Glycogen Storage Disease Ii

A number sign (#) is used with this entry because glycogen storage disease II (GSD2) is caused by homozygous or compound heterozygous mutation in the GAA gene (606800), which encodes acid alpha-1,4-glucosidase, also known as acid maltase, on chromosome 17q25.

Description

Glycogen storage disease II, an autosomal recessive disorder, is the prototypic lysosomal storage disease. In the classic infantile form (Pompe disease), cardiomyopathy and muscular hypotonia are the cardinal features; in the juvenile and adult forms, involvement of skeletal muscles dominates the clinical picture Matsuishi et al. (1984).

Clinical Features

Infantile Onset (Pompe Disease)

In classic cases of Pompe disease, affected children are prostrate and markedly hypotonic with large hearts. The tongue may be enlarged. Although the enzyme is deficient in all tissues, muscle weakness and heart involvement are the most common features. The liver is rarely enlarged, except as a result of heart failure, and hypoglycemia and acidosis do not occur as they do in glycogen storage disease I (232200). Death usually occurs in the first year of life in the classic form of the disorder and cardiac involvement is striking. Indeed, Pompe (1932) reported this condition as 'idiopathic hypertrophy of the heart,' and 'cardiomegalia glycogenica' is a synonym.

Slonim et al. (2000) proposed a second, milder subtype of the infantile form. They reported 12 infants who showed less severe cardiomyopathy, absence of left ventricular outflow obstruction, and traces (less than 5%) of residual acid maltase activity; 9 of the 12 had longer survival with assisted ventilation and intubation.

Smith et al. (1967) reported a boy with a myotonic form of disease and survival to the age of almost 11 years. The heart was not significantly involved. Alpha-1,4-glucosidase was absent from liver and muscle. There were heavy glycogen deposits and an anomalous polysaccharide with short outer chains was identified. Smith et al. (1966) reported a similar case in a boy who survived to the age of 4.5 years. Zellweger et al. (1965) described brothers, aged 15 and 4.5 years, with minimal manifestations limited to skeletal muscle. A deficiency of muscle alpha-1,4-glucosidase was demonstrated. Muscle showed abnormal accumulations of glycogen. A maternal uncle may have also been affected.

On analysis of questionnaire data from 255 children and adults with Pompe disease, Hagemans et al. (2005) found that disease severity, including wheelchair use and use of respiratory support, increased with disease duration, but was not related to the age of the patients. However, there was a subset of patients under age 15 years with a more severe disease, requiring increased use of ventilatory support, wheelchair support, and nutritional support. All within this patient subgroup had onset of symptoms within the first 2 years of life.

Forsha et al. (2011) studied the prevalence of cardiovascular abnormalities and the efficacy and safety of enzyme replacement therapy in patients with late-onset Pompe disease. Ninety patients were randomized 2:1 to enzyme replacement therapy or placebo in a double-blinded protocol. ECGs and echocardiograms were obtained at baseline and scheduled intervals during the 78-week study period. Eighty-seven patients were included. Median age was 44 years, and half were men. At baseline, a short PR interval was present in 10%, 7% had decreased left ventricular systolic function, and 5% had elevated left ventricular mass on echocardiogram (all in the mild range). There was no change in cardiovascular status associated with enzyme replacement therapy. No significant safety concerns were identified. Although some patients with late-onset Pompe disease had abnormalities on ECG or echocardiogram, those classically seen in infantile Pompe disease, such as significant ventricular hypertrophy, were not noted.

Banugaria et al. (2011) retrospectively analyzed 34 infants with Pompe disease; 11 were cross-reactive immunologic material (CRIM)-negative patients; 9 were high-titer CRIM-positive patients; and 14 were low-titer CRIM-positive patients. Clinical outcome measures included survival, ventilator-free survival, left ventricular mass index, the Alberta Infant Motor Scale score, and urine Glc4 levels. Clinical outcomes in the high-titer CRIM-positive group were poor across all areas evaluated relative to the low-titer CRIM-positive group. For the CRIM-negative and high-titer CRIM-positive groups, no statistically significant differences were observed for any outcome measures, and both patient groups did poorly. Banugaria et al. (2011) concluded that, irrespective of CRIM material status, patients with infantile Pompe disease and high sustained antibody titer have an attenuated therapeutic response to enzyme replacement therapy. Banugaria et al. (2011) concluded that with the advent of immunomodulation therapies, identification of patients at risk for developing high sustained antibody titer is critical.

