Distinct mutations in the glycogen debranching enzyme found in glycogen storage disease type III lead to impairment in diverse cellular functions

Maltooligosaccharides: Properties, Production and Applications

Maltooligosaccharides (MOS) are homooligosaccharides that consist of 3-10 glucose molecules linked by α-1,4 glycosidic bonds. As they have physiological functions, they are commonly used as ingredients in nutritional products and functional foods. Many researchers have investigated the potential applications of MOS and their derivatives in the pharmaceutical industry. In this review, we summarized the properties and methods of fabricating MOS and their derivatives, including sulfated and non-sulfated alkylMOS. For preparing MOS, different enzymatic strategies have been proposed by various researchers, using α-amylases, maltooligosaccharide-forming amylases, or glycosyltransferases as effective biocatalysts. Many researchers have focused on using immobilized biocatalysts and downstream processes for MOS production. This review also provides an overview of the current challenges and future trends of MOS production.

New approach to prepare fluorogenic branched dextrins for assaying glycogen debranching enzyme

Glycogen debranching enzyme (GDE), together with glycogen phosphorylase (GP), is responsible for the complete degradation of glycogen. GDE has distinct catalytic sites for 4-α-glucanotransferase and amylo-α-1,6-glucosidase. For the GDE sensitive assay, we previously developed the GP limit fluorogenic branched dextrin Glcα1-4Glcα1-4Glcα1-4Glcα1-4(Glcα1-4Glcα1-4Glcα1-4Glcα1-6)Glcα1-4Glcα1-4Glcα1-4GlcPA (B4/84, where Glc = D-glucose and GlcPA = 1-deoxy-1-[(2-pyridyl)amino]-D-glucitol). However, B4/84 is not widely available because of difficulties in its chemical synthesis and positional-isomer separation (0.33% yield by α-1,6-coupling of maltotetraose with Glc₇-GlcPA). In this study, we attempted to develop an efficient method for the preparation of Glcα1-4Glcα1-4Glcα1-4Glcα1-4(Glcα1-4Glcα1-4Glcα1-4Glcα1-6)Glcα1-4Glcα1-4GlcPA (B3/74), which was designed to have the minimum essential dextrin structure for GDE. First, Glcα1-6Glcα1-4Glcα1-4GlcPA (B3/31) was prepared from commercially available Glcα1-6Glcα1-4Glcα1-4Glc. Using α-cyclodextrin as a donor substrate, cyclodextrin glucanotransferase elongated both the main and side branches on B3/31, while all the glycosidic bonds in B3/31 were left intact. After exhaustive digestion with GP, B3/74 was obtained from B3/31 with 16% yield, a value that is 48-fold greater than that previously reported for B4/84. GDE 4-α-glucanotransferase exhibited high activity toward both B3/74 and B4/84. In addition, we studied the efficient conversion of B3/74 into Glcα1-4Glcα1-4Glcα1-4Glcα1-4(Glcα1-6)Glcα1-4Glcα1-4GlcPA (B3/71), which has the best dextrin structure for the GDE amylo-α-1,6-glucosidase.

Expression of a Functional Recombinant Human Glycogen Debranching Enzyme (hGDE) in N. benthamiana Plants and in Hairy Root Cultures

BACKGROUND

Glycogen storage disease type III (GSDIII, Cori/Forbes disease) is a metabolic disorder due to the deficiency of the Glycogen Debranching Enzyme (GDE), a large monomeric protein (about 176 kDa) with two distinct enzymatic activities: 4-α-glucantransferase and amylo-α-1,6-glucosidase. Several mutations along the amylo-alpha-1,6-glucosidase,4-alphaglucanotransferase (Agl) gene are associated with loss of enzymatic activity. The unique treatment for GSDIII, at the moment, is based on diet. The potential of plants to manufacture exogenous engineered compounds for pharmaceutical purposes, from small to complex protein molecules such as vaccines, antibodies and other therapeutic/prophylactic entities, was shown by modern biotechnology through "Plant Molecular Farming".

