Mutations in the testis/liver isoform of the phosphorylase kinase gamma subunit (PHKG2) cause autosomal liver glycogenosis in the gsd rat and in humans

Fast and deep phosphoproteome analysis with the Orbitrap Astral mass spectrometer

Owing to its roles in cellular signal transduction, protein phosphorylation plays critical roles in myriad cell processes. That said, detecting and quantifying protein phosphorylation has remained a challenge. We describe the use of a novel mass spectrometer (Orbitrap Astral) coupled with data-independent acquisition (DIA) to achieve rapid and deep analysis of human and mouse phosphoproteomes. With this method, we map approximately 30,000 unique human phosphorylation sites within a half-hour of data collection. The technology is benchmarked to other state-of-the-art MS platforms using both synthetic peptide standards and with EGF-stimulated HeLa cells. We apply this approach to generate a phosphoproteome multi-tissue atlas of the mouse. Altogether, we detect 81,120 unique phosphorylation sites within 12 hours of measurement. With this unique dataset, we examine the sequence, structural, and kinase specificity context of protein phosphorylation. Finally, we highlight the discovery potential of this resource with multiple examples of phosphorylation events relevant to mitochondrial and brain biology.

Gene therapy for glycogen storage diseases

Glycogen storage disorders (GSDs) are inherited disorders of metabolism resulting from the deficiency of individual enzymes involved in the synthesis, transport, and degradation of glycogen. This literature review summarizes the development of gene therapy for the GSDs. The abnormal accumulation of glycogen and deficiency of glucose production in GSDs lead to unique symptoms based upon the enzyme step and tissues involved, such as liver and kidney involvement associated with severe hypoglycemia during fasting and the risk of long-term complications including hepatic adenoma/carcinoma and end stage kidney disease in GSD Ia from glucose-6-phosphatase deficiency, and cardiac/skeletal/smooth muscle involvement associated with myopathy +/- cardiomyopathy and the risk for cardiorespiratory failure in Pompe disease. These symptoms are present to a variable degree in animal models for the GSDs, which have been utilized to evaluate new therapies including gene therapy and genome editing. Gene therapy for Pompe disease and GSD Ia has progressed to Phase I and Phase III clinical trials, respectively, and are evaluating the safety and bioactivity of adeno-associated virus vectors. Clinical research to understand the natural history and progression of the GSDs provides invaluable outcome measures that serve as endpoints to evaluate benefits in clinical trials. While promising, gene therapy and genome editing face challenges with regard to clinical implementation, including immune responses and toxicities that have been revealed during clinical trials of gene therapy that are underway. Gene therapy for the glycogen storage diseases is under development, addressing an unmet need for specific, stable therapy for these conditions.

A novel PHKA2 variant in a Chinese boy with glycogen storage diseases type IXa

Background

Glycogen storage diseases (GSDs) are a group of heterogeneous inherited metabolic disorders with an incidence of 4%-5%. There are 19 types of GSDs, making diagnosis one of the greatest challenges.

Methods

The proband and his parents were referred to our hospital for genetic diagnosis. Ultrasound screening suggested hepatomegaly. A novel insertion variant NM_000292 c.1155_1156insT (p. 386N>*) in PHKA2 gene was identified using trio whole exome sequencing (Trio-WES), which resulted in the codon of amino acid 386 from asparagine to termination (p. 386N>*). The 3D mutant protein structure was predicted using AlphaFold, and the results showed that the truncated PHKA2 protein contained 385 of the 1,235 amino acids of the mature protein.

Conclusion

We describe a previously unreported case of a GSDs IXa type Chinese boy caused by a novel PHKA2 variant. This clinical case contributes to the understanding of the characteristics of GSDs type IXa and expands the variants spectrum of genes related to GSDs type IXa. Our findings demonstrated the significance of genetic testing in the diagnosis of GSDs.

Glycogen storage diseases: An update

Glycogen storage diseases (GSDs), also referred to as glycogenoses, are inherited metabolic disorders of glycogen metabolism caused by deficiency of enzymes or transporters involved in the synthesis or degradation of glycogen leading to aberrant storage and/or utilization. The overall estimated GSD incidence is 1 case per 20000-43000 live births. There are over 20 types of GSD including the subtypes. This heterogeneous group of rare diseases represents inborn errors of carbohydrate metabolism and are classified based on the deficient enzyme and affected tissues. GSDs primarily affect liver or muscle or both as glycogen is particularly abundant in these tissues. However, besides liver and skeletal muscle, depending on the affected enzyme and its expression in various tissues, multiorgan involvement including heart, kidney and/or brain may be seen. Although GSDs share similar clinical features to some extent, there is a wide spectrum of clinical phenotypes. Currently, the goal of treatment is to maintain glucose homeostasis by dietary management and the use of uncooked cornstarch. In addition to nutritional interventions, pharmacological treatment, physical and supportive therapies, enzyme replacement therapy (ERT) and organ transplantation are other treatment approaches for both disease manifestations and long-term complications. The lack of a specific therapy for GSDs has prompted efforts to develop new treatment strategies like gene therapy. Since early diagnosis and aggressive treatment are related to better prognosis, physicians should be aware of these conditions and include GSDs in the differential diagnosis of patients with relevant manifestations including fasting hypoglycemia, hepatomegaly, hypertransaminasemia, hyperlipidemia, exercise intolerance, muscle cramps/pain, rhabdomyolysis, and muscle weakness. Here, we aim to provide a comprehensive review of GSDs. This review provides general characteristics of all types of GSDs with a focus on those with liver involvement.

A Mouse Model of Glycogen Storage Disease Type IX-Beta: A Role for Phkb in Glycogenolysis

Glycogen storage disease type IX (GSD-IX) constitutes nearly a quarter of all GSDs. This ketotic form of GSD is caused by mutations in phosphorylase kinase (PhK), which is composed of four subunits (α, β, γ, δ). PhK is required for the activation of the liver isoform of glycogen phosphorylase (PYGL), which generates free glucose-1-phosphate monomers to be used as energy via cleavage of the α -(1,4) glycosidic linkages in glycogen chains. Mutations in any of the PhK subunits can negatively affect the regulatory and catalytic activity of PhK during glycogenolysis. To understand the pathogenesis of GSD-IX-beta, we characterized a newly created PHKB knockout (Phkb−/−) mouse model. In this study, we assessed fasting blood glucose and ketone levels, serum metabolite concentrations, glycogen phosphorylase activity, and gene expression of gluconeogenic genes and fibrotic genes. Phkb−/− mice displayed hepatomegaly with lower fasting blood glucose concentrations. Phkb−/− mice showed partial liver glycogen phosphorylase activity and increased sensitivity to pyruvate, indicative of partial glycogenolytic activity and upregulation of gluconeogenesis. Additionally, gene expression analysis demonstrated increased lipid metabolism in Phkb−/− mice. Gene expression analysis and liver histology in the livers of old Phkb−/− mice (>40 weeks) showed minimal profibrogenic features when analyzed with age-matched wild-type (WT) mice. Collectively, the Phkb−/− mouse recapitulates mild clinical features in patients with GSD-IX-beta. Metabolic and molecular analysis confirmed that Phkb−/− mice were capable of sustaining energy homeostasis during prolonged fasting by using partial glycogenolysis, increased gluconeogenesis, and potentially fatty acid oxidation in the liver.