Prater et al. (2012) described the phenotype of long-term survivors with infantile Pompe disease. Inclusion criteria included ventilator-free status and age less than 6 months at treatment initiation, as well as survival to age greater than 5 years. Eleven of 17 patients met these study criteria; all were CRIM-positive, alive, and invasive ventilator-free at most recent assessment, with a median age of 8.0 years (range 5.4-12.0 years). All had marked improvements in cardiac parameters. Commonly present were gross motor weakness, motor speech deficits, sensorineural and/or conductive hearing loss, osteopenia, gastroesophageal reflux, and dysphagia with aspiration risk. Seven of 11 patients were independently ambulatory and 4 required the use of assistive ambulatory devices. All long-term survivors had low or undetectable anti-alglucosidase alfa antibody titers. Prater et al. (2012) concluded that long-term survivors exhibited sustained improvements in cardiac parameters and gross motor function. Residual muscle weakness, hearing loss, risk for arrhythmias, hypernasal speech, dysphagia with risk for aspiration, and osteopenia were commonly observed findings.

Adult Onset

Hudgson et al. (1968) reported the case of a Portuguese girl who died at age 19 and that of a living 44-year-old housewife. Other experiences suggesting the existence of more than one type of glycogenosis II were reported by Swaiman et al. (1968).

Adult-onset acid maltase deficiency may simulate limb-girdle dystrophy and the only clinical clue may be early involvement of the diaphragm (Engel, 1970; Newsom-Davis et al., 1976; Sivak et al., 1981). Trend et al. (1985) reported 4 of 5 patients who presented with acute respiratory insufficiency or chronic nocturnal ventilatory insufficiency. They reported that long-term domiciliary ventilatory support using a rocking bed or intermittent positive pressure respirations with a tracheostomy permitted patients to return to work. Molho et al. (1987) reported the cases of monozygotic twin brothers who at age 50 developed bilateral paralysis of the diaphragm. Severe dyspnea in the supine position necessitated mechanical ventilation by pneumobelts during the night. The possibility of adult acid maltase deficiency should be considered in these cases.

Francesconi and Auff (1982) described Wolff-Parkinson-White syndrome (194200) and second-degree atrioventricular block in a patient with the adult form of glycogenosis II. Byrne et al. (1986) stated that 'cardiac involvement has only been reported in 1 patient with noninfantile acid maltase deficiency.'

Makos et al. (1987) described 3 brothers with alpha-glucosidase deficiency, each of whom developed a fusiform basilar artery aneurysm as young adults, which was complicated by fatal rupture in 2 of them and by a cerebellar infarction in the third. Postmortem examination demonstrated severe vacuolization of skeletal muscle, liver, and vascular smooth muscle with accumulation of glycogen. In the surviving brother, similar glycogen deposition was demonstrated in the smooth muscle of the superficial temporal artery. Glycogen deposition in vascular smooth muscle had been demonstrated previously in this disorder but had not been considered clinically significant. One of the brothers had onset of weakness at age 19, demonstration at age 27 of basilar artery aneurysm by cerebral angiography, which was performed because of throbbing, occipital headaches, and, at age 32, cerebellar infarction. He had 2 sons who were normal. The patients in this family had normal alpha-glucosidase activity in leukocytes but barely detectable alpha-glucosidase in muscle homogenates at acid pH. Kretzschmar et al. (1990) described a 40-year-old male with adult acid maltase deficiency who, in addition to involvement of the liver and skeletal muscles, had extensive involvement of large and small cerebral arteries with aneurysm formation.

Chancellor et al. (1991) described the case of a 68-year-old man who first developed difficulty walking at the age of 65 and for several months had experienced urinary incontinence with exercise. Chancellor et al. (1991) pointed out that many patients with detrusor instability remain asymptomatic, probably because they augment urethral closure pressure by increasing striatal muscle activity in the sphincter mechanism. They postulated that the inability to withstand increases in detrusor pressure only occurred because of striated pelvic floor muscle fatigue associated with exercise. Alternatively, there may have been a neurogenic component in the muscle weakness because of involvement of spinal motor neurons.