OBJECTIVE AND METHODS

In an attempt to develop novel protein-based therapeutics for GSDIII, the Agl gene, encoding for the human GDE (hGDE) was engineered for expression as a histidinetagged GDE protein both in Nicotiana benthamiana plants by a transient expression approach, and in axenic hairy root in vitro cultures (HR) from Lycopersicum esculentum and Beta vulgaris.

RESULTS

In both plant-based expression formats, the hGDE protein accumulated in the soluble fraction of extracts. The plant-derived protein was purified by affinity chromatography in native conditions showing glycogen debranching activity.

CONCLUSION

These investigations will be useful for the design of a new generation of biopharmaceuticals based on recombinant GDE protein that might represent, in the future, a possible therapeutic option for GSDIII.

Intron retention is among six unreported AGL mutations identified in Malaysian GSD III patients

BACKGROUND

Glycogen storage disease type III is an autosomal recessive disorder that is caused by deficiencies of the glycogen debranching enzyme. Mutations within the AGL gene have been found to be heterogeneous, with some common mutations being reported in certain populations. The mutation spectrum of AGL gene in the multi-ethnic Malaysian population is still unknown.

OBJECTIVE

The present study seeks to determine the mutation spectrum of the AGL gene in Malaysian population.

METHODS

A total of eleven patients (eight Malay, two Chinese and one Bajau) were investigated. Genomic DNA was extracted and subsequently the AGL gene was amplified using specific primers and sequenced. Mutations found were screened in 150 healthy control samples either by restriction enzyme digestion assay or TaqMan^® SNP Genotyping assay.

RESULTS

We identified six unreported mutations (c.1423+1G>T, c.2914_2915delAA, c.3814_3815delAG, c.4333T>G, c.4490G>A, c.4531_4534delTGTC) along with three previously reported mutations (c.99C>T, c.1783C>T, c.2681+1G>A). One of the six unreported mutation causes abnormal splicing and results in retention of intron 12 of the mature transcript, while another is a termination read-through. One of the reported mutation c.2681+1G>A was recurrently found in the Malay patients (n = 7 alleles; 31.8%).

CONCLUSION

The mutation spectrum of the AGL gene in Malaysian patients has shown considerable heterogeneity, and all unreported mutations were absent in all 150 healthy control samples tested.

[Molecular and clinical characterization of Colombian patients suffering from type III glycogen storage disease]

INTRODUCTION

Type III glycogen storage disease (GSD III) is an autosomal recessive disorder in which a mutation in the AGL gene causes deficiency of the glycogen debranching enzyme. The disease is characterized by fasting hypoglycemia, hepatomegaly and progressive myopathy. Molecular analyses of AGL have indicated heterogeneity depending on ethnic groups. The full spectrum of AGL mutations in Colombia remains unclear.

OBJECTIVE

To describe the clinical and molecular characteristics of ten Colombian patients diagnosed with GSD III.

MATERIALS AND METHODS

We recruited ten Colombian children with a clinical and biochemical diagnosis of GSD III to undergo genetic testing. The full coding exons and the relevant exon-intron boundaries of the AGL underwent Sanger sequencing to identify mutation.

RESULTS

All patients had the classic phenotype of the GSD III. Genetic analysis revealed a mutation p.Arg910X in two patients. One patient had the mutation p.Glu1072AspfsX36, and one case showed a compound heterozygosity with p.Arg910X and p.Glu1072AspfsX36 mutations. We also detected the deletion of AGL gene 3, 4, 5, and 6 exons in three patients. The in silico studies predicted that these defects are pathogenic. No mutations were detected in the amplified regions in three patients.

CONCLUSION

We found mutations and deletions that explain the clinical phenotype of GSD III patients. This is the first report with a description of the clinical phenotype and the spectrum of AGL mutations in Colombian patients. This is important to provide appropriate prognosis and genetic counseling to the patient and their relatives.