Expanding the clinicopathological-genetic spectrum of glycogen storage disease type IXd by a Chinese neuromuscular center

Background

Glycogen storage disease (GSDs) is characterized by abnormally inherited glycogen metabolism. GSD IXd, which is caused by mutations in the PHKA1 gene, is an X-linked rare disease with mild myopathic symptoms. To date, only 13 patients with GSD IXd have been reported. In this study, we aimed to expand the clinicopathological-genetic spectrum of GSD IXd at a neuromuscular center in China.

Methods

Data on patients diagnosed with GSD IXd at our neuromuscular center were collected retrospectively. Clinical features, electrophysiology, muscle pathology, and genetic information were analyzed.

Results

Between 2015 and 2021, three patients were diagnosed with GSD IXd based on clinical manifestations, pathological findings, and genetic testing. One patient presented with mitochondrial myopathy. All patients exhibited muscle weakness and elevated levels of creatine kinase. Electromyography-detected myopathic changes were found in two patients, whereas one patient refused to undergo this examination. Pathological examinations in all patients revealed subsarcolemmal accumulation of glycogen under PAS staining. All patients had mutations in the PHKA1 gene and the patient with mitochondrial myopathy also had a mutation in the MT-TL1 gene.

Conclusion

Our study expands the clinicogenotype and phenotype of GSD IXd in a Chinese population. Our study also expands the known mutation spectrum for GSD IXd, contributing to a better characterization and understanding of this ultrarare neuromuscular disorder.

A female patient with GSD IXc developing multiple and recurrent hepatocellular carcinoma: a case report and literature review

Glycogen storage disease type IX (GSD IX), the most common form of GSD, is caused by a defect in phosphorylase kinase (PhK). We describe the case of a female patient with GSD IXc harboring a homozygous mutation in PHKG2 (NM_000294.3; PHKG2 (c.280_282delATC (p. I94del)) definitively diagnosed using the GSD gene panel. She presented with hypoglycemia, hepatomegaly, and short stature and died of cirrhosis and recurrent multiple hepatocellular adenoma at the age of 69 years and 11 months.

Characterization of liver GSD IX γ2 pathophysiology in a novel Phkg2-/- mouse model

INTRODUCTION

Liver Glycogen Storage Disease IX is a rare metabolic disorder of glycogen metabolism caused by deficiency of the phosphorylase kinase enzyme (PhK). Variants in the PHKG2 gene, encoding the liver-specific catalytic γ2 subunit of PhK, are associated with a liver GSD IX subtype known as PHKG2 GSD IX or GSD IX γ2. There is emerging evidence that patients with GSD IX γ2 can develop severe and progressive liver disease, yet research regarding the disease has been minimal to date. Here we characterize the first mouse model of liver GSD IX γ2.

METHODS

A Phkg2-/- mouse model was generated via targeted removal of the Phkg2 gene. Knockout (Phkg2-/-, KO) and wild type (Phkg2+/+, WT) mice up to 3 months of age were compared for morphology, Phkg2 transcription, PhK enzyme activity, glycogen content, histology, serum liver markers, and urinary glucose tetrasaccharide Glcα1-6Glcα1-4Glcα1-4Glc (Glc4).

RESULTS

When compared to WT controls, KO mice demonstrated significantly decreased liver PhK enzyme activity, increased liver: body weight ratio, and increased glycogen in the liver, with no glycogen accumulation observed in the brain, quadricep, kidney, and heart. KO mice demonstrated elevated liver blood markers as well as elevated urine Glc4, a commonly used biomarker for glycogen storage disease. KO mice demonstrated features of liver structural damage. Hematoxylin & Eosin and Masson's Trichrome stained KO mice liver histology slides revealed characteristic GSD hepatocyte architectural changes and early liver fibrosis, as have been reported in liver GSD patients.

DISCUSSION

This study provides the first evidence of a mouse model that recapitulates the liver-specific pathology of patients with GSD IX γ2. The model will provide the first platform for further study of disease progression in GSD IX γ2 as well as for the evaluation of novel therapeutics.

PHKA2 variants expand the phenotype of phosphorylase B kinase deficiency to include patients with ketotic hypoglycemia only

Idiopathic ketotic hypoglycemia (IKH) is a diagnosis of exclusion with glycogen storage diseases (GSDs) as a differential diagnosis. GSD IXa presents with ketotic hypoglycemia (KH), hepatomegaly, and growth retardation due to PHKA2 variants. In our multicenter study, 12 children from eight families were diagnosed or suspected of IKH. Whole‐exome sequencing or targeted next‐generation sequencing panels were performed. We identified two known and three novel (likely) pathogenic PHKA2 variants, such as p.(Pro869Arg), p.(Pro498Leu), p.(Arg2Gly), p.(Arg860Trp), and p.(Val135Leu), respectively. Erythrocyte phosphorylase kinase activity in three patients with the novel variants p.(Arg2Gly) and p.(Arg860Trp) were 15%–20% of mean normal. One patient had short stature and intermittent mildly elevated aspartate aminotransferase, but no hepatomegaly. Family testing identified two asymptomatic children and 18 adult family members with one of the PHKA2 variants, of which 10 had KH symptoms in childhood and 8 had mild symptoms in adulthood. Our study expands the classical GSD IXa phenotype of PHKA2 missense variants to a continuum from seemingly asymptomatic carriers, over KH‐only with phosphorylase B kinase deficiency, to more or less complete classical GSD IXa. In contrast to typical IKH, which is confined to young children, KH may persist into adulthood in the KH‐only phenotype of PHKA2.

Dosage Compensation in Females with X-Linked Metabolic Disorders

Through the use of new genomic and metabolomic technologies, our comprehension of the molecular and biochemical etiologies of genetic disorders is rapidly expanding, and so are insights into their varying phenotypes. Dosage compensation (lyonization) is an epigenetic mechanism that balances the expression of genes on heteromorphic sex chromosomes. Many studies in the literature have suggested a profound influence of this phenomenon on the manifestation of X-linked disorders in females. In this review, we summarize the clinical and genetic findings in female heterozygotic carriers of a pathogenic variant in one of ten selected X-linked genes whose defects result in metabolic disorders.

Benign or not benign? Deep phenotyping of liver Glycogen Storage Disease IX

INTRODUCTION

Liver Glycogen Storage Disease Type IX (GSD IX) is one of the most common forms of GSD. It is caused by a deficiency in enzyme phosphorylase kinase (PhK), a complex, hetero-tetrameric enzyme comprised of four subunits - α, β, γ, and δ - each with tissue specific isoforms encoded by different genes. Until the recent availability of gene panels and exome sequencing, the diagnosis of liver GSD IX did not allow for differentiation of these subtypes. This study presents the first comprehensive literature review for liver GSD IX subtypes - GSD IX α2, β, and γ2. We aim to better characterize the natural history of liver GSD IX and further investigate if there are subtype-specific differences in clinical presentation.