Laforet et al. (2000) reported the clinical features of 21 unrelated patients with juvenile- or adult-onset GAA deficiency. The mean age at onset of obvious muscle complaints was 36 years, although most patients (16 of 21) reported mild muscular symptoms since childhood, including scapular winging, scoliosis, and difficulty running. Most patients had predominant involvement of pelvic girdle muscles without significant distal leg involvement. Eight (40%) patients had severe respiratory muscle involvement, which was not correlated with the severity of limb muscle weakness. Biochemical studies showed residual GAA activity in leukocytes ranging from 0 to 17% of normal values; there was no correlation between leukocyte GAA activity and clinical severity. Genetic analysis identified the common -13T-G transversion in the GAA gene (606800.0006) in 17 patients (16 compound heterozygotes and 1 homozygote). There were no genotype/phenotype correlations.

Anneser et al. (2005) reported a 30-year-old woman with alpha-glucosidase deficiency confirmed by mutation in the GAA gene (606800.0016; 606800.0017). She presented with a 4-year history of progressive proximal muscle weakness, and examination showed marked vacuolar myopathy, marked reduction in GAA enzyme activity, increased serum creatine kinase, and increased transaminase levels. After diagnosis, she experienced 3 stroke-like episodes within 3 months. Brain CT showed dilatative angiopathy of the intracerebral vessels, especially of the basilar artery, with calcifications of the carotid and medial cerebral arteries. MRI showed several white matter lesions. She had no other additional risk factors for atherosclerosis. Anneser et al. (2005) suggested that similar extramuscular vascular changes may be the most relevant prognostic factor for adult patients with slowly progressive Pompe disease.

Groen et al. (2006) found that 4 (33%) of 12 patients with adult-onset GSD II had ptosis, which was the presenting feature in 3 patients. Six (50%) of the 12 had measurable evidence of decreased levator palpebral muscle function. The prevalence of ptosis was significantly higher in patients compared to the general population, suggesting that it may be considered a clinical feature of adult-onset GSD II.

Genotype/Phenotype Correlations

Koster et al. (1978) and Loonen et al. (1981) described a grandfather with acid maltase deficiency leading to difficulty climbing stairs after age 52, and a granddaughter with typical Pompe disease leading to death at 16 weeks. The muscle of both subjects showed residual activity. It seems likely that the grandfather was a genetic compound. In this same family, Hoefsloot et al. (1990) showed that 3 sibs were homozygous for an allele that caused complete deficiency of acid alpha-glucosidase; these patients had a severe infantile form of the disease. The eldest patient in the family, with very mild clinical symptoms, was shown to be a compound heterozygote for this allele and for a second allele characterized by a reduced net production of catalytically active acid alpha-glucosidase, resulting in partial enzyme deficiency. The mutant alleles were segregated in human-mouse somatic cell hybrids to investigate their individual function.

Danon et al. (1986) also reported instances of the probable genetic compound state. Nishimoto et al. (1988) described a family in which the proband, aged 15, had the juvenile muscular dystrophy form of glycogenosis type II, whereas both parents and 2 sisters had pseudodeficiency of acid alpha-glucosidase. It was almost impossible to distinguish the homozygote from the heterozygous members by lymphocyte assays alone. Both parents may have been compound heterozygotes for the pseudodeficiency allele and the allele for the juvenile form.

Allelic heterogeneity was demonstrated further by the patient reported by Suzuki et al. (1988): a male developed cardiomyopathy at 12 years of age and died of heart failure at age 15 years without any sign of skeletal muscle involvement, either clinically or histologically. A Km mutant of acid alpha-glucosidase was demonstrated. Iancu et al. (1988) described an affected 12-year-old boy who presented with a right lumbar mass which appeared to represent local pseudohypertrophy.

Pathogenesis

The defect in type II glycogen storage disease involves acid alpha-1,4-glucosidase (acid maltase), a lysosomal enzyme. Whereas the glycogen is distributed rather uniformly in the cytoplasm in the other glycogen storage diseases (e.g., GSD I; 232200), it is enclosed in lysosomal membranes in this form.