Glycogen debranching enzyme (AGL) is a novel regulator of non-small cell lung cancer growth

Glycogen debranching enzyme (AGL) and Glycogen phosphorylase (PYG) are responsible for glycogen breakdown. We have earlier shown that AGL is a regulator of bladder tumor growth. Here we investigate the role of AGL in non-small cell lung cancers (NSCLC). Short hairpin RNA (shRNA) driven knockdown of AGL resulted in increased anchorage independent and xenograft growth of NSCLC cells. We further establish that an increase in hyaluronic acid (HA) synthesis driven by Hyaluronic Acid Synthase 2 (HAS2) is critical for anchorage independent growth of NSCLC cells with AGL loss. Using gene knockdown approach against HAS2 and by using 4-methylumbelliferone (4MU), an inhibitor of HA synthesis, we show that HA synthesis is critical for growth of NSCLC cells that have lost AGL. We further show NSCLC cells without AGL expression are dependent on RHAMM for HA signaling and growth. Analysis of NSCLC patient datasets established that patients with low AGL/high HAS2 or low AGL/high RHAMM mRNA expression have poor overall survival compared to patients with high AGL/low HAS2 or high AGL/low RHAMM expression. We show for the first time that loss of AGL promotes anchorage independent growth of NSCLC cells. We further show that HAS2 driven HA synthesis and signaling via RHAMM is critical in regulating growth of these cancer cells with AGL loss. Further patients presenting with low AGL and HAS2 or RHAMM over expressing tumors might present the ideal cohort who would respond to inhibitors of HA synthesis and signaling.

Amylose recognition and ring-size determination of amylomaltase

Starch is a major carbon and energy source throughout all kingdoms of life. It consists of two carbohydrate polymers, branched amylopectin and linear amylose, which are sparingly soluble in water. Hence, the enzymatic breakdown by glycoside hydrolases (GHs) is of great biological and societal importance. Amylomaltases (AMs) are GHs specialized in the hydrolysis of α-1,4-linked sugar chains such as amylose. They are able to catalyze an intramolecular transglycosylation of a bound sugar chain yielding polymeric sugar rings, the cycloamyloses (CAs), consisting of 20 to 100 glucose units. Despite a wealth of data on short oligosaccharide binding to GHs, no structural evidence is available for their interaction with polymeric substrates that better represent the natural polysaccharide. We have determined the crystal structure of Thermus aquaticus AM in complex with a 34-meric CA-one of the largest carbohydrates resolved by x-ray crystallography and a mimic of the natural polymeric amylose substrate. In total, 15 glucose residues interact with the protein in an extended crevice with a length of more than 40 Å. A modified succinimide, derived from aspartate, mediates protein-sugar interactions, suggesting a biological role for this nonstandard amino acid. The structure, together with functional assays, provides unique insights into the interaction of GHs with their polymeric substrate and reveals a molecular ruler mechanism for minimal ring-size determination of CA products.

Glycogen metabolism in humans

In the human body, glycogen is a branched polymer of glucose stored mainly in the liver and the skeletal muscle that supplies glucose to the blood stream during fasting periods and to the muscle cells during muscle contraction. Glycogen has been identified in other tissues such as brain, heart, kidney, adipose tissue, and erythrocytes, but glycogen function in these tissues is mostly unknown. Glycogen synthesis requires a series of reactions that include glucose entrance into the cell through transporters, phosphorylation of glucose to glucose 6-phosphate, isomerization to glucose 1-phosphate, and formation of uridine 5'-diphosphate-glucose, which is the direct glucose donor for glycogen synthesis. Glycogenin catalyzes the formation of a short glucose polymer that is extended by the action of glycogen synthase. Glycogen branching enzyme introduces branch points in the glycogen particle at even intervals. Laforin and malin are proteins involved in glycogen assembly but their specific function remains elusive in humans. Glycogen is accumulated in the liver primarily during the postprandial period and in the skeletal muscle predominantly after exercise. In the cytosol, glycogen breakdown or glycogenolysis is carried out by two enzymes, glycogen phosphorylase which releases glucose 1-phosphate from the linear chains of glycogen, and glycogen debranching enzyme which untangles the branch points. In the lysosomes, glycogen degradation is catalyzed by α-glucosidase. The glucose 6-phosphatase system catalyzes the dephosphorylation of glucose 6-phosphate to glucose, a necessary step for free glucose to leave the cell. Mutations in the genes encoding the enzymes involved in glycogen metabolism cause glycogen storage diseases.