METHODS

A comprehensive literature review was performed with the help of a medical librarian at Duke University Medical Center to gather all published patients of liver GSD IX. Our refined search yielded 74 articles total. Available patient data were compiled into an excel spreadsheet. Data were analyzed via descriptive statistics. The number of patients with specific symptoms were individually summed and reported as a percentage of the total number of patients for which data were available or were averaged and reported as a mean numerical value. Published pathology reports were scored using the International Association of the Study of the Liver Scale.

RESULTS

There were a total of 183 GSD IX α2 patients, 17 GSD IX β patients, and 30 GSD IX γ2 patients. Average age at diagnosis was 4 years for GSD IX α2 patients, 2.34 years for GSD IX β patients, and 1.81 years for GSD IX γ2 patients. Hepatomegaly was reported in 164/176 (93.2%) of GSD IX α2 patients, 16/17 (94.1%) of GSD IX β patients, and 30/30 (100%) of GSD IX γ2 patients. Fasting hypoglycemia was reported in 53/121 (43.8%) of GSD IX α2 patients, 8/16 (50%) of GSD IX β patients, and 18/19 (94.7%) of GSD IX γ2 patients. Liver biopsy pathology reports were available and interpreted for 46 GSD IX α2 patients, 3 GSD IX β patients, and 24 GSD IX γ2 patients. 22/46 (47.8%) GSD IX α2 patients, 1/3 (33.3%) GSD IX β patients, and 23/24 (95.8%) GSD IX γ2 patients with available pathology reports documented either some degree of fibrosis or cirrhosis.

CONCLUSION

Our comprehensive review demonstrates quantitatively that the clinical presentation of GSD IX γ2 patients is more severe than that of GSD IX α2 or β patients. However, our study also shows the existence of a severe phenotype in GSD IX α2, evidenced by early onset liver pathology in conjunction with clinical symptoms. There is need for a more robust natural history study to better understand the variability in liver pathophysiology within liver GSD IX; in addition, further study of mutations and gene mapping could bring a better understanding of the relationship between genotype and clinical presentation.

Rat models of human diseases and related phenotypes: a systematic inventory of the causative genes

The laboratory rat has been used for a long time as the model of choice in several biomedical disciplines. Numerous inbred strains have been isolated, displaying a wide range of phenotypes and providing many models of human traits and diseases. Rat genome mapping and genomics was considerably developed in the last decades. The availability of these resources has stimulated numerous studies aimed at discovering causal disease genes by positional identification. Numerous rat genes have now been identified that underlie monogenic or complex diseases and remarkably, these results have been translated to the human in a significant proportion of cases, leading to the identification of novel human disease susceptibility genes, helping in studying the mechanisms underlying the pathological abnormalities and also suggesting new therapeutic approaches. In addition, reverse genetic tools have been developed. Several genome-editing methods were introduced to generate targeted mutations in genes the function of which could be clarified in this manner [generally these are knockout mutations]. Furthermore, even when the human gene causing a disease had been identified without resorting to a rat model, mutated rat strains (in particular KO strains) were created to analyze the gene function and the disease pathogenesis. Today, over 350 rat genes have been identified as underlying diseases or playing a key role in critical biological processes that are altered in diseases, thereby providing a rich resource of disease models. This article is an update of the progress made in this research and provides the reader with an inventory of these disease genes, a significant number of which have similar effects in rat and humans.

A novel frameshift PHKA2 mutation in a family with glycogen storage disease type IXa: A first report in Vietnam and review of literature

BACKGROUND

Glycogen storage diseases (GSDs) are clinically and genetically heterogeneous disorders. Overlapping features between liver GSDs are a major challenge in the clinical diagnosis of them. Genetic testing can provide an early and accurate diagnosis of patients suspected with GSDs.

CASE PRESENTATION

In this study, we report two siblings born to healthy, non-consanguineous Vietnamese parents with hepatomegaly. The proband presented with hepatomegaly, normal spleen, elevated transaminases, without hypoglycemia, normal lactate dehydrogenase and creatine kinase. Liver biopsy revealed degeneration and swollen hepatocytes, suggesting a diagnosis with GSDs.

METHODS

Whole exome sequencing was applied to identify genetic variants in the proband. Variant validation and familial co-segregation analysis were examined using Sanger sequencing.

RESULTS

A novel frameshift duplication mutation c.3308_3312dupATGTC (p.L1105Mfs*11) of the PHKA2 gene was identified in the proband and his elder brother at the hemizygous state. This mutation was inherited from their mother. Their father and younger brother were normal genotype.

CONCLUSIONS

The two siblings were accurately diagnosed with GSD type XIa. This is the first case report of GSD type IXa in Vietnamese patients with a mutation in the PHKA2 gene. This finding may support for genetics diagnosis of unknown cause of hepatomegaly.

Molecular diagnosis of glycogen storage disease type IX using a glycogen storage disease gene panel

Glycogen storage disease type IX (GSD IX) is caused by a deficiency of hepatic phosphorylase kinase. The aim of this study was to clarify the clinical features, long term outcomes, and genetic analysis of GSD IX in Korea. A GSD gene panel was created and hybridization capture-based next-generation sequencing was performed. We investigated clinical laboratory data, results of molecular genetic analysis, liver biopsy findings, and long-term outcomes. Ten children were diagnosed with GSD IX at Seoul National University Children's Hospital. Hypoglycemia, hyperlactacidemia, hypertriglyceridemia, hyperuricemia, liver fibrosis on liver biopsy, and short stature was found in 30%, 56%, 100%, 60%, 80% and 50% of the children, respectively. Seven PHKA2 variants were identified in eight children with GSD IXa-one nonsense (c.2268dupT; p.(Asp757Ter)), two splicing (c.918+1G > A, c.718-2A > G), one frameshift (c.405_419delinsTCCTGGCC; p.(Asp136ProfsTer11)), and three missense variants (c.3628G > A; p.(Gly1210Arg), c.1245G > T and c.2746C > T; p.(Arg916Trp)). Two variants of PHKG2 were identified in two children with GSD IXc-one frameshift (c.783delC; p.(Ser262AlafsTer6)) and one missense (c.661G > A; p.(Val221Met)). Elevated liver enzymes and hypertriglyceridemia in children with GSD IXa tended to improve with age. For the first time, we report hepatocellular carcinoma in a patient with GSD IXc. The GSD gene panel is a useful diagnostic tool to confirm GSD IX. The clinical phenotype of GSD IXc is severe and monitoring for the development of hepatocellular carcinoma should be implemented.

Mutation in PHKA2 leading to childhood glycogen storage disease type IXa: A case report and literature review

INTRODUCTION

Glycogen storage disease (GSD) type IX, characterized by liver enlargement and elevated aminotransferase levels, is the most frequent type of GSD. The global incidence of GSD type IXa is only about 1/100,000 individuals. Case reports of GSD type IX are rare in China. We present the first case report of GSD type IXa in Northeast China caused by mutation of PHKA2.