In a case of infantile acid alpha-glucosidase deficiency, Beratis et al. (1978) concluded that the defect was a structural mutation causing synthesis of a catalytically inactive, cross-reacting material (CRM)-positive, enzyme protein. On the other hand, the mutation in the adult form causes a reduction in the amount of enzyme protein. Of 9 fibroblast lines from patients with the infantile form of acid alpha-glucosidase deficiency, Beratis et al. (1983) found that 8 were CRM-negative and 1 was CRM-positive. No difference in apparent enzyme activity was detected between the 2 forms. In 2 fibroblast strains from the adult form, rocket immunoelectrophoresis showed a reduction in the amount of enzyme protein that was directly proportional to the reduction in enzyme activity. In another 'adult' fibroblast line, enzyme activity was in the same range as in the infantile form and no CRM was identified. Fibroblasts with phenotype 2 of acid alpha-glucosidase, considered a normal variant, showed reduction both in the amount of enzyme protein and in the ability to cleave glycogen; catalytic activity for maltose was normal, however.

Reuser et al. (1978) studied fibroblasts from the infantile, juvenile, and adult forms of acid alpha-glucosidase deficiency. An inverse correlation was found between the severity of clinical manifestations and the level of residual enzyme activity in fibroblasts. The kinetic and electrophoretic properties of residual enzyme in fibroblasts from adult patients were identical to those from controls. The mutation may, therefore, affect the production or degradation of enzyme rather than its catalytic function. Complementation studies by fusion of fibroblasts from different types yielded no sign of nonallelism of the several forms.

Reuser et al. (1987) investigated the nature of the acid alpha-glucosidase deficiency in cultured fibroblasts from 30 patients. Deficiency of catalytically active mature enzyme in lysosomes was common to all clinical phenotypes but, in most cases, was more profound in early-onset than in late-onset forms of the disease. The role of secondary factors cannot be excluded, however, because 3 adult patients were found with very low activity and little enzyme in the lysosomes.

Diagnosis

Angelini et al. (1972) showed that the adult form of the disease can be diagnosed in cultured skin fibroblasts. Askanas et al. (1976) established muscle tissue cultures from a 34-year-old patient with the adult-onset myopathy. Morphologically and biochemically, the newly grown fibers of cultured muscle showed the same changes as did biopsied muscle.

Ausems et al. (1999) found that creatine kinase (CK) elevation is a sensitive marker of GSD II. CK levels were elevated in all 18 patients in their cohort and in 94.3% of GSD II patients reported in the literature. They proposed a diagnostic protocol for adult-onset GSD II. In patients presenting with a slowly progressive proximal muscle weakness or with respiratory insufficiency, they recommended measurement of serum levels of CK, followed by measurement of acid alpha-glucosidase activity in leukocytes, using glycogen as a substrate. To rule out the pseudodeficiency state seen in carriers of the GAA2 allele, they recommended that patients with depressed leukocyte activity have a repeat assay in cultured fibroblasts using artificial substrate.

Kallwass et al. (2007) reported a simple and reliable method to measure alpha-glucosidase activity in dried blood spots using Acarbose, a highly selective alpha-glucosidase inhibitor, to eliminate isoenzyme interference. The authors demonstrated that this method efficiently detected late-onset Pompe patients who were frequently misdiagnosed by conventional methods due to residual GAA activity in other tissue types.

Bembi et al. (2008) provided a detailed guide to the diagnosis of GSD II, with emphasis on the importance of early recognition of clinical manifestations. Diagnosis is confirmed by biochemical assays showing absent or decreased GAA enzyme and enzyme activity in peripheral blood cells, skin fibroblasts, or muscle biopsy. Affected adults usually present with skeletal muscle weakness and cramps and may often have respiratory failure. Progression is usually slow. Muscle imaging may be useful to assess the extent of involvement in older patients. Affected infants can present with hypertrophic cardiomyopathy in the first months of life and show rapid progression, often leading to death within the first 2 years. Patients with juvenile onset have a more attenuated course compared to infantile onset, and do not have cardiomyopathy. Other features include generalized hypotonia and hepatomegaly.