Crystal structure of glycogen debranching enzyme and insights into its catalysis and disease-causing mutations

Glycogen is a branched glucose polymer and serves as an important energy store. Its debranching is a critical step in its mobilization. In animals and fungi, the 170 kDa glycogen debranching enzyme (GDE) catalyses this reaction. GDE deficiencies in humans are associated with severe diseases collectively termed glycogen storage disease type III (GSDIII). We report crystal structures of GDE and its complex with oligosaccharides, and structure-guided mutagenesis and biochemical studies to assess the structural observations. These studies reveal that distinct domains in GDE catalyse sequential reactions in glycogen debranching, the mechanism of their catalysis and highly specific substrate recognition. The unique tertiary structure of GDE provides additional contacts to glycogen besides its active sites, and our biochemical experiments indicate that they mediate its recruitment to glycogen and regulate its activity. Combining the understanding of the GDE catalysis and functional characterizations of its disease-causing mutations provides molecular insights into GSDIII.

Debranching of glycogen is an important step in its use as an energy source. Here, the authors describe the crystal structures of glycogen debranching enzyme alone and in complex with oligosaccharides and provide molecular insights into the function, and into associated diseases.

Molecular diagnosis of glycogen storage disease and disorders with overlapping clinical symptoms by massive parallel sequencing

PURPOSE

Glycogen storage disease (GSD) is an umbrella term for a group of genetic disorders that involve the abnormal metabolism of glycogen; to date, 23 types of GSD have been identified. The nonspecific clinical presentation of GSD and the lack of specific biomarkers mean that Sanger sequencing is now widely relied on for making a diagnosis. However, this gene-by-gene sequencing technique is both laborious and costly, which is a consequence of the number of genes to be sequenced and the large size of some genes.

METHODS

This work reports the use of massive parallel sequencing to diagnose patients at our laboratory in Spain using either a customized gene panel (targeted exome sequencing) or the Illumina Clinical-Exome TruSight One Gene Panel (clinical exome sequencing (CES)). Sequence variants were matched against biochemical and clinical hallmarks.

RESULTS

Pathogenic mutations were detected in 23 patients. Twenty-two mutations were recognized (mostly loss-of-function mutations), including 11 that were novel in GSD-associated genes. In addition, CES detected five patients with mutations in ALDOB, LIPA, NKX2-5, CPT2, or ANO5. Although these genes are not involved in GSD, they are associated with overlapping phenotypic characteristics such as hepatic, muscular, and cardiac dysfunction.

CONCLUSIONS

These results show that next-generation sequencing, in combination with the detection of biochemical and clinical hallmarks, provides an accurate, high-throughput means of making genetic diagnoses of GSD and related diseases.Genet Med 18 10, 1037-1043.

Glycogen metabolism has a key role in the cancer microenvironment and provides new targets for cancer therapy

Metabolic reprogramming is a hallmark of cancer cells and contributes to their adaption within the tumour microenvironment and resistance to anticancer therapies. Recently, glycogen metabolism has become a recognised feature of cancer cells since it is upregulated in many tumour types, suggesting that it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells under stress conditions such as hypoxia, glucose deprivation and anticancer treatment. The various methods to detect glycogen in tumours in vivo as well as pharmacological modulators of glycogen metabolism are also reviewed. Finally, we discuss the therapeutic value of targeting glycogen metabolism as a strategy for combinational approaches in cancer treatment.

Molecular and clinical delineation of 12 patients with glycogen storage disease type III in Western Turkey

BACKGROUND

Glycogen storage disease type III (GSD III; MIM #232400) is an autosomal recessive inherited disorder characterized by fasting hypoglycemia, growth retardation, hepatomegaly, progressive myopathy, and cardiomyopathy. GSD III is caused by deficiency in the glycogen debranching enzyme (gene symbol: AGL). Molecular analyses of AGL have indicated heterogeneity depending on ethnic groups. In Turkey we reported 13 different AGL mutations from GSD III patients in the Eastern region; however, the full spectrum of AGL mutations in Turkish population remains unclear. Here we investigated 12 GSD III patients mostly from Western Turkey.

METHODS

The full coding exons, their relevant exon-intron boundaries, and the 5'- and 3'-flanking regions of the patients' AGL were sequenced. AGL haplotypes were determined. Splicing mutations were characterized by RNA transcript analysis.