PATIENT CONCERNS

An 11-year-old boy was referred to our hospital because of liver enlargement with consistently elevated transaminase levels over 6 months.

DIAGNOSIS

Histopathological results following an ultrasound-guided liver biopsy confirmed a diagnosis of GSD. Further genetic testing showed that the patient had GSD type IXa caused by the c.133C>T mutation in PHAK2.

INTERVENTIONS

We placed the patient on a high-protein and high-starch diet and provided hepatoprotective and supportive therapy.

OUTCOMES

The patient's transaminase levels decreased significantly and were nearly normal at 10-month follow-up.

CONCLUSION

This is the first reported case of GSD type IXa in Northeast China. We hope that the detailed and complete report of this case will provide a reference for the diagnosis of liver enlargement of unknown etiology in future clinical practice.

A novel PHKA2 mutation in a Chinese child with glycogen storage disease type IXa: a case report and literature review

Background

PHKA2 gene mutations can cause liver phosphorylase kinase (PhK) deficiency, resulting in glycogen storage disease type IXa (GSD IXa). Elevated liver transaminase levels and liver enlargement are the most frequent phenotypes of GSD IXa. However, whether the phenotypes are applicable to Chinese patients remains unclear.

Case report

A boy aged 2 years and 8 months with a history of episodic fatigue and weakness since he was 2 years old was referred to our endocrinology clinic. Apart from symptomatic ketotic hypoglycemic episodes (palpitation, hand shaking, sweating, etc.), no abnormalities of liver transaminase levels or liver size were found. To identify the aetiology of his clinically diagnosed hypoglycaemia, the proband and his parents were screened for PHKA2 gene mutations by next-generation sequencing. A heterozygous mutation (c.2972C > G, p.G991A) in PHKA2 was found in the proband and his mother. Twenty-one Chinese cases with GSD IXa have been reported in the literature to date, and elevated liver transaminase levels (95%) and liver enlargement (91%) are the most frequent phenotypes of GSD IXa in Chinese patients. Hypoglycaemia may be one of the early onset symptoms in infants with GSD IXa.

Conclusions

This study enriches our knowledge of the PHKA2 gene mutation spectrum and provides further information about the phenotypic characteristics of Chinese GSD IXa patients.

Diagnosis and management of glycogen storage diseases type VI and IX: a clinical practice resource of the American College of Medical Genetics and Genomics (ACMG)

PURPOSE

Glycogen storage disease (GSD) types VI and IX are rare diseases of variable clinical severity affecting primarily the liver. GSD VI is caused by deficient activity of hepatic glycogen phosphorylase, an enzyme encoded by the PYGL gene. GSD IX is caused by deficient activity of phosphorylase kinase (PhK), the enzyme subunits of which are encoded by various genes: ɑ (PHKA1, PHKA2), β (PHKB), ɣ (PHKG1, PHKG2), and δ (CALM1, CALM2, CALM3). Glycogen storage disease types VI and IX have a wide spectrum of clinical manifestations and often cannot be distinguished from each other, or from other liver GSDs, on clinical presentation alone. Individuals with GSDs VI and IX can present with hepatomegaly with elevated serum transaminases, ketotic hypoglycemia, hyperlipidemia, and poor growth. This guideline for the management of GSDs VI and IX was developed as an educational resource for health-care providers to facilitate prompt and accurate diagnosis and appropriate management of patients.

METHODS

A national group of experts in various aspects of GSDs VI and IX met to review the limited evidence base from the scientific literature and provided their expert opinions. Consensus was developed in each area of diagnosis, treatment, and management. Evidence bases for these rare disorders are largely based on expert opinion, particularly when targeted therapeutics that have to clear the US Food and Drug Administration (FDA) remain unavailable.

RESULTS

This management guideline specifically addresses evaluation and diagnosis across multiple organ systems involved in GSDs VI and IX. Conditions to consider in a differential diagnosis stemming from presenting features and diagnostic algorithms are discussed. Aspects of diagnostic evaluation and nutritional and medical management, including care coordination, genetic counseling, and prenatal diagnosis are addressed.

CONCLUSION

A guideline that will facilitate the accurate diagnosis and optimal management of patients with GSDs VI and IX was developed. This guideline will help health-care providers recognize patients with GSDs VI and IX, expedite diagnosis, and minimize adverse sequelae from delayed diagnosis and inappropriate management. It will also help identify gaps in scientific knowledge that exist today and suggest future studies.

Development of diagnostic SCAR markers for genomic DNA amplifications in breast carcinoma by DNA cloning of high-GC RAMP-PCR fragments

Cancer is genetically heterogeneous regarding to molecular genetic characteristics and pathogenic pathways. A wide spectrum of biomarkers, including DNA markers, is used in determining genomic instability, molecular subtype determination and disease prognosis, and estimating sensitivity to different drugs in clinical practice. In a previous study, we developed highly effective DNA markers using improved random amplified polymorphic DNA (RAPD) with high-GC primers, which is a valuable approach for the genetic authentication of medicinal plants. In this study, we applied this effective DNA marker technique to generate genetic fingerprints that detect genomic alterations in human breast cancer tissues and then developed sequence-characterized amplified region (SCAR) markers. Three SCAR markers (BC10-1, BC13-4 and BC31-2) had high levels of genomic DNA amplification in breast cancer. The PHKG2 and RNF40 genes are either overlapping or close to the sequences of SCAR marker BC13-4, while SCAR marker BC10-1 is in the intron and overlap the DPEP1 gene, suggesting that alterations in the expression of these genes could contribute to cancer progression. Screening of breast cancer cell lines showed that the mRNA expression levels for the PHKG2 and DPEP1 were lower in non-tumorigenic mammary epithelial cell MCF10A, but elevated in other cell lines. The DPEP1 mRNA level in invasive ductal carcinoma specimens was significantly higher than that of the adjacent normal tissues in women. Taken together, high-GC RAMP-PCR provides greater efficacy in measuring genomic DNA amplifications, deletion or copy number variations. Furthermore, SCAR markers BC10-1 and BC13-4 might be useful diagnostic markers for breast cancer carcinomas.

PHKG2 mutation spectrum in glycogen storage disease type IXc: a case report and review of the literature

BACKGROUND

PHKG2 gene mutation can lead to liver phosphorylase kinase (PhK) deficiency, which is related to glycogen storage disease type IX (GSD IX). GSD IXc due to PHKG2 mutation is the second most common GSD IX.

METHODS

We identified a novel mutation (c.553C>T, p.Arg185X) in PHKG2 in a Chinese family and verified it by next-generation and Sanger sequencing. The mutation spectrum of the PHKG2 gene was summarized based on 25 GSD IXc patients with PHKG2 mutations.

RESULTS

We found that missense mutation (39%) was the most common type of mutation, followed by nonsense mutation (23%). Mutations were more prevalent in Asian (12/25) and European (9/25) populations than in populations from elsewhere. The exons had more sites of mutation than the introns, and exons 3 and 6 were the most frequent sites of mutations.

