with identical mutations in methylmalonyl CoA mutase

We have previously identified a mutation in the gene for methylmalonyl CoA mutase in a patient with the mut- phenotype of methylmalonic aciduria. This mutation (G717V) interferes with the binding of the deoxyadenosylcobalamin cofactor to the apoenzyme producing a mutant holoenzyme that is defective, but not completely inactive, in vitro. This report describes the clinical phenotype associated with this mutation in the original patient and two additional patients who are homozygous for this allele. All three patients presented in the first years of life with multiple episodes of life-threatening organic acidosis and hyperammonemia. None had evidence of disease in the perinatal period, and all three have low-normal intelligence. These three children exhibit a distinctive phenotype of disease that is intermediate between the fulminant and benign forms of methylmalonic aciduria. These data suggest that this phenotype is the specific consequence of the G717V mutation, and that the degree of residual enzyme activity associated with the G717V mutation is close to the threshold required in vivo for maintaining metabolic homeostasis.

Cobalamin deficiency associated with methylmalonic acidemia in a cat

A 9-month-old sexually intact male longhair cat was examined because of lethargy, anorexia, cold intolerance, and failure to thrive since acquisition at an early age. Clinical signs of disease were less pronounced when the cat was fed a low-protein diet. Anemia, hypoglycemia, low total CO2 content, and hyperammonemia were detected. The cat was euthanatized. Urine obtained immediately before euthanasia contained a large amount of methylmalonic acid. Total serum cobalamin concentration was low. Hepatic methylmalonic-CoA mutase activity, with and without the addition of coenzyme adenosylcobalamin, was consistent with a cobalamin deficiency. Methylmalonic acidemia secondary to a putative defect in cobalamin absorption was diagnosed.

Somatic gene therapy in gastroenterology: approaches and applications

Somatic gene therapy represents a new approach for treating a variety of genetic and acquired diseases and has many potential applications in gastroenterology. Somatic gene therapy is based on the ability to transfer recombinant genes efficiently into somatic tissues, such as the liver or intestine, and achieve expression of the recombinant gene product. Gene transfer could be used to replace genetically defective gene functions or prevent the progression of acquired or multifactorial diseases. The therapeutic application of gene transfer technologies requires development of methods for achieving efficient transfer and expression of recombinant gene products, research directed at developing strategies for altering the course of disease, and clinical research aimed at assessing both the technical feasibility and therapeutic efficacy of this approach.

Cloning and expression of a mutant methylmalonyl coenzyme A mutase with altered cobalamin affinity that causes mut- methylmalonic aciduria

Distinct genotypic and phenotypic forms of methylmalonyl CoA mutase (MCM) apoenzyme deficiency can be delineated by biochemical analysis of mutant fibroblasts. One form, designated mut-, expresses a phenotype in which residual enzyme activity is evident in cultured cells exposed to high concentrations of hydroxycobalamin. We describe cloning of an MCM cDNA from cells exhibiting a mut- phenotype and characterization of the mutant gene product overexpressed in primary muto human fibroblasts and Saccharomyces cerevisiae. Three novel base changes were observed. Recombinant clones containing one of these base changes (G717V) express four characteristics of the mut- phenotype: failure to constitute [14C]propionate incorporation activity in fibroblasts assayed under basal cell culture conditions, constitution of [14C]propionate incorporation activity in fibroblasts stimulated with 0.1-1.0 micrograms/ml hydroxycobalamin, interallelic complementation with alleles bearing an R93H mutation, and an apparent Km (adenosylcobalamin) 1,000-fold higher than normal. These results demonstrate that the G717V mutation produces the mut- phenotype and localizes determinants for adenosylcobalamin binding near the carboxyl terminus of MCM.

Hepatocellular transplantation in acute hepatic failure and targeting genetic markers to hepatic cells