RESULTS

Twelve different mutations were identified: 7 novel AGL mutations [69-base pair deletion (c.1056_1082+42del69), 21-base par deletion (c.3940_3949+11del21), two small duplications (c.364_365dupCT and c.1497_1500dupAGAG), and 3 splicing mutations (c.1736-11A>G, c.3259+1G>A and c.3588+2T>G)], along with 5 known mutations (c.1019delA, c.958+1G>A, c.4161+5G>A, p.R864X and p.R1218X). Transcripts of splicing mutations (c.1736-11A>G, c.3588+2T>G and c.4161+5G>A) were shown to cause aberrant splicing. AGL haplotype analyses suggested that c.1019delA and c.958+1G>A are founder mutations in Turkish patients, while p.R864X is a recurrent mutation.

CONCLUSIONS

Our study broadens the spectrum of AGL mutations and demonstrates that mutations in Western Turkey are different from those in the Eastern region.

Phylogenomic analysis of glycogen branching and debranching enzymatic duo

BACKGROUND

Branched polymers of glucose are universally used for energy storage in cells, taking the form of glycogen in animals, fungi, Bacteria, and Archaea, and of amylopectin in plants. Some enzymes involved in glycogen and amylopectin metabolism are similarly conserved in all forms of life, but some, interestingly, are not. In this paper we focus on the phylogeny of glycogen branching and debranching enzymes, respectively involved in introducing and removing of the α(1-6) bonds in glucose polymers, bonds that provide the unique branching structure to glucose polymers.

RESULTS

We performed a large-scale phylogenomic analysis of branching and debranching enzymes in over 400 completely sequenced genomes, including more than 200 from eukaryotes. We show that branching and debranching enzymes can be found in all kingdoms of life, including all major groups of eukaryotes, and thus were likely to have been present in the last universal common ancestor (LUCA) but have been lost in seemingly random fashion in numerous single-celled eukaryotes. We also show how animal branching and debranching enzymes evolved from their LUCA ancestors by acquiring additional domains. Furthermore, we show that enzymes commonly perceived as orthologous, such as human branching enzyme GBE1 and E. coli branching enzyme GlgB, are in fact related by a gene duplication and consequently paralogous.

CONCLUSIONS

Despite being usually associated with animal liver glycogen and plant starch, energy storage in the form of branched glucose polymers is clearly an ancient process and has probably been present in the last universal common ancestor of all present life. The evolution of the enzymes enabling this form of energy storage is more complex than previously thought and illustrates the need for explicit phylogenomic analysis in the study of even seemingly "simple" metabolic enzymes. Patterns of conservation in the evolution of the glycogen/starch branching and debranching enzymes hint at some as yet unknown mechanisms, as mutations disrupting these patterns lead to a variety of genetic diseases in humans and other mammals.

Glycogen storage disease type III: A novel Agl knockout mouse model

Glycogen storage disease type III is an autosomal recessive disease characterized by a deficiency in the glycogen debranching enzyme, encoded by AGL. Essential features of this disease are hepatomegaly, hypoglycemia, hyperlipidemia, and growth retardation. Progressive skeletal myopathy, neuropathy, and/or cardiomyopathy become prominent in adults. Currently, there is no available cure. We generated an Agl knockout mouse model by deletion of the carboxy terminus of the protein, including the carboxy end of the glucosidase domain and the glycogen-binding domain. Agl knockout mice presented serious hepatomegaly, but we did not observe signs of cirrhosis or adenomas. In affected tissues, glycogen storage was higher than in wild-type mice, even in the central nervous system which has never been tested in GSDIII patients. The biochemical findings were in accordance with histological data, which clearly documented tissue impairment due to glycogen accumulation. Indeed, electron microscopy revealed the disruption of contractile units due to glycogen infiltrations. Furthermore, adult Agl knockout animals appeared less prompt to move, and they exhibited kyphosis. Three-mo-old Agl knockout mice could not run, and adult mice showed exercise intolerance. In addition, older affected animals exhibited an accelerated respiratory rate even at basal conditions. This observation was correlated with severe glycogen accumulation in the diaphragm. Diffuse glycogen deposition was observed in the tongues of affected mice. Our results demonstrate that this Agl knockout mouse is a reliable model for human glycogenosis type III, as it recapitulates the essential phenotypic features of the disease.

