Therapeutic advances in the management of Pompe disease and other metabolic myopathies

ERT

The first effective approach to ERT was recombinant human acid alfa-glucosidase (rhGAA) derived from hamster ovary cells. The treatment was soon demonstrated to be effective in infant patients in markedly reducing left ventricular mass and improving cardiomyopathy, which are characteristic of the infantile form, and the primary cause of death in infant patients. Furthermore, rhGAA treatment reduced the risk of death by 99%, and markedly reduced the risk of invasive ventilation compared with an untreated historical control group [Kishnani et al. 2006b]. In contrast to the natural course in untreated patients, most treated infants acquired motor and functional skills, and some patients remained ambulant at the age of 11 years [van der Ploeg, 2010a]. It was thought that ERT, started early in life in infant patients, would lead to a shift toward a juvenile or ‘late-onset’ phenotype, but this was not the case: these long-term survivor infant patients presented peculiar clinical characteristics that need further investigation to be completely understood [Ebbink et al. 2012; Prater et al. 2012].

The slowly progressive nature of adult GSDII patients and the wide variability of organ involvement make it difficult to establish a prognosis, to predict the clinical evolution of the disease, and to define the impact of therapies such as ERT on its nature. The only randomized placebo-controlled study is the Late Onset Treatment Study, in which 90 patients between 10 years and 70 years of age were treated with rhGAA (60 patients), or placebo (30 cases), for 18 months [van der Ploeg et al. 2010b]. The treatment improved walking distance measured with the 6-min walk test, and stabilized pulmonary function in the rhGAA group compared with a slight worsening of walked distance and respiratory function in the placebo group. Other open-label studies showed a rather variable course of neuromuscular deficits in chronic adult-onset GSDII patients during long observational periods of ERT, demonstrating a variable effect in prolonging walked distance and respiratory function [Angelini et al. 2012a; Regnery et al. 2012]. For a better evaluation of natural course and treatment efficacy, work has been carried out to define accurate quantitative outcome measures. Apart from the manual muscle testing for muscle strength, examination of patients usually includes functional tests, such as the 6-min walk test, and timed tests include, for example, the gait, climbing stairs, Gowers’ maneuver and arise from a chair (GSGC) score. The 6-min walk test is a useful measurement of functional endurance during prolonged ambulation, widely used for nonwheelchair-dependent patients [van der Ploeg et al. 2010b; Angelini et al. 2012a]. The GSGC score was validated in the clinical follow up of GSDII cases [Angelini et al. 2012b], as well as in other myopathies. It can be used for patients at different stages of the disease; it is well related to daily life activities, and sensitive to even small differences in the clinical course of treated and untreated patients [Angelini et al. 2012b]. An increasing role in the evaluation of muscle disease has been acquired by magnetic resonance imaging of muscle, an accurate and noninvasive test that can evaluate muscle morphology and fibro-fatty muscle degeneration, quantify glycogen, and detail the changes with spectroscopic studies [Carlier et al. 2011]. The glucose tetrasaccharide, Glca1-6Glca1-4Glca1-4Glc (Glc4), is a glycogen-derived limited dextrin that correlates with the extent of glycogen accumulation in skeletal muscle. It is used in GSDII as a biomarker of tissue damage, and to monitor the response to ERT with very good sensitivity and specificity, but it does not provide information on the location and distribution of excess glycogen accumulation [Young et al. 2012].

What is new in GSDII?

