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

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.

Novel mutations in the PHKB gene in an iranian girl with severe liver involvement and glycogen storage disease type IX: a case report and review of literature

BACKGROUND

Glycogen storage disease (GSD) type IXb is one of the rare variants of GSDs. It is a genetically heterogeneous metabolic disorder due to deficient hepatic phosphorylase kinase activity. Diagnosis of GSD can be difficult because of overlapping manifestations. Mutation analysis of the genes related to each type of GSD is supposed to be problem-solving, however, the presence of novel mutations can be confusing. In this case report, we will describe our experience with a young girl with the diagnosis of GSD and two novel mutations related to GSD type IXb.

CASE PRESENTATION

A 3-year- old girl presented with short stature, hepatomegaly, and liver cirrhosis. No specific diagnosis was made based on laboratory data, so liver biopsy and targeted-gene sequencing (TGS) were performed to find out the specific molecular basis of her disease. It was confirmed that the patient carries two novel variants in the PHKB gene. The variant in the PHKB gene was classified as pathogenic.

CONCLUSIONS

This is the first reported case of a dual molecular mutation of glycogen storage disease type IXb in the same patient. Two novel variants in PHKB were identified and one of them was a pathogenic split-site mutation. In conclusion, for the first time, identification of the novel variants in this patient expands the molecular and the phenotype basis of dual variants in GSD-IXb.

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.

Family-specific, novel, deleterious germline variants provide a rich resource to identify genetic predispositions for BRCAx familial breast cancer

Background

Genetic predisposition is the primary risk factor for familial breast cancer. For the majority of familial breast cancer, however, the genetic predispositions remain unknown. All newly identified predispositions occur rarely in disease population, and the unknown genetic predispositions are estimated to reach up to total thousands. Family unit is the basic structure of genetics. Because it is an autosomal dominant disease, individuals with a history of familial breast cancer must carry the same genetic predisposition across generations. Therefore, focusing on the cases in lineages of familial breast cancer, rather than pooled cases in disease population, is expected to provide high probability to identify the genetic predisposition for each family.

Methods

In this study, we tested genetic predispositions by analyzing the family-specific variants in familial breast cancer. Using exome sequencing, we analyzed three families and 22 probands with BRCAx (BRCA-negative) familial breast cancer.

Results

We observed the presence of family-specific, novel, deleterious germline variants in each family. Of the germline variants identified, many were shared between the disease-affected family members of the same family but not found in different families, which have their own specific variants. Certain variants are putative deleterious genetic predispositions damaging functionally important genes involved in DNA replication and damaging repair, tumor suppression, signal transduction, and phosphorylation.

Conclusions

Our study demonstrates that the predispositions for many BRCAx familial breast cancer families can lie in each disease family. The application of a family-focused approach has the potential to detect many new predispositions.

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.

Liver glycogen storage diseases due to phosphorylase system deficiencies: diagnosis thanks to non invasive blood enzymatic and molecular studies

Glycogen storage disease (GSD) due to a deficient hepatic phosphorylase system defines a genetically heterogeneous group of disorders that mainly manifests in children. We investigated 45 unrelated children in whom a liver GSD VI or IX was suspected on the basis of clinical symptoms including hepatomegaly, increased serum transaminases, postprandial lactatemia and/or mild fasting hypoglycemia. Liver phosphorylase and phosphorylase b kinase activities studied in peripheral blood cells allowed to suspect diagnosis in 37 cases but was uninformative in 5. Sequencing of liver phosphorylase genes was useful to establish an accurate diagnosis. Causative mutations were found either in the PYGL (11 patients), PHKA2 (26 patients), PHKG2 (three patients) or in the PHKB (three patients) genes. Eleven novel disease causative mutations, five missense (p.N188K, p.D228Y, p.P382L, p.R491H, p.L500R) and six truncating mutations (c.501_502ins361pb, c.528+2T>C, c.856-29_c.1518+614del, c.1620+1G>C, p.E703del and c.2313-1G>T) were identified in the PYGL gene. Seventeen novel disease causative mutations, ten missense (p.A42P, p.Q95R, p.G131D, p.G131V, p.Q134R, p.G187R, p.G300V, p.G300A, p.C326Y, p.W820G) and seven truncating (c.537+5G>A, p.G396DfsX28, p.Q404X, p.N653X, p.L855PfsX87, and two large deletions) were identified in the PHKA2 gene. Four novel truncating mutations (p.R168X, p.Q287X, p.I268PfsX12 and c.272-1G>C) were identified in the PHKG2 gene and three (c.573_577del, p.R364X, c.2427+3A>G) in the PHKB gene. Patients with PHKG2 mutations evolved towards cirrhosis. Molecular analysis of GSD VI or IX genes allows to confirm diagnosis suspected on the basis of enzymatic analysis and to establish diagnosis and avoid liver biopsy when enzymatic studies are not informative in blood cells.

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.

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.

Glycogen storage myopathies

The glycogen storage myopathies are caused by enzyme defects in the glycogenolytic or in the glycolytic pathway affecting skeletal muscle alone or in conjunction with other tissues. The authors review recent findings in this area, including a new entity, aldolase deficiency, and the wealth of molecular genetic data that are rapidly accumulating. Despite this progress, genotype-phenotyp3 correlations are still murky in most glycogen storage myopathies.

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.

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.