Galactosemia: Towards Pharmacological Chaperones

Galactosemia is a rare inherited metabolic disease resulting from mutations in the four genes which encode enzymes involved in the metabolism of galactose. The current therapy, the removal of galactose from the diet, is inadequate. Consequently, many patients suffer lifelong physical and cognitive disability. The phenotype varies from almost asymptomatic to life-threatening disability. The fundamental biochemical cause of the disease is a decrease in enzymatic activity due to failure of the affected protein to fold and/or function correctly. Many novel therapies have been proposed for the treatment of galactosemia. Often, these are designed to treat the symptoms and not the fundamental cause. Pharmacological chaperones (PC) (small molecules which correct the folding of misfolded proteins) represent an exciting potential therapy for galactosemia. In theory, they would restore enzyme function, thus preventing downstream pathological consequences. In practice, no PCs have been identified for potential application in galactosemia. Here, we review the biochemical basis of the disease, identify opportunities for the application of PCs and describe how these might be discovered. We will conclude by considering some of the clinical issues which will affect the future use of PCs in the treatment of galactosemia.ERDF/Spanish Ministry of Science, Innovation and Universities-State Research Agency RTI2018-096246-B-I00FEDER/Junta de Andalucía - Consejería de Transformación Económica, Industria, Conocimiento y Universidades P18-RT-241

Galactose epimerase deficiency: lessons from the GalNet registry.

BACKGROUND Galactose epimerase (GALE) deficiency is a rare hereditary disorder of galactose metabolism with only a few cases described in the literature. This study aims to present the data of patients with GALE deficiency from different countries included through the Galactosemia Network to further expand the existing knowledge and review the current diagnostic strategy, treatment and follow-up of this not well characterized entity. METHODS Observational study collecting medical data from December 2014 to April 2022 of 22 not previously reported patients from 14 centers in 9 countries. Patients were classified as generalized or non-generalized based on their genotype, enzyme activities in different tissues and/or clinical picture and professional judgment of the treating physician. RESULTS In total 6 patients were classified as generalized and 16 as non-generalized. In the generalized group, acute neonatal illness was reported in 3, cognitive and developmental delays were present in 5 and hearing problems were reported in 3. Four generalized patients were homozygous for the genetic variant NM_001008216.2:c.280G > A (p.Val94Met). In the non-generalized group, no clearly related symptoms were found. Ten novel genetic variants were reported in this study population. CONCLUSION The phenotypic spectrum of GALE deficiency ranges from asymptomatic to severe. The generalized patients have a phenotype that is in line with the 9 described cases in the literature and prescribing dietary interventions is the cornerstone for treatment. In the non-generalized group, treatment advice is more difficult. To be able to offer proper counseling, in addition to red blood cell enzyme activity, genetic studies, transferrin glycoform analysis and enzymatic measurements in fibroblasts are recommended. Due to lack of facilities, additional enzymatic testing is not common practice in many centers nor a tailored long-term follow-up is performed

UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase- 6 (pp-GalNAc-T6): Role in Cancer and Prospects as a Drug Target

UDP-N-acetyl-d-galactosamine: polypeptide N-acetylgalactosaminyl transferase-6 (pp-GalNAc-T6) is a member of the N-acetyl-d-galactosamine transferase family. It catalyzes the addition of N-acetyl-d-galactosamine to proteins, often the first step in O-glycosylation of proteins. Glycosylated proteins play important roles in vivo in the cell membrane. These are often involved in cell-cell adhesion, cytoskeleton regulation and immune recognition. pp-GalNAc-T6 has been shown to be upregulated in a number of types of cancer. Abnormally glycosylated forms of mucin 1 (substrate of the enzyme), are used clinically as a biomarker for breast cancer. There is potential for other products of the pp-GalNAc-T6 catalyzed reaction to be used. It is also possible that pp-GalNAc-T6 itself could be used as a biomarker, since levels of this protein tend to be low in non-malignant tissues. pp-GalNAc-T6 has been implicated in malignant transformation and metastasis of cancer cells. As such, it has considerable potential as a target for chemotherapy. To date, no selective inhibitors of the enzyme have been identified. However, general inhibitors of the enzyme family result in reduced cell surface O-linked glycosylation and induce apoptosis in cultured cells. Thus, a selective inhibitor of pp-GalNAc-T6 is likely to target cancer cells and could be developed into a novel anticancer therapy

