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

doi:10.1016/j.ymgme.2013.12.008

Molecular Genetics and Metabolism

Volume 111, Issue 3, March 2014, Pages 309-313

https://doi.org/10.1016/j.ymgme.2013.12.008 Get rights and content

Highlights

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We report the clinical and laboratory features for 5 patients with PHKG2 mutations.
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A literature review of reported patients with PHKG2 mutations is presented.
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We identified five novel and two previously reported mutations in the PHKG2 gene.
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There is variable clinical severity of symptoms associated with PHKG2 mutations

Abstract

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.

Introduction

Deficiency of liver phosphorylase b kinase (PhK), also known as glycogen storage disease type IX (GSD IX), is one of the most common forms of glycogen storage disease. It accounts for about 25% of cases and has an estimated frequency of 1 in 100,000 [1]. PhK is a key regulatory enzyme in glycogen breakdown and metabolism. In response to physiological conditions, PhK activates glycogen phosphorylase which catalyzes the sequential cleavage of glucosyl units from glycogen to release glucose 1-phosphate [2], [3]. PhK is a heterotetramer that is composed of four copies each of alpha, beta, gamma, and delta (calmodulin) subunits [4]. The gamma subunit contains the catalytic site. Its activity is regulated by the phosphorylation state of the alpha and beta subunits, as well as by calmodulin via calcium levels [4]. Different isoforms of PhK exist due to tissue-specific expression and alternative splicing of different subunit genes. In liver, the alpha, beta, and gamma subunits are encoded by the PHKA2 gene (OMIM# 300798; Xp22.2-p22.1), the PHKB gene (OMIM# 172490; 16q12-q13), and PHKG2 gene (OMIM# 172471; 16p12.1-p11.2) respectively. However, recent studies have suggested that these genes are more widely expressed than previously thought [5]. To date, mutations in each of these genes have been identified in individuals with liver PhK deficiency [1], [6], [7], [8], [9], [10].

Children with liver PhK deficiency typically present in the first two years of life with hepatomegaly, growth retardation, and elevated serum transaminases and plasma triglycerides. Ketotic hypoglycemia and hypotonia may also be present. Available literature suggests that the clinical course is usually mild when compared to other common liver GSDs, and that symptoms typically improve with age [3], [6], [11], [12]. Liver cirrhosis and adenomas, which are common in some other types of liver GSD, have been reported less frequently in individuals with liver PhK deficiency [13]. Over recent years, however, a wide variation in clinical severity among patients with liver PhK deficiency has been recognized and some general correlations between the gene defect and clinical severity have been made [7], [8], [14], [15]. Mutations in the X-linked PHKA2 gene are the most common cause of liver PhK deficiency, accounting for about 75% of cases [7], [8]. Although historically reported to have a relatively benign course, a wide spectrum of clinical severity resulting from mutations in PHKA2 has emerged recently [7], [14], [15], [16], [17], [18]. Mutations in the PHKB gene cause an autosomal recessive form of liver PhK deficiency that, so far, has been associated with mild symptoms [7], [8], [10], [19], [20]. By contrast, mutations in the PHKG2 gene often cause more pronounced biochemical and clinical abnormalities including an increased risk for developing liver cirrhosis in childhood [1], [7], [8], [21], [22], [23], [24]. Here, we report the clinical symptoms, laboratory findings, and PHKG2 gene changes in five individuals with liver PhK deficiency in order to further delineate the variability in clinical phenotype caused by mutations in this gene.