In the last few years, new diagnostic tests have been carried out, mostly using enzyme analysis on dried blood spots with fluorometric or mass spectrometry methods [Burton, 2012]. These techniques seem useful for early diagnosis, or as diagnostic tools in the newborn screening programs that are being carried out by various national health institutions. At the same time, the accurate clinical evaluation of Pompe patients in the last few years has led to the identification of unusual phenotypes, widening the clinical spectrum of the disease. In a recent German case series, Schuller and colleagues identified up to 16% of patients presenting a combination of severe scoliosis with lumbar hyperlordosis, incomplete rigid spine syndrome, and a low mean body mass index, apart from the more frequent limb girdle weakness associated with diaphragmatic weakness [Schuller et al. 2012]. Cardiac involvement in adult patients is not uncommon, as previously mentioned, and seems quite responsive to ERT [Angelini et al. 2012a; Fernandez et al. 2012]. Tongue weakness may be present as an axial sign of late-onset GSDII, even relatively early in the course of the disease, and it may contribute to the differential diagnosis of this now treatable condition, along with dysphagia and dysarthria [Dubrovsky et al. 2011]. The involvement of smooth muscle is well known, manifesting, for example, with vascular abnormalities (e.g. intracranial aneurysms) [Schüller et al. 2012; Hobson-Webb et al. 2012]. Urinary and fecal incontinence can be due either to the involvement of striated and smooth pelvic floor muscle, or lower motor neuron or autonomic involvement, which was occasionally observed in late-onset adult patients [Remiche et al. 2012]. Incontinence is associated with a poorer quality of life, and can be present at all stages of the disease. For these reasons, it should be investigated regularly, and used as an additional parameter to be considered in the decision to start ERT, the more so because incontinence seems to be responsive to ERT.

Gene modifiers

It is known that modifying factors have a sizeable effect on the clinical course of GSDII [Kroos et al. 2012]. de Filippi and colleagues observed a correlation between the presence of a specific polymorphism in the angiotensin-converting enzyme (ACE) and the phenotype, being the DD genotype of the ACE gene associated with an earlier age of onset [de Filippi et al. 2010]. The mechanism by which variation in ACE activity may influence muscle function and exercise performance is complex. The DD genotype is associated with a higher ACE activity that results in a lower half-life of bradykinin and increased production of angiotensin II, with a consequent vasoconstriction. Thus, the physiological impact of the ACE genotype may take place by reduced endothelium-dependent vasodilatation and decreased substrate delivery to the working muscles. This point may be of particular relevance in a disorder primarily due to the inability of muscle to use stored glycogen, so that the muscle energy supply becomes fully dependent on circulating glucose. The D allele is also associated with an increase in type II fibers, which are less rich in mitochondria and thus more sensitive to oxidative stress (Shang et al. 2003). Other proposed mechanisms include an influence of bradykinin on increasing muscle blood flow and glucose extraction rate, and a stimulation of protein synthesis [Woods et al. 2000].

Autophagy and GSDII

Autophagy, the major degradation pathway for long-lived proteins and organelles, is essential for cell homeostasis and its dysfunction has been linked to a number of muscle disorders that are typically characterized by massive autophagic buildup. This was first shown in Pompe disease in a mouse model, where the areas of cellular debris, seen predominantly in type IIB myofibers, were associated with resistance to ERT [Fukuda et al. 2006]. In patients, as in the mouse model, the enormous buildup, rather than the enlarged, glycogen-filled lysosomes outside the autophagic regions, appears to cause the muscle damage [Raben et al. 2007]. What is emerging from recent studies on GSDII patients is that autophagy impairment contributes to disease progression and atrophy [Nascimbeni et al. 2012]. A residual functional autophagic flux has been shown to be important also for efficient ERT and GAA maturation. Indeed, most of the cellular trafficking, including endosomal trafficking, needed for recombinant GAA uptake and delivery to lysosomes, uses proteins that also participate in autophagy, and the autophagosome accumulation that characterizes GSDII can sequester these factors. The maturation steps of GAA from the synthesis of the immature protein in the endoplasmic reticulum to the final cleaved active protein in lysosomes are complex and require a functional system of vesicle trafficking.

These findings highlight the importance of developing new drugs to restore the autophagic flux in these patients in order to improve the uptake of recombinant GAA.