Galactosemia: Towards Pharmacological Chaperones

Galactosemia is a rare inherited metabolic disease resulting from mutations in the four genes which encode enzymes involved in the metabolism of galactose. The current therapy, the removal of galactose from the diet, is inadequate. Consequently, many patients suffer lifelong physical and cognitive disability. The phenotype varies from almost asymptomatic to life-threatening disability. The fundamental biochemical cause of the disease is a decrease in enzymatic activity due to failure of the affected protein to fold and/or function correctly. Many novel therapies have been proposed for the treatment of galactosemia. Often, these are designed to treat the symptoms and not the fundamental cause. Pharmacological chaperones (PC) (small molecules which correct the folding of misfolded proteins) represent an exciting potential therapy for galactosemia. In theory, they would restore enzyme function, thus preventing downstream pathological consequences. In practice, no PCs have been identified for potential application in galactosemia. Here, we review the biochemical basis of the disease, identify opportunities for the application of PCs and describe how these might be discovered. We will conclude by considering some of the clinical issues which will affect the future use of PCs in the treatment of galactosemia

UDP-galactose 4'-epimerase from the liver fluke, Fasciola hepatica: biochemical characterization of the enzyme and identification of inhibitors

The Leloir pathway enzyme UDP-galactose 4’-epimerase from the common liver fluke Fasciola hepatica (FhGALE) was identified and characterised. The enzyme can be expressed in, and purified from, Escherichia coli. The recombinant enzyme is active: the K(m) (470 µM) is higher than the corresponding human enzyme (HsGALE), whereas the k(cat) (2.3 s(−1)) is substantially lower. FhGALE binds NAD(+) and was shown to be dimeric by analytical gel filtration. Like the human and yeast GALEs, FhGALE is stabilised by the substrate UDP-galactose. Molecular modelling predicted that FhGALE adopts a similar overall fold to HsGALE and that tyrosine 155 is likely to be the catalytically critical residue in the active site. In silico screening of the NCI DTP library identified 40 potential inhibitors of FhGALE which were tested in vitro. Of these, six showed concentration-dependent inhibition of FhGALE, some with nanomolar IC(50) values. Two inhibitors (5-fluoroorotate and N-[(benzyloxy)carbonyl]leucyltryptophan) demonstrated selectivity for FhGALE over HsGALE. These compounds also thermally destabilised FhGALE in a concentration-dependent manner. Interestingly the selectivity of 5-fluoroorotate was not shown by orotic acid, which differs in structure by one fluorine atom. These results demonstrate that, despite the structural and biochemical similarities of FhGALE and HsGALE, it is possible to discover compounds which preferentially inhibit FhGALE

UDP-galactose 4′-epimerase from the liver fluke, Fasciola hepatica

Leloir pathway enzyme uridine diphosphate (UDP)-galactose 4'-epimerase from the common liver fluke Fasciola hepatica (FhGALE) was identified and characterized. The enzyme can be expressed in, and purified from, Escherichia coli. The recombinant enzyme is active: the K(m) (470 μM) is higher than the corresponding human enzyme (HsGALE), whereas the k(cat) (2.3 s(-1)) is substantially lower. FhGALE binds NAD(+) and has shown to be dimeric by analytical gel filtration. Like the human and yeast GALEs, FhGALE is stabilized by the substrate UDP-galactose. Molecular modelling predicted that FhGALE adopts a similar overall fold to HsGALE and that tyrosine 155 is likely to be the catalytically critical residue in the active site. In silico screening of the National Cancer Institute Developmental Therapeutics Program library identified 40 potential inhibitors of FhGALE which were tested in vitro. Of these, 6 showed concentration-dependent inhibition of FhGALE, some with nanomolar IC50 values. Two inhibitors (5-fluoroorotate and N-[(benzyloxy)carbonyl]leucyltryptophan) demonstrated selectivity for FhGALE over HsGALE. These compounds also thermally destabilized FhGALE in a concentration-dependent manner. Interestingly, the selectivity of 5-fluoroorotate was not shown by orotic acid, which differs in structure by 1 fluorine atom. These results demonstrate that, despite the structural and biochemical similarities of FhGALE and HsGALE, it is possible to discover compounds which preferentially inhibit FhGALE.</p