Glycogen storage diseases (GSDs) are a heterogeneous group of inherited storage disorders characterized by the accumulation of glycogen in various tissues, caused by deficiencies of enzymes or transport proteins involved in glycogen metabolism. GSD IX is caused by deficiency of the phosphorylase b kinase (PhK) enzyme, which can be present in liver, liver and muscle, or muscle only in rare cases (1). Patients with hepatic PhK deficiency have excessive amounts of glycogen in the liver and typically present in early childhood with short stature, hepatomegaly, and, in some cases, ketotic hypoglycemia. Elevated liver transaminases and triglycerides are also common. Changes in the PHKA2 gene, which resides on the X chromosome, account for approximately 75% of cases. The X-linked subtype of liver PhK deficiency has variable clinical severity, but, to our knowledge, liver cirrhosis has not been reported in these patients (1,2). We report a case of X-linked liver GSD IX with liver cirrhosis in which diagnosis could only be confirmed by DNA analyses.
The patient, a white boy, was born at term by normal spontaneous vaginal delivery. The birth weight was 7 lb 8 oz (3.4 kg). The prenatal or postnatal history was unremarkable and the developmental milestones were normal. He was breast-fed for the first 6 months of life, after which solid food was introduced. At 18 months, he presented to the family's pediatrician with massive hepatomegaly and was referred to a pediatric gastroenterologist. The parents reported that the child had a tendency to wake up in the early mornings irritable, often sweaty, and wanting to be fed. Failure to feed him promptly resulted in emesis. At the initial gastrointestinal clinical evaluation, the patient was an active, healthy-looking toddler with an obvious protuberant abdomen. His weight, height, and head circumference were 90th, 50th, and 75th to 90th percentile, respectively. Apart from nasal congestion and a dry cough, his respiratory and cardiovascular systems were normal. The liver measured 10 cm below the right costal margin along the right nipple line, and was firm and not tender. The spleen was not palpable, and there was no ascites. Muscle tone and power, and deep tendon reflexes were normal. It was suspected that the child's early morning behavior was due to hypoglycemia; however, fasting (8 hour) blood sugar level was 81 mg/dL (normal 70–105). Early morning cornstarch feeding did not help.
The patient's mother reported having 2 male maternal relatives with hepatomegaly who are brothers and are her mother's first cousins. Review of the medical records revealed that a suspected diagnosis of hepatic PhK deficiency had been made in one of the brothers in 1978 when he was 34 years old. He is alive and reportedly doing well.
Complete blood count was normal, but there was lymphocytosis probably due to an ongoing viral infection. Basic metabolic panel, serum total protein, and albumin were also normal. Random blood sugar was 150 mg/dL (normal 65–110). Wilson disease, α1-antitrypsin deficiency, and iron storage disease were ruled out by appropriate blood tests. Serological tests showed no evidence of infection with hepatitis virus A, B, or C, Epstein-Barr virus, or Cytomegalovirus. Results of liver function tests, serum triglyceride, cholesterol, lactate, prothrombin time, partial thromboplastin time, and international normalized ratio are shown in Table 1. Results of enzyme studies for evaluation for GSD are shown in Table 2.
IMAGING STUDIES AND EKG
Abdominal sonogram at 18 months of age confirmed profound hepatomegaly with no focal masses and normal spleen size. Repeat abdominal ultrasound at age 71 months showed persisting significant hepatomegaly and splenomegaly, but without ascites. The liver showed heterogeneous echotexture with no focal hepatic or splenic lesion. Electrocardiogram was normal at 21 months of age.
Initial percutaneous liver biopsy done at 18 months of age showed stage 3 portal-portal fibrosis confirmed by trichrome stain and areas greatly suggestive of micronodular cirrhosis. There were only minimal chronic inflammatory cells. There were no hepatocyte periodic-acid Schiff-positive inclusions, but there was periodic-acid Schiff-positive diastase-sensitive material in the cytoplasm consistent with glycogen. Electron microscopy revealed abundant hepatocellular glycogen with cytoplasmic lipid. The glycogen was present in β particles and not membrane bound or fibrillar. An open liver biopsy was performed after 3 years when the patient was 56 months old. Histopathology again revealed micronodular cirrhosis, abundant glycogen, no steatosis, necrosis, inflammation, or cholestasis. The electron micrographs confirmed the findings in the initial biopsy.
DNA sequence analyses of the PHKA2 gene revealed a previously unreported missense change, p.Arg298Pro (c.893G>C). Sequencing of the coding exons and splice junctions of the PHKG2 gene revealed no pathogenic changes. Targeted DNA analyses for common mutations causing GSD Ia, GSD Ib, and GSD IIIb were normal.
We report a case with X-linked liver PhK deficiency that highlights some interesting findings. First, the diagnosis was not revealed by enzyme assay and required molecular testing. Our patient had normal PhK activity in erythrocytes and liver. He was later found to have a previously unreported amino acid change (p.Arg298Pro) in the PHKA2 gene, which is predicted to be disease causing by the PolyPhen-2 algorithm (3), and is not listed as a common genetic variant in the Single Nucleotide Polymorphism database (http://www.ncbi.nlm.nih.gov/projects/SNP). In addition, the finding of a change in an X-linked gene is consistent with the family history. Patients with X-linked liver PhK deficiency, also known as X-linked liver glycogenosis (XLG), can be divided into 2 biochemical groups: XLG1, in which the enzyme deficiency is detectable in erythrocytes and liver tissue in vitro, and XLG2, in which PhK activity is normal or elevated in erythrocytes and variable (deficient–normal) in liver (4). Based on our finding of normal enzyme activity in vitro, our case is classified with the XLG2 biochemical phenotype. The correlation between underlying PHKA2 mutations and XLG2 is not well understood; however, other missense mutations clustering near p.Arg298Pro within the predicted glycoside-binding site of PHKA2 have also been associated with XLG2 (5). Second, our patient had micronodular cirrhosis on liver biopsies performed at the ages of 18 and 56 months. Although liver cirrhosis has been reported in patients with liver PhK deficiency, it has not been associated with PHKA2 gene mutations to our knowledge. All of the patients with PhK deficiency and liver cirrhosis who have been characterized molecularly have had changes in the PHKG2 gene (6). No pathogenic alterations in this gene were found in our patient. The finding of splenomegaly and hepatomegaly in a follow-up abdominal sonogram is bothersome because it may herald the onset of portal hypertension. There was no laboratory or histopathological evidence of other causes of chronic hepatitis in childhood. Finally, growth retardation, albeit with catch-up growth pattern, has been considered one of the characteristics of GSD type IX (7); however, our patient's growth has been normal and he did not have documented hypoglycemia in infancy. This is consistent with the extreme clinical variability in this condition.
In conclusion, the present case demonstrates that normal liver PhK activity in vitro does not rule out X-linked liver PhK deficiency in vivo; genetic and molecular analyses may be necessary to confirm the diagnosis. The present case also represents the first known reported case of liver PhK deficiency with an PHKA2 mutation and liver cirrhosis.
We thank the Association for Glycogen Storage Disease for research funding; the Amarillo Pathology Associates and Milton Finegold, MD, from the Texas Children's Hospital for the light and electron microscopic examinations of liver tissues; Catherine Rehder, PhD, from the Molecular Diagnostics Laboratory, Department of Pathology, and Keri Boyette, MS, from the Division of Medical Genetics, Department of Pediatrics, both at Duke University, Durham, NC, for technical assistance in performing the DNA sequence analyses.
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