The carbohydrate-binding domain of overexpressed STBD1 is important for its stability and protein-protein interactions

STBD1 (starch-binding domain-containing protein 1) belongs to the CBM20 (family 20 carbohydrate binding module) group of proteins, and is implicated in glycogen metabolism and autophagy. However, very little is known about its regulation or interacting partners. Here, we show that the CBM20 of STBD1 is crucial for its stability and ability to interact with glycogen-associated proteins. Mutation of a conserved tryptophan residue (W293) in this domain abolished the ability of STBD1 to bind to the carbohydrate amylose. Compared with the WT (wild-type) protein, this mutant exhibited rapid degradation that was rescued upon inhibition of the proteasome. Furthermore, STBD1 undergoes ubiquitination when expressed in COS cells, and requires the N-terminus for this process. In contrast, inhibition of autophagy did not significantly affect protein stability. In overexpression experiments, we discovered that STBD1 interacts with several glycogen-associated proteins, such as GS (glycogen synthase), GDE (glycogen debranching enzyme) and Laforin. Importantly, the W293 mutant of STBD1 was unable to do so, suggesting an additional role for the CBM20 domain in protein–protein interactions. In HepG2 hepatoma cells, overexpressed STBD1 could associate with endogenous GS. This binding increased during glycogenolysis, suggesting that glycogen is not required to bridge this interaction. Taken together, our results have uncovered new insights into the regulation and binding partners of STBD1.

STBD1 is a protein with a carbohydrate-binding domain that is implicated in autophagy and glycogen metabolism. Here we show the carbohydrate-binding domain is crucial for its stability and ability to bind to several glycogen-associated proteins.

Role in tumor growth of a glycogen debranching enzyme lost in glycogen storage disease

BACKGROUND

Bladder cancer is the most common malignancy of the urinary system, yet our molecular understanding of this disease is incomplete, hampering therapeutic advances.

METHODS

Here we used a genome-wide functional short-hairpin RNA (shRNA) screen to identify suppressors of in vivo bladder tumor xenograft growth (n = 50) using bladder cancer UMUC3 cells. Next-generation sequencing was used to identify the most frequently occurring shRNAs in tumors. Genes so identified were studied in 561 patients with bladder cancer for their association with stratification of clinical outcome by Kaplan-Meier analysis. The best prognostic marker was studied to determine its mechanism in tumor suppression using anchorage-dependent and -independent growth, xenograft (n = 20), and metabolomic assays. Statistical significance was determined using two-sided Student t test and repeated-measures statistical analysis.

RESULTS

We identified the glycogen debranching enzyme AGL as a prognostic indicator of patient survival (P = .04) and as a novel regulator of bladder cancer anchorage-dependent (P < .001), anchorage-independent (mean ± standard deviation, 180 ± 23.1 colonies vs 20±9.5 in control, P < .001), and xenograft growth (P < .001). Rescue experiments using catalytically dead AGL variants revealed that this effect is independent of AGL enzymatic functions. We demonstrated that reduced AGL enhances tumor growth by increasing glycine synthesis through increased expression of serine hydroxymethyltransferase 2.

CONCLUSIONS

Using an in vivo RNA interference screen, we discovered that AGL, a glycogen debranching enzyme, has a biologically and statistically significant role in suppressing human cancer growth.

Laforin-malin complex degrades polyglucosan bodies in concert with glycogen debranching enzyme and brain isoform glycogen phosphorylase