Orthotopic liver transplantation (OLT) represents the only therapeutic option for many patients with end-stage liver disease as well as many inborn genetic errors of hepatic metabolism. Despite dramatic progress in methods for OLT, the utilization of this procedure is limited by its considerable morbidity and mortality, by a chronic shortage of organs for transplant, and by difficulty arranging funding for many patients. Many children with fulminant hepatic failure do not receive OLT because this technology is unavailable or unaffordable. Hepatocellular transplantation (HCT), in which isolated, heterologous hepatocytes from a donor liver would be infused into the diseased organ in order to provide essential hepatic functions, could provide a much needed therapeutic alternative to OLT in the treatment of some causes of hepatic insufficiency. Experiments in animals have demonstrated that several genetic deficiencies of hepatic metabolism as well as experimental induced hepatic failure in animals can be reversed by HCT. Despite this experience, HCT has never been attempted in human subjects. This protocol represents the first proposed clinical trial of HCT. We are proposing a clinical trial in which HCT would be attempted as a therapeutic intervention in children with acute hepatic failure who have no other medical or surgical options. This proposal is intended to establish surgical methods for HCT and to evaluate the feasibility of this procedure for treating hepatic disease in humans. It is our expectation that HCT may provide short-term support for patients awaiting organ availability, a "bridge to recovery" allowing patients with fulminant hepatic failure to recover, or a long-term repopulation of the patient's liver with healthy donor cells. One of the major limitations of many animal studies in HCT is that, since the donor hepatocytes are often indistinguishable from those of the host, it has often been difficult to demonstrate a clear correlation between engraftment and the therapeutic effect. In order to verify engraftment independent of the therapeutic response, we propose to "mark" the donor hepatocytes by transducing these cells with a recombinant retroviral vector (LNL6) carrying a marker gene (NEO-R, neomycin phosphoribosyl transferase). The presence of this marker will enhance the ability to identify transplanted cells in the host using assays for the NEO-R gene or transcribed NEO-R mRNA. The LNL6 vector has been approved for human use and has been used as a marker gene for transplanted cells in human subjects without any reported adverse effects. We would like to emphasize that this is a proposal with therapeutic intent.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical application of genotypic diagnosis for phenylketonuria: theoretical considerations

Identification of mutations within the phenylalanine hydroxylase gene which cause phenylketonuria has introduced the possibility of diagnosing phenylketonuria by direct analysis of the genome. Genotypic analysis could be used for identifying homozygotes in the newborn period, for prenatal diagnosis, or for heterozygote detection in general populations. Establishing the clinical utility of genotypic diagnosis, however, will require characterization of the cohort of patients identified by genotypic diagnosis, correlation of mutant genotypes with specific biochemical and developmental phenotypes, and consideration of how genotypic diagnosis might contribute to improving the clinical outcome in individuals at risk for mental retardation due to hyperphenylalaninemia.

Identification of a point mutation in the topoisomerase II gene from a human leukemia cell line containing an amsacrine-resistant form of topoisomerase II

HL-60/AMSA is a human leukemia cell line that is 50- to 100-fold more resistant to the cytotoxic actions of the topoisomerase II-reactive intercalator amsacrine than is its drug-sensitive HL-60 parent line. Previously, we have shown that the topoisomerase II from HL-60/AMSA is also resistant to inhibition by amsacrine and other intercalating agents. We therefore sought the molecular basis for the resistance of the topoisomerase II of HL-60/AMSA and, by inference, of the HL-60/AMSA line itself. We report the cloning and sequencing of the topoisomerase II genes from both the sensitive and resistant leukemia cell lines using polymerase chain reaction technology. We have identified a single base change associated with the drug-resistant form of topoisomerase II. This mutation is present in both cloned HL-60/AMSA complementary DNA and extracted HL-60/AMSA genomic DNA. A rapid assay for this mutation in clinical samples has been developed and applied to the DNA of cells from both normal volunteers and leukemia patients. Thus far, the HL-60/AMSA genotype has not been identified in the cells from any individual, suggesting that this genotype is indeed a mutation and not an allelic form of topoisomerase II. The novel assay developed will allow a rapid search for the prevalence of this mutation in clinical samples from patients with leukemia who have relapsed following intercalator therapy.

Is there methylmalonyl CoA mutase in Aspergillus nidulans?

In most animal species and many prokaryotes, methylmalonyl CoA mutase catalyzes isomerization between methylmalonyl CoA and succinyl CoA using adenosylcobalamin as a cofactor. We describe the absence of this enzyme in Aspergillus nidulans based on the absence of enzyme activity in vitro and the failure to metabolize methylmalonate or grow in media containing this organic acid as the sole carbon source. These data contrast previous assumptions that propionate may be metabolized through propionyl CoA and methylmalonyl CoA to the TCA cycle in this organism. This is consistent with the separate evolution of these pathways in animals and lower eukaryotes due to the distinct endosymbiotic origin of their mitochondria.

Clinical considerations in the design of protocols for somatic gene therapy

Despite two decades of investigating the potential for somatic gene therapy in curing human disease, few clinical trials are being proposed. This delay is due in part to limitations of existing methods for gene transfer and to the recognized need to proceed judiciously into this controversial arena. Delay is also caused by a disjunction between the traditional precedents and principles of clinical investigation and the procedures instituted to regulate somatic gene therapy. The premise of this report is twofold: first, that more extensive clinical investigation of gene transfer technologies would be beneficial to patients, medicine, and basic science and second, that clinical investigations could be expedited by appealing to the established experience in clinical investigation without compromising the scientific excellence and discipline essential for this highly public process. This report develops a clinical perspective on potential applications of existing gene transfer technologies and the issues involved in developing experimental protocols. The initiation of clinical trials should be a primary goal of gene therapy research programs.

