Analysis of the 5' flanking region of the human galactocerebrosidase (GALC) gene

The galactocerebrosidase enzyme contributes to the maintenance of a functional hematopoietic stem cell niche

The balance between survival and death in many cell types is regulated by small changes in the intracellular content of bioactive sphingolipids. Enzymes that either produce or degrade these sphingolipids control this equilibrium. The findings here described indicate that the lysosomal galactocerebrosidase (GALC) enzyme, defective in globoid cell leukodystrophy, is involved in the maintenance of a functional hematopoietic stem/progenitor cell (HSPC) niche by contributing to the control of the intracellular content of key sphingolipids. Indeed, we show that both insufficient and supraphysiologic GALC activity-by inherited genetic deficiency or forced gene expression in patients' cells and in the disease model-induce alterations of the intracellular content of the bioactive GALC downstream products ceramide and sphingosine, and thus affect HSPC survival and function and the functionality of the stem cell niche. Therefore, GALC and, possibly, other enzymes for the maintenance of niche functionality and health tightly control the concentration of these sphingolipids within HSPCs.

AAV-Mediated expression of galactocerebrosidase in brain results in attenuated symptoms and extended life span in murine models of globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD) or Krabbe disease is a neurodegenerative disorder caused by a deficiency of galactocerebrosidase (GALC) activity. GALC is required for the lysosomal degradation of galactosylceramide, psychosine, and possibly other galactolipids. This process is extremely important during active myelination. In the absence of functional GALC, psychosine accumulates, resulting in the apoptotic death of myelin-producing cells. While most patients are infants who do not survive beyond 2 years of age, some older patients are also diagnosed. Hematopoietic stem cell transplantation has proven to have a positive effect on the course of some patients with late-onset Krabbe disease. Murine models of this disease provide an excellent opportunity to evaluate therapeutic alternatives including gene therapy. In this study we used serotype 1 AAV to express mouse GALC under the control of the human cytomegalovirus promoter. Direct administration of these viral particles into the brains of neonatal mice with GLD resulted in sustained expression of GALC activity, improved myelination, attenuated symptoms, and prolonged life span. While this treatment also resulted in significant pathological improvements, the treated mice died with symptoms similar to those of the untreated mice. Additional initiatives may be required to prevent the onset of disease and reverse the course of the disease in animal models and human patients.

Gene expression profiling in polycythaemia vera: overexpression of transcription factor NF-E2

Summary The molecular aetiology of polycythaemia vera (PV) remains unknown and the differential diagnosis between PV and secondary erythrocytosis (SE) can be challenging. Gene expression profiling can identify candidates involved in the pathophysiology of PV and generate a molecular signature to aid in diagnosis. We thus performed cDNA microarray analysis on 40 PV and 12 SE patients. Two independent data sets were obtained: using a two-step training/validation design, a set of 64 genes (class predictors) was determined, which correctly discriminated PV from SE patients. Separately 253 genes were identified to be upregulated and 391 downregulated more than 1.5-fold in PV compared with healthy controls (P < 0.01). Of the genes overexpressed in PV, 27 contained Sp1 sites: we therefore propose that altered activity of Sp1-like transcription factors may contribute to the molecular aetiology of PV. One Sp1 target, the transcription factor NF-E2 [nuclear factor (erythroid-derived 2)], is overexpressed 2- to 40-fold in PV patients. In PV bone marrow, NF-E2 is overexpressed in megakaryocytes, erythroid and granulocytic precursors. It has been shown that overexpression of NF-E2 leads to the development of erythropoietin-independent erythroid colonies and that ectopic NF-E2 expression can reprogram monocytic cells towards erythroid and megakaryocytic differentiation. Transcription factor concentration may thus control lineage commitment. We therefore propose that elevated concentrations of NF-E2 in PV patients lead to an overproduction of erythroid and, in some patients, megakaryocytic cells/platelets. In this model, the level of NF-E2 overexpression determines both the severity of erythrocytosis and the concurrent presence or absence of thrombocytosis.