CONCLUSIONS

This study expands our knowledge of the PHKG2 gene mutation spectrum, providing a molecular basis for GSD IXc.

Clinical and Molecular Variability in Patients with PHKA2 Variants and Liver Phosphorylase b Kinase Deficiency

Glycogen storage disease (GSD) type IX is a rare disease of variable clinical severity affecting primarily the liver tissue. Individuals with liver phosphorylase b kinase (PhK) deficiency (GSD IX) can present with hepatomegaly with elevated serum transaminases, ketotic hypoglycemia, hyperlipidemia, and poor growth with considerable variation in clinical severity. PhK is a cAMP-dependent protein kinase that phosphorylates the inactive form of glycogen phosphorylase, phosphorylase b, to produce the active form, phosphorylase a. PhK is a heterotetramer; the alpha 2 subunit in the liver is encoded by the X-linked PHKA2 gene. About 75% of individuals with liver PhK deficiency have mutations in the PHKA2 gene; this condition is also known as X-linked glycogenosis (XLG). Here we report the variability in clinical severity and laboratory findings in 12 male patients from 10 different families with X-linked liver PhK deficiency caused by mutations in PHKA2. We found that there is variability in the severity of clinical features, including hypoglycemia and growth. We also report additional PHKA2 variants that were identified in 24 patients suspected to have liver PhK deficiency. The basis of the clinical variation in GSDIX due to X-linked PHKA2 gene mutations is currently not well understood. Creating systematic registries, and collecting longitudinal data may help in better understanding of this rare, but common, glycogen storage disorder.

SYNOPSIS

Liver phosphorylase b kinase (PhK) deficiency caused due to mutations in X-linked PHKA2 is highly variable.

A new variant in PHKA2 is associated with glycogen storage disease type IXa

Glucogenosis type IX is caused by pathogenic variants of the PHKA2 gene. Herein, we report a patient with clinical symptoms compatible with Glycogen Storage Disease type IXa. PYGL, PHKA1, PHKA2, PHKB and PHKG2 genes were analyzed by Next Generation Sequencing (NGS). We identified the previously undescribed hemizygous missense variant NM_000292.2(PHKA2):c.1963G > A, p.(Glu655Lys) in PHKA2 exon 18. In silico analyses showed two possible pathogenic consequences: it affects a highly conserved amino acid and disrupts the exon 18 canonical splice donor site. The variant was found as a "de novo" event.

Elucidation of the molecular mechanisms underlying adverse reactions associated with a kinase inhibitor using systems toxicology

BACKGROUND/OBJECTIVES

Targeted kinase inhibitors are an important class of agents in anticancer therapeutics, but their limited tolerability hampers their clinical performance. Identification of the molecular mechanisms underlying the development of adverse reactions will be helpful in establishing a rational method for the management of clinically adverse reactions. Here, we selected sunitinib as a model and demonstrated that the molecular mechanisms underlying the adverse reactions associated with kinase inhibitors can efficiently be identified using a systems toxicological approach.

METHODS

First, toxicological target candidates were short-listed by comparing the human kinase occupancy profiles of sunitinib and sorafenib, and the molecular mechanisms underlying adverse reactions were predicted by sequential simulations using publicly available mathematical models. Next, to evaluate the probability of these predictions, a clinical observation study was conducted in six patients treated with sunitinib. Finally, mouse experiments were performed for detailed confirmation of the hypothesized molecular mechanisms and to evaluate the efficacy of a proposed countermeasure against adverse reactions to sunitinib.

RESULTS

In silico simulations indicated the possibility that sunitinib-mediated off-target inhibition of phosphorylase kinase leads to the generation of oxidative stress in various tissues. Clinical observations of patients and mouse experiments confirmed the validity of this prediction. The simulation further suggested that concomitant use of an antioxidant may prevent sunitinib-mediated adverse reactions, which was confirmed in mouse experiments.

CONCLUSIONS

A systems toxicological approach successfully predicted the molecular mechanisms underlying clinically adverse reactions associated with sunitinib and was used to plan a rational method for the management of these adverse reactions.

Salt-inducible Kinase 3 Signaling Is Important for the Gluconeogenic Programs in Mouse Hepatocytes

Salt-inducible kinases (SIKs), members of the 5'-AMP-activated protein kinase (AMPK) family, are proposed to be important suppressors of gluconeogenic programs in the liver via the phosphorylation-dependent inactivation of the CREB-specific coactivator CRTC2. Although a dramatic phenotype for glucose metabolism has been found in SIK3-KO mice, additional complex phenotypes, dysregulation of bile acids, cholesterol, and fat homeostasis can render it difficult to discuss the hepatic functions of SIK3. The aim of this study was to examine the cell autonomous actions of SIK3 in hepatocytes. To eliminate systemic effects, we prepared primary hepatocytes and screened the small compounds suppressing SIK3 signaling cascades. SIK3-KO primary hepatocytes produced glucose more quickly after treatment with the cAMP agonist forskolin than the WT hepatocytes, which was accompanied by enhanced gluconeogenic gene expression and CRTC2 dephosphorylation. Reporter-based screening identified pterosin B as a SIK3 signaling-specific inhibitor. Pterosin B suppressed SIK3 downstream cascades by up-regulating the phosphorylation levels in the SIK3 C-terminal regulatory domain. When pterosin B promoted glucose production by up-regulating gluconeogenic gene expression in mouse hepatoma AML-12 cells, it decreased the glycogen content and stimulated an association between the glycogen phosphorylase kinase gamma subunit (PHKG2) and SIK3. PHKG2 phosphorylated the peptides with sequences of the C-terminal domain of SIK3. Here we found that the levels of active AMPK were higher both in the SIK3-KO hepatocytes and in pterosin B-treated AML-12 cells than in their controls. These results suggest that SIK3, rather than SIK1, SIK2, or AMPKs, acts as the predominant suppressor in gluconeogenic gene expression in the hepatocytes.

Clinical, Biochemical, and Genetic Characterization of Glycogen Storage Type IX in a Child with Asymptomatic Hepatomegaly

Glycogen storage disease type IX (GSD IX) is caused by a defect in phosphorylase b kinase (PhK) that results from mutations in the PHKA2, PHKB, and PHKG2 genes. Patients usually manifest recurrent ketotic hypoglycemia with growth delay, but some may present simple hepatomegaly. Although GSD IX is one of the most common causes of GSDs, its biochemical and genetic diagnosis has been problematic due to its rarity, phenotypic overlap with other types of GSDs, and genetic heterogeneities. In our report, a 22-month-old boy with GSD IX is described. No other manifestations were evident except for hepatomegaly. His growth and development also have been proceeding normally. Diagnosed was made by histologic examination, an enzyme assay, and genetic testing with known c.3210_3212del (p.Arg1070del) mutation in PHKA2 gene.