Clinical Management

Slonim et al. (1983) and Margolis and Hill (1986) concluded that a high-protein diet is effective therapy in adults with acid maltase deficiency. Striking improvement in respiratory function was observed. The effect was serendipitously discovered when a high-protein diet for weight reduction was given. Correction of obesity was not thought to be the exclusive or even the major mechanism of the respiratory improvement. Isaacs et al. (1986) observed benefit from a high-protein, low-carbohydrate diet in a patient with adult acid maltase deficiency.

Amalfitano et al. (2001) reported the results of a phase I/II open-label single-dose study of recombinant human alpha-glucosidase infused intravenously twice weekly in 3 infants with infantile GSD II. The results of more than 250 infusions showed that recombinant human GAA was generally well tolerated. Steady decreases in heart size and maintenance of normal cardiac function for more than 1 year were observed in all 3 infants. These infants lived well past the critical age of 1 year (16, 18, and 22 months old at the time of this study) and continued to have normal cardiac function. Improvements of skeletal muscle functions were also noted; 1 patient showed marked improvement and had normal muscle tone and strength as well as normal neurologic and developmental evaluations.

Van den Hout et al. (2003) studied the natural course of infantile Pompe disease in 20 Dutch patients and reviewed the findings in 133 published cases. They concluded that survival, decrease of the diastolic thickness of the left ventricular posterior wall, and achievement of major motor milestones are valid endpoints for therapeutic studies.

Bembi et al. (2008) provided a detailed review of the clinical management of GSD II and emphasized a multidisciplinary approach. Enzyme replacement therapy with alglucosidase-alpha has been shown to be effective, particularly in infants.

Wang et al. (2011) described the ACMG standards and guidelines for the diagnostic confirmation and management of presymptomatic individuals with lysosomal storage diseases.

Inheritance

Glycogen storage disease type II is inherited as an autosomal recessive trait.

Smith et al. (2007) studied sib phenotype discordance in classic infantile Pompe disease by reviewing the medical literature for affected sibships in which at least 1 sib had clinical or biochemical findings consistent with infantile Pompe disease, including symptoms beginning in infancy, early hypotonia, cardiomegaly by 6 months of age, and early death. Since 1931, the literature has documented 13 families with 31 affected infants (11 probands; 20 affected sibs). The median age at symptom onset for all affected infants was 3 months (range, 0 to 6 months) with a significant correlation between probands and affected sibs (R = 0.60, p = 0.04). The median age at death for all affected infants was 6 months (range, 1.5 to 13 months); probands were slightly older at death than their sibs. The median length of disease course for all affected infants was 3 months (range, 0 to 10 months) and was slightly longer for probands. There was phenotypic concordance, particularly with respect to cardiomyopathy. Smith et al. (2007) concluded that there is minimal phenotypic and life span variation among sibs with infantile Pompe disease, which is important for genetic counseling.

Molecular Genetics

Multiple mutations in the acid maltase gene have been shown to cause glycogen storage disease II. Martiniuk et al. (1990) demonstrated a single basepair substitution of G to A at position 271 (606800.0001). Wokke et al. (1995) found a single mutation in intron 1 of the acid maltase (606800.0006) in 16 patients with adult-onset acid maltase deficiency.

Lam et al. (2003) reported compound heterozygosity for mutations in the GAA gene in a 16-year-old Chinese boy with juvenile-onset GSD II. The patient had mild symptoms in early childhood, but his condition worsened at age 12 years, with severe weakness, sleep-disordered breathing, and respiratory difficulties. His asymptomatic 13-year-old brother, who had the same mutations, had only biochemical abnormalities suggestive of disease (elevated CK, lack of GAA activity in leukocytes). The authors commented on the intrafamilial variability.