Generation of a mouse with low galactocerebrosidase activity by gene targeting: a new model of globoid cell leukodystrophy (Krabbe disease)

Globoid cell leukodystrophy (Krabbe disease) is a severe leukodystrophy caused by mutations in the galactocerebrosidase (GALC) gene leading to extremely low (less than 5% of normal activity) GALC activity. Human patients include primarily severely affected infants as well as patients with a later onset of symptoms. The infants usually die before 2 years of age, but it is difficult to predict the clinical course in older patients. In addition to these patients, additional individuals identified in this laboratory have 10--20% of normal GALC activity measured in accessible tissues. These individuals have a wide range of clinical presentations involving neurological degeneration. On molecular analysis of the GALC gene they all have three or more mutations considered to be normal polymorphisms resulting in amino acid changes in the two copies of the GALC gene. In order to investigate the role these amino acid changes may play on clinical, biochemical, and pathological findings, a new transgenic mouse was generated by homologous recombination. After preliminary studies determined what effect each amino acid change had on mouse GALC activity in transient transfection experiments, mice containing a cysteine residue at codon 168 instead of histidine (H168C) were produced. These mice developed symptoms, but they were delayed by 10--15 days from the well-characterized twitcher (twi) mouse. They accumulated psychosine slightly slower than twi mice, showed pathological changes less severe than twi mice in the central and peripheral nervous systems, and live about 15 days longer than twi mice. They have large litters and will play a role in therapy trials using new procedures currently under development.

Krabbe disease: genetic aspects and progress toward therapy

Krabbe disease or globoid cell leukodystrophy is a disorder involving the white matter of the peripheral and central nervous systems. Mutations in the gene for the lysosomal enzyme galactocerebrosidase (GALC) result in low enzymatic activity and decreased ability to degrade galactolipids found almost exclusively in myelin. The pathological changes observed, including the presence of globoid cells and decreased myelin, appear to result from the toxic nature of psychosine and accumulation of galactosylceramide that cannot be degraded due to the GALC deficiency. Over 60 mutations have been identified in this gene. The great majority are disease-causing; however, a few are considered polymorphisms. While most patients present with symptoms within the first 6 months of life, others present later in life including adulthood. Even patients with the same genotype can have very different clinical presentations and course. The reason for this is not known. Treatment at this time is limited to hematopoietic stem cell transplantation that appears to slow the progression of the disease and improve the magnetic resonance images. Studies using stem cells and viral vectors to transduce transplantable cells are under way in model systems. In culture, oligodendrocytes from the twitcher mouse model can assume a normal appearance after differentiation if GALC activity is provided via viral transduction or uptake from donor cells. Therefore continued myelination and/or remyelination in patients will require supplying GALC activity by transplanted cells or viral vectors to still functional endogenous oligodendrocytes or transplantation of normal oligodendrocytes or stem cells that can differentiate into oligodendrocytes. Using the animal models these options can be explored.

Transcriptional repression of the human galactocerebrosidase gene in squamous cell carcinomas of the larynx

Alterations of gene expression in squamous cell carcinoma (SCC) cell lines derived from the larynx and keratinocytes derived from adjacent normal mucosa of the larynx have been studied using the mRNA differential display technique. Lane-to-lane comparison of reverse transcribed mRNA showed a strong repression of a 148 bp fragment in SCC cells. The fragment was reamplified and cloned. Sequencing revealed a 99.3% homology with a region in exon 17 of the human galactocerebrosidase (GALC) gene. Northern blot analysis confirmed the differential expression of this gene in both carcinoma cell lines and laryngeal SCC biopsies in contrast with corresponding normal mucosa. To provide further evidence for the differential expression rate, both types of cells were transiently transfected with a 152 bp (-176 to -24) high regulatory promoter element of the 5' flanking region of the GALC gene. Results of 3 independent transfection experiments indicated a 16-fold repression of the GALC gene expression in SCC cells compared with benign keratinocytes. However, neither mutation nor other alterations of the promoter sequence were detected. Expression of the GALC gene is thus greatly affected in SCCs of the larynx.

Reporter gene technology: the future looks bright

Reporter gene technology is widely used to monitor the cellular events associated with signal transduction and gene expression. Based upon the splicing of transcriptional control elements to a variety of reporter genes (with easily measurable phenotypes), it "reports" the effects of a cascade of signalling events on gene expression inside cells. The principal advantage of these assays is their high sensitivity, reliability, convenience, and adaptability to large-scale measurements. This review summarises the current status of reporter gene technology including its role in monitoring gene transfer and expression and its development as a biological screen. With the advances in this technology and in detection methods, it is likely that luciferase and green fluorescent protein will become increasingly popular for the non-invasive monitoring of gene expression in living tissues and cells. Such techniques will be important in defining the molecular events associated with gene transcription, which has implications for our understanding of the molecular basis of disease and will influence our approach to gene therapy and drug development.