Section snippets

Patients

The patients were recruited under a research protocol in accordance with the Institutional Review Board requirements of Duke University or the University of Florida Health Center. PHKG2 gene sequence analysis was performed by the Duke Molecular Diagnostics Laboratory (patients 1, 2 and 5) or in another clinical diagnostic laboratory (patients 3 and 4). Medical records, including clinic notes and reports from clinical laboratory testing, were reviewed. Laboratory data obtained included blood

Clinical history

A summary of the clinical and laboratory features for the five patients included in this study is shown in Table 1. All patients in our case series had hepatomegaly, elevated serum transaminases, and fasting hypoglycemia, although disease severity was variable. Ketosis was observed in some cases. Over time, with age and treatment, there was general improvement in the biochemical abnormalities for all of the patients. In four of the patients, hepatomegaly has lessened, or even normalized

Discussion

Mutations in PHKG2 have been associated with more severe clinical and biochemical abnormalities. These may include extremely low PhK activity in liver, pronounced tendencies to hypoglycemia, abnormal glucagon response, and increased risk of liver cirrhosis [1], [7], [8], [21], [22], [23], [24], [30]. To date, 17 individuals (16 probands and one sibling) with PhK deficiency, caused by PHKG2 mutations, have been reported in full length papers in the medical literature (Supplementary Table 1).

Acknowledgments

We would like to thank the patients and families who participated in this research study. We are grateful to the Association for Glycogen Storage Diseases, US and the YT and Alice Chen Pediatric Genetics and Genomics Center at Duke for funding this research. Support for this project was also provided by the National Institutes of Health (NIH)/NCATS Clinical and Translational Science Award to the University of Florida UL1 TR000064, Matthew's GSD Type IX Fund, and the Sturtz GSD Research Fund