Amartino et al. (2006) reported severe infantile and asymptomatic adult forms of GSD II in 2 generations of the same family. The proband was a 2-month-old male infant of nonconsanguineous Argentinian parents who was admitted to the hospital at 5 days with cyanosis and found to have cardiomegaly, an elevated CK level, high-voltage QRS complexes on ECG, and a thick interventricular septum and hypertrophic ventricular walls on echocardiogram. Pompe disease was suspected and confirmed by measuring GAA activity in leukocytes, and Amartino et al. (2006) identified homozygosity for mutations in the GAA gene, inherited from the parents, respectively. The asymptomatic father was found to have a second mutation on his other allele, the common adult-onset IVS1 splice site mutation (606800.0006). Subsequent evaluation revealed a normal physical examination with no neuromuscular complaints and normal ECG and echocardiogram, but he had elevated CK, short duration potentials on electromyography, and reductions in maximal expiratory and inspiratory pressures on spirometry.

Among 40 Italian patients with late-onset GSD II, Montalvo et al. (2006) identified 26 different mutations, including 12 novel mutations, in the GAA gene. The most common mutation was a splice site mutation in intron 1 (606800.0006), present in heterozygosity in 34 (85%) of 40 patients (allele frequency 42.3%).

Modifier Genes

De Filippi et al. (2010) studied 38 patients with late-onset Pompe disease, aged 44.6 +/- 19.8 years, and compared the distribution of angiotensin I-converting enzyme (ACE) polymorphism (106180.0001) according to demographic and disease parameters. The distribution of ACE polymorphism was in line with the general population, with 16% of patients carrying the II genotype, 37% carrying the DD genotype, and the remaining patients with the ID genotype. The 3 groups did not differ in mean age, disease duration, Walton score, and other scores used to measure disease severity. The DD polymorphism was associated with earlier onset of disease (P = 0.041), higher creatine kinase levels at diagnosis (P = 0.024), presence of muscle pain (P = 0.014), and more severe rate of disease progression (P = 0.037, analysis of variance test for interaction).

Population Genetics

In Israel, almost all cases of Pompe disease have occurred in Palestinian Arabs (Bashan et al., 1988).

On the basis of Hardy-Weinberg equilibrium and the fact that 7 mutations they tested represented only 29% of the total, Martiniuk et al. (1998) estimated the actual carrier frequency to be about 1 in 100. Mutant gene frequency, q, was calculated to be 0.005. The expected number of individuals born with GSD II was estimated to be 1 in 40,000 births.

Three mutations in the GAA gene are common in the Dutch patient population: IVS1-13T-G (606800.0006), 525delT (606800.0014), and EX18DEL (606800.0012). Sixty-three percent of Dutch GSD II patients carry 1 or 2 of these mutations, and the genotype-phenotype correlation is known (Kroos et al., 1995). To determine the frequency of GSD II, Ausems et al. (1999) screened an unselected sample of neonates for these 3 mutations. Based on the calculated carrier frequencies so derived, the predicted frequency of the disease was 1 in 40,000, divided into 1 in 138,000 for infantile GSD II and 1 in 57,000 for adult GSD II. This was about 2 to 4 times higher than previously suggested.

Animal Model

Acid maltase-deficient Japanese quails exhibit progressive myopathy and cannot lift their wings, fly, or right themselves from the supine position in the flip test. Kikuchi et al. (1998) injected 6 4-week-old acid maltase-deficient quails, with the clinical symptoms listed, with 14 or 4.2 mg/kg of the precursor form of recombinant human GAA enzyme or buffer alone every 2 to 3 days for 18 days (7 injections). On day 18, both high dose-treated birds (14 mg/kg) scored positive flip tests and flapped their wings, and 1 bird flew up more than 100 cm. GAA activity increased in most of the tissues examined. In heart and liver, glycogen levels dropped to normal and histopathology was normal. In pectoralis muscle, morphology was essentially normal, except for increased glycogen granules. In sharp contrast, sham-treated quail muscle had markedly increased glycogen granules, multivesicular autophagosomes, and inter- and intrafascicular fatty infiltrations. Low dose-treated birds (4.2 mg/kg) improved less biochemically and histopathologically than high dose birds, indicating a dose-dependent response. Additional experiments with intermediate doses and extended treatment halted the progression of the disease. Data were claimed to be the first to show that an exogenous protein can target to muscle and produce muscle improvement. The data also suggested that enzyme replacement with recombinant human GAA is a promising therapy for human Pompe disease.