Differential diagnosis of mut and cbl methylmalonic aciduria by DNA-mediated gene transfer in primary fibroblasts

Methylmalonic aciduria can be caused by mutations in the gene encoding the methylmalonyl coenzyme A mutase apoenzyme (mut) or genes required for the provision of cofactor B12 (cbl). The mut and cbl forms are classically differentiated by somatic cell complementation. We describe a novel method for differential diagnosis of mut and cbl methylmalonic aciduria using DNA-mediated gene transfer of a methylmalonyl CoA mutase cDNA clone. Gene transfer of a functional methylmalonyl CoA mutase cDNA clone into mut fibroblasts reconstitutes holoenzyme activity measured by metabolism of [14C]-propionate in culture. Identical gene transfers into cbl fibroblasts have no effect. This method is used for the differential diagnosis of mut and cbl genotypes in cells from patients with a clinical diagnosis of methylmalonic aciduria and is shown to be a facile, sensitive, and specific method for genetic diagnosis. This work establishes the principle of using DNA-mediated gene transfer to identify the genotype of diseases which can result from mutations at several different genetic loci. This type of differential genotypic diagnosis will be particularly important for establishing the applicability of somatic gene therapy in individual patients.

Mouse hepatocytes migrate to liver parenchyma and function indefinitely after intrasplenic transplantation

One approach to gene therapy for hepatic diseases is to remove hepatocytes from an affected individual, genetically alter them in vitro, and reimplant them into a receptive locus. Although returning hepatocytes to the liver itself would be advantageous, the feasibility of this approach has never been evaluated due to the inability to distinguish donor from host hepatocytes. To unambiguously identify transplanted hepatocytes after transplantation, and to better quantitate their number and degree of liver function, two transgenic mouse lines were generated in a C57BL/6 background. The first expresses the Escherichia coli beta-galactosidase gene from the relatively liver-specific human alpha 1-antitrypsin (hAAT) promoter and allows transgenic hepatocytes to be readily identified after 5-bromo-4-chloro-3-indolyl beta-D-galactoside staining; the second produces the hAAT protein under control of the same promoter, which enables hepatocyte survival and maintenance of liver function to be quantitated by measuring the serum levels of hAAT. Hepatocytes isolated from transgenic donors were transplanted into nontransgenic C57BL/6 recipients by intrasplenic injection. Surprisingly, a large fraction of these cells were identified within the liver parenchyma but not the spleen at 2 months after transplantation. The high levels of serum hAAT detected in transplant recipients were stable for greater than 6 months, suggesting that established cells will survive indefinitely. These results have important implications for liver organogenesis and hepatic gene therapy.

Genetic characterization of a MUT locus mutation discriminating heterogeneity in mut0 and mut- methylmalonic aciduria by interallelic complementation

Genetic complementation of fibroblasts from patients with methylmalonic aciduria (MMA) defines a unique class of allelic mutations arising from mutations at the locus encoding the methylmalonyl coenzyme A (CoA) mutase apoenzyme. Various phenotypes of MMA have been delineated including complete absence of enzyme activity (mut0) and abnormal enzyme activity with an elevated Km for adenosylcobalamin (mut-). We describe genetic studies on a cell line (WG1130) from a patient with mut0 MMA which exhibited an unusual complementation phenotype, complementing with three of nine mut0 cell lines and four of five mut- cell lines. This suggests that interallelic complementation occurs between mutant alleles in WG1130 and subsets of alleles associated with both mut0 and mut- phenotypes. The methylmalonyl CoA mutase cDNA was cloned from WG1130 and found to contain a G354----A (Arg93----His) mutation. Gene transfer of this mutant clone into primary fibroblasts from patients with MMA confirms that this mutation expresses a mut0 phenotype when transferred into a mut0 cell line with low levels of mRNA but can contribute to apoenzyme function when transferred into mut cell lines which show correction with WG1130 by somatic cell complementation. These results point to further heterogeneity within both mut0 and mut- and may enable identification of mutations affecting discrete components of apoenzyme function.

Structure of the human methylmalonyl-CoA mutase (MUT) locus

The MUT locus encoding the enzyme methylmalonyl-CoA mutase is defective in mut forms of methylmalonic acidemia. This locus has been mapped to chromosome 6p12-21.1. We report cloning and characterization of this locus which comprises 13 exons spanning greater than 35 kb of the genome. The MUT locus exhibits consensus sequences for transcription, splicing, and polyadenylation. The putative promoter region was localized in a CG island 5' to exon I and was shown to direct expression of a beta-galactosidase reporter gene in cultured cells. Of interest is the observation that the first intron occurs within the 5' untranslated region, and no introns separate the mitochondrial targeting sequences and the mature apoenzyme. An informative HindIII polymorphism was localized within the coding sequence and can be assayed using the polymerase chain reaction. These studies describe the structure of the MUTlocus and provide a foundation for characterization of mutations in mut methylmalonic acidemia.