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With increasing societal development and the concurrent improvement in people's quality of life, meat consumption has gradually changed from a focus on “quantity” to “quality”. Broiler production is increasingly used as a means to improve meat quality by altering various characteristics, especially its genetic factors. However, until now, little has been known about the genetic variants related to meat quality traits in Chinese purebred chicken populations. To better understand these genetic underpinnings, a total of 17 traits related to meat quality and carcass were measured in 325 Chinese Ningdu yellow chickens. We performed DNA sequencing to detect nucleotide mutations, after which we conducted association studies between PHKG1 gene polymorphisms and traits related to meat quality and carcass. Results indicated a large phenotypic variation in meat quality traits. More specifically, the single nucleotide polymorphism (SNP) rs15845448 was significantly associated with drip loss at 24 h (P = 8.04 × 10⁻⁶) and 48 h (P = 5.47 × 10⁻⁶), pH (P = 2.39 × 10⁻³), and meat color L* (P = 9.88 × 10⁻³). Moreover, the SNP rs15845448 reduced 24 h and 48 h drip loss by 3.62 and 5.97%, respectively. However, no significant associations were found between rs15845448 and carcass traits (P > 0.05). Furthermore, a haplotype block containing 2 adjacent SNPs (rs15845448 and rs15845450) was identified. This block displayed 4 distinct haplotypes that had significant association with drip loss at 24 h and 48 h, pH, and meat color L*. Collectively, these results provide new insights into the genetic basis of meat quality in Chinese Ningdu yellow chickens. Moreover, the significance of SNP rs15845448 could be incorporated into the selection programs involving this breed.
Characterization of liver GSD IX γ2 pathophysiology in a novel Phkg2<sup>−/−</sup> mouse model
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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.
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).
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.
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.
Liver histology in children with glycogen storage disorders type VI and IX
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Glycogen storage diseases (GSD) type VI and IX are caused by liver phosphorylase system deficiencies and the two types are clinically indistinguishable.
As the role of liver biopsy is increasingly questioned, we aim to assess its current value in clinical practice.
We retrospectively reviewed children with diagnosis of GSD VI and IX at a paediatric liver centre between 2001 and 2018. Clinical features, molecular analysis and imaging were reviewed. Liver histology was reassessed by a single histopatologist.
Twenty-two cases were identified (9 type VI, 9 IXa, 1 IXb and 3 IXc). Features at presentation were hepatomegaly (95%), deranged AST (81%), short stature (50%) and failure to thrive (4%). Liver biopsy was performed in 19 patients. Fibrosis varied in GSD IXa with METAVIR score between F1-F3 and ISHAK score of F2-F5. METAVIR score was F2-F3 in GSD VI and F3-F4 in GSD IXc. Hepatocyte glycogenation, mild steatosis, lobular inflammatory activity and periportal copper binding protein staining were also demonstrated.
Although GSD VI and IX are considered clinically mild, chronic histological changes of varying severity could be seen in all liver biopsies. Histopathological assessment of the liver involvement is superior to biochemical parameters, but definitive classification requires a mutational analysis.
Benign or not benign? Deep phenotyping of liver Glycogen Storage Disease IX
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Citation Excerpt :
Most liver GSD IX patients present between infancy and two years with hepatomegaly, elevated liver transaminases, elevated triglycerides, motor delay, growth delay, and/or episodes of ketotic hypoglycemia [10]. The clinical presentation of the majority of GSD IX α2 and GSD IX β patients is reported to become milder with age as metabolic demands decrease; however, the symptoms of GSD IX γ2 patients have been shown to persist with age, potentially progressing to cirrhosis, liver failure, hepatocellular carcinoma, and death [4,13–22]. We present the first comprehensive literature review of liver GSD IX in order to characterize the natural history of GSD IX α2, GSD IX γ2 and GSD IX β, and further investigate genotype-phenotype correlations.
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.
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.
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.
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.
Molecular diagnosis of glycogen storage disease type IX using a glycogen storage disease gene panel
2020, European Journal of Medical Genetics
Citation Excerpt :
Less than 30 pathogenic variants of PHKG2 have been described till date (Albash et al., 2014; Bali et al., 2014; Roscher et al., 2014). Several studies have reported that patients with GSD IXc often show evidence of fibrosis, and even cirrhosis (Albash et al., 2014; Bali et al., 2014; Burwinkel et al., 2003; Tsilianidis et al., 2013). Furthermore, several patients with PHKG2 variants have been reported to have liver adenomas (Bali et al., 2014; Burwinkel et al., 1998, 2003).
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.
Disorders of Carbohydrate Metabolism
2020, Emery and Rimoin’s Principles and Practice of Medical Genetics and Genomics: Metabolic Disorders
Inborn errors of carbohydrate metabolism covered in this chapter include disaccharidase deficiencies, disorders of monosaccharide metabolism, glycogen storage diseases (GSDs), and gluconeogenic disorders. This chapter focuses mainly on clinical aspects, genetics, and current treatments for these disorders. The defective digestion of dietary disaccharides lactose and sucrose is due to deficiencies of lactase and sucrase–isomaltase, respectively. Disorders of monosaccharide metabolism include defects in transport (glucose–galactose malabsorption), enzymatic defects in galactose, fructose and pentose metabolism. Disorders of glycogen metabolism include over 12 glycogenoses. Glycogen storage diseases, a major category of glycogenoses, are categorized by the type of tissue involved: liver, muscle, and/or cardiac. Gluconeogenic disorders entail deficiencies of fructose-1,6-diphosphatase, pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and pyruvate dehydrogenase complex.

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¹: These authors contributed equally.

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

Highlights

Abstract

Introduction

Section snippets

Patients

Clinical history

Discussion

Acknowledgments

Mol. Genet. Metab.

Mol. Genet. Metab.

Mol. Genet. Metab.

Endocrinol Metab. Clin. North. Am.

Mol. Genet. Metab.

Mol. Genet. Metab.

Am. J. Hum. Genet.

Biochem. Biophys. Res. Commun.

Biochim. Biophys. Acta

Genet. Med.

Mol. Genet. Metab.

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

Nat. Genet.

The family of glycogen phosphorylases: structure and function

Crit. Rev. Biochem. Mol. Biol.

Phosphorylase kinase: the complexity of its regulation is reflected in the complexity of its structure

Front. Biosci.

Phosphorylase kinase deficiency

Mutations in the phosphorylase kinase gene PHKA2 are responsible for X-linked liver glycogen storage disease

Hum. Mol. Genet.

Autosomal glycogenosis of liver and muscle due to phosphorylase kinase deficiency is caused by mutations in the phosphorylase kinase beta subunit (PHKB)

Hum. Mol. Genet.