Novel PHKG2 mutation causing GSD IX with prominent liver disease: report of three cases and review of literature

Glycogen storage disease type IX (GSD IX) is a common form of glycogenosis due to mutations in PHKA1, PHKA2, or PHKB and PHKG2 genes resulting in the deficiency of phosphorylase kinase. The first two genes are X-linked while the latter two follow an autosomal recessive inheritance. The majority of cases of GSD IX are attributed to defects in PHKA2 which usually cause a mild disease. We report three patients with PHKG2-related GSD IX presenting with significant hepatic involvement, fibrosis, and cirrhosis. Interestingly, the homozygosity mapping resolved a dilemma about an erroneously normal phosphorylase kinase activity in patient 1. The novel mutation found in all the three patients (p.G220E) affects the catalytic subunit of the phosphorylase kinase. Increasing evidence shows that patients with PHKG2 mutations have a severe hepatic phenotype within the heterogeneous GSD IX disorder. Therefore, defect in PHKG2 should be considered in patients with suspected glycogenosis associated with significant liver fibrosis and cirrhosis.

Variability of disease spectrum in children with liver phosphorylase kinase deficiency caused by mutations in the PHKG2 gene

Liver phosphorylase b kinase (PhK) deficiency (glycogen storage disease type IX), one of the most common causes of glycogen storage disease, is caused by mutations in the PHKA2, PHKB, and PHKG2 genes. Presenting symptoms include hepatomegaly, ketotic hypoglycemia, and growth delay. Clinical severity varies widely. Autosomal recessive mutations in the PHKG2 gene, which cause about 10-15% of cases, have been associated with severe symptoms including increased risk of liver cirrhosis in childhood. We have summarized the molecular, biochemical, and clinical findings in five patients, age 5-16 years, diagnosed with liver PhK deficiency caused by PHKG2 gene mutations. We have identified five novel and two previously reported mutations in the PHKG2 gene in these five patients. Clinical severity was variable among these patients. Histopathological studies were performed for four of the patients on liver biopsy samples, all of which showed signs of fibrosis but not cirrhosis. One of the patients (aged 9 years) developed a liver adenoma which later resolved. All patients are currently doing well. Their clinical symptoms have improved with age and treatment. These cases add to the current knowledge of clinical variability in patients with PHKG2 mutations. Long term studies, involving follow-up of these patients into adulthood, are needed.

Glycogen storage disease type IX: High variability in clinical phenotype

Glycogen storage disease type IX (GSD type IX) results from a deficiency of hepatic phosphorylase kinase activity. The phosphorylase kinase holoenzyme is made up of four copies of each of four subunits (alpha, beta, gamma and delta). The liver isoforms of the alpha-, beta- and gamma-subunits are encoded by PHKA2, PHKB and PHKG2, respectively. Mutation within these genes has been shown to result in GSD type IX. The diagnosis of GSD type IX is complicated by the spectrum of clinical symptoms, variation in tissue specificity and severity, and its inheritance, either X-linked or autosomal recessive. We investigated 15 patients from 12 families with suspected GSD type IX. Accurate diagnosis had been hampered by enzymology not being diagnostic in five cases. Clinical symptoms included combinations of hypoglycaemia, hepatosplenomegaly, short stature, hepatopathy, weakness, fatigue and motor delay. Biochemical findings included elevated lactate, urate and lipids. We characterised causative mutations in the PHKA2 gene in ten patients from eight families, in PHKG2 in two unrelated patients and in the PHKB gene in three patients from two families. Seven novel mutations were identified in PHKA2 (p.I337X, p.P498L, p.P869R, p.Y116_T120dup, p.R1070del, p.R916W and p.M113I), two in PHKG2 (p.L144P and p.H48QfsX5) and two in PHKB (p.Y419X and c.2336+965A>C). There was a severe phenotype in patients with PHKG2 mutations, a mild phenotype with patients PHKB mutations and a broad spectrum associated with PHKA2 mutations. Molecular analysis allows accurate diagnosis where enzymology is uninformative and identifies the pattern of inheritance permitting counselling and family studies.

Glycogen storage diseases: New perspectives

Glycogen storage diseases (GSD) are inherited metabolic disorders of glycogen metabolism. Different hormones, including insulin, glucagon, and cortisol regulate the relationship of glycolysis, gluconeogenesis and glycogen synthesis. The overall GSD incidence is estimated 1 case per 20000-43000 live births. There are over 12 types and they are classified based on the enzyme deficiency and the affected tissue. Disorders of glycogen degradation may affect primarily the liver, the muscle, or both. Type Ia involves the liver, kidney and intestine (and Ib also leukocytes), and the clinical manifestations are hepatomegaly, failure to thrive, hypoglycemia, hyperlactatemia, hyperuricemia and hyperlipidemia. Type IIIa involves both the liver and muscle, and IIIb solely the liver. The liver symptoms generally improve with age. Type IV usually presents in the first year of life, with hepatomegaly and growth retardation. The disease in general is progressive to cirrhosis. Type VI and IX are a heterogeneous group of diseases caused by a deficiency of the liver phosphorylase and phosphorylase kinase system. There is no hyperuricemia or hyperlactatemia. Type XI is characterized by hepatic glycogenosis and renal Fanconi syndrome. Type II is a prototype of inborn lysosomal storage diseases and involves many organs but primarily the muscle. Types V and VII involve only the muscle.

Effects of Scutellariae Radix on gene expression in HEK 293 cells using cDNA microarray

The aim of the present study was to elucidate the molecular mechanisms underlying the anti-inflammatory effect of Scutellaria Radix (SR). The complementary DNA (cDNA) microarray method was used to survey the effects of SR on the changes of gene expression profile in HEK293 cells. Based on differential expression, 66 genes were selected for further analysis from 9,600 candidate genes in the microarray; 23 genes were validated by RT-PCR. The broad spectrum of the differentially expressed genes, including those associated with inflammation, immune response, energy metabolism, as well as others, such as ISGF3G, IL6ST, CD98, ATP5G2, PHKG2, YB-1 and SLC7A4, indicate overall cellular response to SR treatment. Our results suggest that the anti-inflammatory effect of SR may be related to IL6ST down-expression, and over-expression of CD98. Moreover, SR-related improvement in immune response may be related to the ISGF3G over-expression.

Differential expression pattern of the novel serine/threonine kinase, STK33, in mice and men

Serine/threonine kinase 33 (STK33/Stk33) is a recently discovered gene whose inferred amino acid sequence translation displays characters typical for a calcium/calmodulin dependent kinase (CAMK). In this study we analysed the STK33/Stk33 RNA and protein distribution and the localization of the protein. The STK33/Stk33 expression pattern resembles those of some related members of the CAMK group. STK33/Stk33 displays a nonubiquitous and, in most tissues, low level of expression. It is highly expressed in testis, particularly in cells from the spermatogenic epithelia. Moreover, significant expression is detected in lung epithelia, alveolar macrophages, horizontal cells in the retina and in embryonic organs such as heart, brain and spinal cord. A possible role of STK33/Stk33 in spermatogenesis and organ ontogenesis is discussed.