In Lafora disease (LD), the deficiency of either EPM2A or NHLRC1, the genes encoding the phosphatase laforin and E3 ligase, respectively, causes massive accumulation of less-branched glycogen inclusions, known as Lafora bodies, also called polyglucosan bodies (PBs), in several types of cells including neurons. The biochemical mechanism underlying the PB accumulation, however, remains undefined. We recently demonstrated that laforin is a phosphatase of muscle glycogen synthase (GS1) in PBs, and that laforin recruits malin, together reducing PBs. We show here that accomplishment of PB degradation requires a protein assembly consisting of at least four key enzymes: laforin and malin in a complex, and the glycogenolytic enzymes, glycogen debranching enzyme 1 (AGL1) and brain isoform glycogen phosphorylase (GPBB). Once GS1-synthesized polyglucosan accumulates into PBs, laforin recruits malin to the PBs where laforin dephosphorylates, and malin degrades the GS1 in concert with GPBB and AGL1, resulting in a breakdown of polyglucosan. Without fountional laforin-malin complex assembled on PBs, GPBB and AGL1 together are unable to efficiently breakdown polyglucosan. All these events take place on PBs and in cytoplasm. Deficiency of each of the four enzymes causes PB accumulation in the cytoplasm of affected cells. Demonstration of the molecular mechanisms underlying PB degradation lays a substantial biochemical foundation that may lead to understanding how PB metabolizes and why mutations of either EPM2A or NHLRC1 in humans cause LD. Mutations in AGL1 or GPBB may cause diseases related to PB accumulation.

Mutation Analysis in Glycogen Storage Disease Type III Patients in the Netherlands: Novel Genotype-Phenotype Relationships and Five Novel Mutations in the AGL Gene

Glycogen Storage Disease type III (GSD III) is an autosomal recessive disorder in which a mutation in the AGL gene causes deficiency of the glycogen debranching enzyme. In childhood, it is characterized by hepatomegaly, keto-hypoglycemic episodes after short periods of fasting, and hyperlipidemia. In adulthood, myopathy, cardiomyopathy, and liver cirrhosis are the main complications. To determine the genotype of the GSD III patients (n = 14) diagnosed and treated in our center, mutation analysis was performed by either denaturing gradient gel electrophoresis or full gene sequencing. We developed, validated and applied both methods, and in all patients a mutation was identified on both alleles. Five novel pathogenic mutations were identified in seven patients, including four missense mutations (c.643G>A, p.Asp215Asn; c.655A>G, p.Asn219Asp; c.1027C>T, p.Arg343Trp; c.1877A>G, p.His626Arg) and one frameshift mutation (c.3911delA, p.Asn1304fs). The c.643G>A, p.Asp215Asn mutation is related with type IIIa, as this mutation was found homozygously in two type IIIa patients. In addition to five novel mutations, we present new genotype-phenotype relationships for c.2039G>A, p.Trp680X; c.753_756delCAGA, p.Asp251fs; and the intron 32 c.4260-12A>G splice site mutation. The p.Trp680X mutation was found homozygously in four patients, presenting a mild IIIa phenotype with mild skeletal myopathy, elevated CK values, and no cardiomyopathy. The p.Asp251fs mutation was found homozygously in one patient presenting with a severe IIIa phenotype, with skeletal myopathy, and severe symptomatic cardiomyopathy. The c.4260-12A>G mutation was found heterozygously, together with the p.Arg343Trp mutation in a severe IIIb patient who developed liver cirrhosis and hepatocellular carcinoma, necessitating an orthotopic liver transplantation.

Egyptian glycogen storage disease type III - identification of six novel AGL mutations, including a large 1.5 kb deletion and a missense mutation p.L620P with subtype IIId

BACKGROUND

Glycogen storage disease type III (GSD III) is caused by mutations in AGL which encodes for a single protein with two enzyme activities: oligo-1, 4-1, 4-glucantransferase (transferase) and amylo-1, 6-glucosidase. Activity of both enzymes is lost in most patients with GSD III, but in the very rare subtype IIId, transferase activity is deficient. Since the spectrum of AGL mutations is dependent on the ethnic group, we investigated the clinical and molecular characteristics in Egyptian patients with GSD III.

METHODS

Clinical features were examined in five Egyptian patients. AGL was sequenced and AGL haplotypes were determined.

RESULTS

Six novel AGL mutations were identified: a large deletion (c.3481-3588+1417del1525 bp), two insertions (c.1389insG and c.2368insA), two small deletions (c.2223-2224delGT and c.4041delT), and a missense mutation (p.L620P). p.L620P was found in a patient with IIId. Each mutation was located on a different AGL haplotype.

CONCLUSIONS

Our results suggest that there is allelic and phenotypic heterogeneity of GSD III in Egypt. This is the second description of a large deletion in AGL. p.L620P is the second mutation found in GSD IIId.