In mice in whom the Gaa gene was disrupted by gene targeting in embryonic stem cells, Raben et al. (1998) found that homozygosity for the knockout was associated with lack of enzyme activity and accumulation of glycogen in cardiac and skeletal muscle lysosomes by 3 weeks of age, with a progressive increase thereafter. By 3.5 weeks of age, these mice had markedly reduced mobility and strength. They grew normally, however, reached adulthood, remained fertile, and, as in the human adult disease, older mice accumulated glycogen in the diaphragm. By 8 to 9 months of age, the animals developed obvious muscle wasting and a weak, waddling gait. In contrast, in a second model, mutant mice with deletion of exon 6, like the knockout mice with disruption of exon 13 reported by Bijvoet et al., 1998, had unimpaired strength and mobility (up to 6.5 months of age) despite indistinguishable biochemical and pathologic changes.

Bijvoet et al. (1999) produced recombinant human acid alpha-glucosidase on an industrial scale in the milk of transgenic rabbits, and administered the purified enzyme intravenously to knockout mice. Full correction of acid alpha-glucosidase activity was obtained in all tissues except brain after a single dose of 17mg/kg. Weekly enzyme infusions over a period of 6 months resulted in normalization of hepatic glycogen, but only partial degradation of lysosomal glycogen in heart, skeletal and smooth muscle. The tissue morphology improved substantially despite the advanced state of disease at the start of treatment. The authors stated that although neurologic symptoms had not been documented in human GSD II patients, the inability of the enzyme to cross the blood-brain barrier in the mouse model remained a point of concern.

Dennis et al. (2000) identified mutations in the bovine Gaa gene that led to generalized glycogenosis in Brahman and Shorthorn bovine breeds. All 3 mutations resulted in premature termination of translation. The authors also presented evidence for a missense mutation segregating with the Brahman population, which is responsible for a 70 to 80% reduction in alpha-glucosidase activity.

Using Gaa-knockout mice and transgenes containing cDNA for the human enzyme under muscle- or liver-specific promoters controlled by tetracycline, Raben et al. (2001) demonstrated that the liver provided enzyme far more efficiently. The achievement of therapeutic levels with skeletal muscle transduction required the entire muscle mass to produce high levels of enzyme of which little found its way to the plasma, whereas liver, comprising less than 5% of body weight, secreted 100-fold more enzyme, all of which was in the active 110-kD precursor form. Skeletal and cardiac muscle pathology was completely reversible if the treatment was begun early.

DeRuisseau et al. (2009) found that Gaa-null mice had increased glycogen levels in cervical spinal cord motor neurons and larger soma size of phrenic neurons. Gaa-null mice had decreased ventilation during quiet breathing and hypercapnic challenge compared to wildtype mice, indicating respiratory insufficiency. Mice with skeletal muscle-specific Gaa expression (MTP) showed normal diaphragm force generation similar to wildtype mice, but decreased ventilation during quiet breathing, similar to Gaa-null mice. The compromised ventilation observed in both mutant mouse models was associated with decreased phrenic nerve motor output. Spinal cord samples from a patient with Pompe disease showed increased neuronal glycogen. DeRuisseau et al. (2009) suggested that respiratory impairment in individuals with Pompe disease results from a combination of muscular and neural deficits.

Douillard-Guilloux et al. (2010) analyzed the effect of a complete genetic elimination of glycogen synthesis in a murine GSDII model. Gaa/Gys1 (138570) double-knockout mice exhibited a profound reduction of the amount of glycogen in the heart and skeletal muscles, a significant decrease in lysosomal swelling and autophagic build-up as well as a complete correction of cardiomegaly. In addition, the abnormalities in glucose metabolism and insulin tolerance observed in the GSDII model were corrected in Gaa/Gys1 double-knockout mice. Muscle atrophy observed in 11-month-old GSDII mice was less pronounced in Gaa/Gys1 double-knockout mice, resulting in improved exercise capacity. Douillard-Guilloux et al. (2010) concluded that long-term elimination of muscle glycogen synthesis leads to a significant improvement of structural, metabolic and functional defects in the GSDII mouse model and offers a novel perspective for the treatment of Pompe disease.