Myopathy and phosphorylase kinase deficiency caused by a mutation in thePHKA1 gene

Phosphorylase kinase (PhK) deficiency is the underlying cause of variable clinical symptoms depending on the tissues involved. Until today, only a few cases of myopathy associated with muscle PhK deficiency caused by a mutation in the gene encoding the alpha subunit of phosphorylase kinase (PHKA1) have been reported. We describe a male patient with myopathy and absent muscle PhK activity caused by a frameshift mutation in the gene encoding the alpha subunit of PhK on chromosome Xq12-q13.

Severe phenotype of phosphorylase kinase-deficient liver glycogenosis with mutations in the PHKG2 gene

Phosphorylase kinase-deficient liver glycogenosis manifests in infancy with hepatomegaly, growth retardation, and elevated plasma aminotransferases and lipids. It can be caused by mutations in three different genes of phosphorylase kinase subunits: PHKA2, PHKB, and PHKG2. It is usually a benign condition, often with complete resolution of symptoms during puberty. A minority of patients displays a more severe phenotype with symptomatic fasting hypoglycemia and abnormal liver histology that may progress to cirrhosis. Three patients with liver cirrhosis in childhood analyzed previously all had PHKG2 mutations. This suggested that this genotype may generally cause a more severe clinical manifestation, but to date PHKG2 mutations have been identified in only seven patients. Here, we report mutation analysis in three new patients with liver phosphorylase kinase deficiency and recurrent hypoglycemia, liver fibrosis, and lack of glucagon response but no overt cirrhosis. In all three patients, PHKG2 mutations were found (H89fs[insC], E157K, D215N, W300X). Three of these mutations are novel, bringing the total number of distinct human PHKG2 mutations to 11, found in 10 patients. We conclude that liver phosphorylase kinase deficiency with a severe phenotype, with or without cirrhosis, is indeed often caused by PHKG2 mutations. These patients require active measures to maintain normoglycemia (raw cornstarch, nocturnal tube feeding), which may also alleviate growth retardation and the development of abnormal liver histology.

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

Muscle-specific deficiency of phosphorylase kinase (Phk) causes glycogen storage disease, clinically manifesting in exercise intolerance with early fatiguability, pain, cramps and occasionally myoglobinuria. In two patients and in a mouse mutant with muscle Phk deficiency, mutations were previously found in the muscle isoform of the Phk alpha subunit, encoded by the X-chromosomal PHKA1 gene (MIM # 311870). No mutations have been identified in the muscle isoform of the Phk gamma subunit (PHKG1). In the present study, we determined Q1the structure of the PHKG1 gene and characterized its relationship to several pseudogenes. In six patients with adult- or juvenile-onset muscle glycogenosis and low Phk activity, we then searched for mutations in eight candidate genes. The coding sequences of all six genes that contribute to Phk in muscle were analysed: PHKA1, PHKB, PHKG1, CALM1, CALM2 and CALM3. We also analysed the genes of the muscle isoform of glycogen phosphorylase (PYGM), of a muscle-specific regulatory subunit of the AMP-dependent protein kinase (PRKAG3), and the promoter regions of PHKA1, PHKB and PHKG1. Only in one male patient did we find a PHKA1 missense mutation (D299V) that explains the enzyme deficiency. Two patients were heterozygous for single amino-acid replacements in PHKB that are of unclear significance (Q657K and Y770C). No sequence abnormalities were found in the other three patients. If these results can be generalized, only a fraction of cases with muscle glycogenosis and a biochemical diagnosis of low Phk activity are caused by coding, splice-site or promoter mutations in PHKA1, PHKG1 or other Phk subunit genes. Most patients with this diagnosis probably are affected either by elusive mutations of Phk subunit genes or by defects in other, unidentified genes.

Detection of PHKA2 gene mutation in four Japanese patients with hepatic phosphorylase kinase deficiency

We analyzed the PHKA2 gene in four Japanese families with hepatic phosphorylase kinase (PhK) deficiency. Mutational analysis of PHKA2 cDNA was performed by reverse-transcribed polymerase chain reaction (RT-PCR) and direct sequencing, and each mutation was confirmed on the genomic DNA. In boys with low erythrocyte PhK activity (i.e., x-linked liver glycogenosis [XLG] type I), deletion of exon 2 (splice site mutation of 79-1 G > T) or nonsense mutation of Q1169X or R497X was identified. However, missense mutation of R295C was identified in one boy with normal erythrocyte PhK activity (i.e., XLG type II). This mutation was not found in 100 control alleles, and was considered responsible for presentation of the XLG type II phenotype. Excluding Q1169X, all mutations detected in this study represented novel mutations. All mothers were found to be heterozygous carriers of the mutations. Gene analysis was confirmed to represent a useful procedure for diagnosing XLG type II, for which liver biopsy had previously been required to detect hepatic PhK deficiency.

Diagnosis of inherited disorders of liver metabolism

Diagnosis of the metabolic disorder responsible for liver disease can sometimes be straightforward but it can also present a major challenge, particularly if the liver is sufficiently damaged to produce secondary biochemical abnormalities such as galactosuria, hypoglycaemia with hypoketonaemia, or excretion of 3-oxo-delta4 bile acids. It is important to consider the age of the patient, the nature of the liver disease, any extrahepatic clinical features, the imaging and the first-line laboratory tests when prioritizing diagnostic investigations. This article gives some examples of diagnoses made in our unit for patients with liver disease presenting in utero, in the neonatal period, in infancy and the preschool years, and in the school years. The differential diagnoses that should be considered for different clinical presentations are discussed.

A novel serine/threonine kinase gene, STK33, on human chromosome 11p15.3

Human chromosomal region 11p15 is known to be associated with several diseases including predispositions to develop various tumor types. In search of candidate genes, a novel human kinase gene is described, STK33, which codes for a serine/threonine protein kinase. The gene was discovered by comparative genome analysis of human chromosome 11p15.3 and its orthologous region on distal mouse chromosome 7. Human STK33 gene contains 12 exons as has been determined by the comparison to the full-length transcript amplified from human uterus RNA. Transcripts are found in a variety of tissues in at least two alternatively spliced forms as revealed by reverse transcriptase-polymerase chain reaction, cDNA sequencing and expressed sequence tag clustering. Phylogenetic analysis suggests that STK33 may belong to the calcium/calmodulin-dependent protein kinase group, even though, like several other members of the group, it lacks the calcium/calmodulin binding domain [FASEB J. 9 (1995) 576]. STK33 shows a differential expression in a variety of normal and malignant tissues.

Glycogen storage diseases. Phenotypic, genetic, and biochemical characteristics, and therapy

The glycogen storage diseases are caused by inherited deficiencies of enzymes that regulate the synthesis or degradation of glycogen. In the past decade, considerable progress has been made in identifying the precise genetic abnormalities that cause the specific impairments of enzyme function. Likewise, improved understanding of the pathophysiologic derangements resulting from individual enzyme defects has led to the development of effective nutritional therapies for each of these disorders. Meticulous adherence to dietary therapy prevents hypoglycemia, ameliorates the biochemical abnormalities, decreases the size of the liver, and results in normal or nearly normal physical growth and development. Nevertheless, serious long-term complications, including nephropathy that can cause renal failure and hepatic adenomata that can become malignant, are a major concern in GSD-I. In GSD-III, the risk for hypoglycemia diminishes with age, and the liver decreases in size during puberty. Cirrhosis develops in some adult patients, and progressive myopathy and cardiomyopathy occur in patients with absent GDE activity in muscle. It remains unclear whether these complications of glycogen storage disease can be prevented by dietary therapy. Glycogen storage diseases caused by lack of phosphorylase activity are milder disorders with a good prognosis. The liver decreases in size, and biochemical abnormalities disappear by puberty.

The molecular background of glycogen metabolism disorders

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

Specific features of glycogen metabolism in the liver

Although the general pathways of glycogen synthesis and glycogenolysis are identical in all tissues, the enzymes involved are uniquely adapted to the specific role of glycogen in different cell types. In liver, where glycogen is stored as a reserve of glucose for extrahepatic tissues, the glycogen-metabolizing enzymes have properties that enable the liver to act as a sensor of blood glucose and to store or mobilize glycogen according to the peripheral needs. The prime effector of hepatic glycogen deposition is glucose, which blocks glycogenolysis and promotes glycogen synthesis in various ways. Other glycogenic stimuli for the liver are insulin, glucocorticoids, parasympathetic (vagus) nerve impulses and gluconeogenic precursors such as fructose and amino acids. The phosphorolysis of glycogen is mainly mediated by glucagon and by the orthosympathetic neurotransmitters noradrenaline and ATP. Many glycogenolytic stimuli, e.g. adenosine, nucleotides and NO, also act indirectly, via secretion of eicosanoids from non-parenchymal cells. Effectors often initiate glycogenolysis cooperatively through different mechanisms.

Mutations in the liver glycogen phosphorylase gene (PYGL) underlying glycogenosis type VI

Deficiency of glycogen phosphorylase in the liver gives rise to glycogen-storage disease type VI (Hers disease; MIM 232700). We report the identification of the first mutations in PYGL, the gene encoding the liver isoform of glycogen phosphorylase, in three patients with Hers disease. These are two splice-site mutations and two missense mutations. A mutation of the 5' splice-site consensus of intron 14 causes the retention of intron 14 and the utilization of two illegitimate 5' splice sites, whereas a mutation of the 3' splice-site consensus of intron 4 causes the skipping of exon 5. Two missense mutations, N338S and N376K, both cause nonconservative replacements of amino acids that are absolutely conserved even in yeast and bacterial phosphorylases. We also report corrections of the PYGL coding sequence, sequence polymorphisms, and a partial PYGL gene structure with introns in the same positions as in PYGM, the gene of the muscle isoform of phosphorylase. Our findings demonstrate that PYGL mutations cause Hers disease, and they may improve laboratory diagnosis of deficiencies of the liver phosphorylase system.

Liver glycogenosis due to phosphorylase kinase deficiency: PHKG2 gene structure and mutations associated with cirrhosis

Mutations in three different genes of phosphorylase kinase (Phk) subunits, PHKA2, PHKB and PHKG2, can give rise to glycogen storage disease of the liver. The autosomal-recessive, liver-specific variant of Phk deficiency is caused by mutations in the gene encoding the testis/liver isoform of the catalytic gamma subunit, PHKG2. To facilitate mutation detection and to improve our understanding of the molecular evolution of Phk subunit isoforms, we have determined the structure of the human PHKG2 gene. The gene extends over 9.5 kilonucleotides and is divided into 10 exons; positions of introns are highly conserved between PHKG2 and the gene of the muscle isoform of the gamma subunit, PHKG1. The beginning of intron 2 harbors a highly informative GGT/GT microsatellite repeat, the first polymorphic marker in the PHKG2 gene at human chromosome 16p11.2-p12.1. Employing the gene sequence, we have identified homozygous translation-terminating mutations, 277delC and Arg44ter, in the two published cases of liver Phk deficiency who developed cirrhosis in childhood. As liver Phk deficiency is generally a benign condition and progression to cirrhosis is very rare, this finding suggests that PHKG2 mutations are associated with an increased cirrhosis risk.

The crystal structure of a phosphorylase kinase peptide substrate complex: kinase substrate recognition

The structure of a truncated form of the gamma-subunit of phosphorylase kinase (PHKgammat) has been solved in a ternary complex with a non-hydrolysable ATP analogue (adenylyl imidodiphosphate, AMPPNP) and a heptapeptide substrate related in sequence to both the natural substrate and to the optimal peptide substrate. Kinetic characterization of the phosphotransfer reaction confirms the peptide to be a good substrate, and the structure allows identification of key features responsible for its high affinity. Unexpectedly, the substrate peptide forms a short anti-parallel beta-sheet with the kinase activation segment, the region which in other kinases plays an important role in regulation of enzyme activity. This anchoring of the main chain of the substrate peptide at a fixed distance from the gamma-phosphate of ATP explains the selectivity of PHK for serine/threonine over tyrosine as a substrate. The catalytic core of PHK exists as a dimer in crystals of the ternary complex, and the relevance of this phenomenon to its in vivo recognition of dimeric glycogen phosphorylase b is considered.

Autosomal recessive phosphorylase kinase deficiency in liver, caused by mutations in the gene encoding the beta subunit (PHKB)

The association of autosomal recessive phosphorylase kinase deficiency in liver of a 3 1/2-year-old female child with mutations in the gene encoding the common part of the beta subunit of phosphorylase kinase is reported. The proband had a severe deficiency of phosphorylase kinase in liver, while the phosphorylase kinase activity in erythrocytes was only slightly diminished. She had no symptoms of muscle involvement. The complete coding sequences of the liver gamma subunit and of the beta subunit of phosphorylase kinase of the proband were analyzed for the presence of mutations, by either reverse-transcribed PCR or SSCP analysis. Three deviations from the normal sequence were found in the region encoding the common part of the beta subunit of phosphorylase kinase-namely, a 1827G-->A (W609X) transition, a 2309A-->G (Y770C) transition, and a deletion of nucleotides 2896-2911-whereas no mutations were detected in the sequence encoding the liver gamma subunit of phosphorylase kinase. The 1827G-->A mutation and the deletion both result in the formation of early stop codons. Investigation of DNA showed that the deletion is caused by a splice-acceptor site mutation (IVS30(-1),g-->t). Family analysis revealed that the 1827G-->A and IVS30(-1),g-->t substitutions are located on different parental chromosomes and that compound heterozygosity for these mutations segregates with the disease. The 2309A-->G mutation was detected in 2%-3% of the normal population. Thus, it is concluded that the deficiency of phosphorylase kinase in this proband is caused by compound heterozygosity for the 1827G-->A and the IVS30(-1),g-->t mutations and that the 2309A-->G mutation is a polymorphism. This implies that a defect in the sequence encoding the common part of the beta subunit of phosphorylase kinase may present as liver phosphorylase kinase deficiency.