GSDIII or debrancher enzyme deficiency

The clinical forms of the disease include a predominantly hepatic form (i.e. hepatomegaly, hypoglycemia, and hypertriglyceridemia), and a muscular form (i.e. myopathy and cardiomyopathy) [Okubo et al. 2011; Dagli et al. 2010]. Usual recommendations include frequent high-carbohydrate meals in the day and raw cornstarch or continuous feeding during the night, which prevent fasting hypoglycemia but do not influence the course of cardiac and muscular manifestations (Kishnani et al. 2010). Recently, Valayannopoulos and colleagues proposed an experimental treatment for a 2-month-old infant with a severe familial form of GSDIII complicated by severe cardiomyopathy, whose sister had died from severe cardiomyopathy on a standard nutritional protocol [Valayannopoulos et al. 2011]. The authors combined the use of synthetic ketone bodies (D,L-3-OH butyrate) as an alternative energy substrate for the heart, a 2:1 ketogenic diet to reduce glucose intake, and a high-protein diet to enhance gluconeogenesis. After 2 years of treatment, echocardiography showed an improvement in the cardiomyopathy. Growth and liver size remained normal, and no side effects were observed. These preliminary results of a case report need confirmation in a larger series.

GSDV or McArdle disease

McArdle disease is a rare disease and is due to a deficiency of the glycogenolytic enzyme myophosphorylase. The disease is clinically characterized by fatigue and exercise-induced myalgia, and, in some cases, by myoglobinuria with acute renal failure due to recurrent rhabdomyolysis. A small proportion of patients develop a progressive weakness of the proximal muscles, especially in the upper limbs [Quinlivan et al. 2010a; Lucia et al. 2012b]. Due to the rarity of the disease, its high variability (and paucity) of symptoms, and the difficulties in defining precise outcome measures, no trial could demonstrate efficacy of any drug, dietary treatment, or exercise scheme. One of the most interesting treatments is vitamin B6. In normal individuals the skeletal muscle contains 80% of the total pool of vitamin B6 bound as pyridoxal 50-phosphate (PLP) to muscle phosphorylase, playing an important role in enzyme activity. In McArdle patients decreased phosphorylase substantially diminishes PLP in skeletal muscle [Haller et al. 1983]. Recently, a Japanese patient was treated for more than 2 years with vitamin B6 supplementation, and the authors could demonstrate an amelioration in symptoms, increase of enzymatic levels on muscle biopsy, and increase of lactate in response to effort during the grip test [Sato et al. 2012]; it is worth noting that previous studies including small case series could not demonstrate a real efficacy of this supplementation. Along with methodological limits (i.e. outcome measures, numbers in cohort), Sato and colleagues postulated that the action of vitamin B6 supplementation may require the presence of some residual muscle phosphorylase and probably would not have been seen in patients with null mutations, including the R50X mutation, which is most common among Whites [Sato et al. 2012]. They hypothesized that vitamin B6 supplementation can restore some stability to the mutant enzyme and enhance the residual phosphorylase activity in skeletal muscle, followed by improvement in insufficient anaerobic glycolysis of skeletal muscle. These hypotheses need further confirmation in larger controlled studies. The intervention that demonstrated some amelioration in clinical symptoms included dietary treatment and physical training [Quinlivan et al. 2010b, 2011]. The infusion of sucrose before exercise can markedly improve exercise tolerance, probably by increased availability of bloodborne glucose, which partially rescues muscle oxidative metabolism in myophosphorylase-deficient muscle early in exercise [Andersen et al. 2008]. A carbohydrate-rich diet seems more effective than a high-protein diet [Andersen and Vissing, 2008]. A regular aerobic training program can potentially improve metabolic capacity and exercise tolerance by conditioning muscles for oxidative phosphorylation. Moderate aerobic exercise (30 min/day, 4 days a week, at an intensity corresponding to 60–70% of maximal heart rate) was tested in eight patients for 14 weeks [Lucia et al. 2012a]. This training program was safe, increased average work capacity, oxygen uptake, cardiac output, and mitochondrial enzyme levels without causing pain or cramps or increasing serum creatine kinase. Randomized studies are needed to confirm these preliminary results. The impact of ACE polymorphism on the disease, as discussed for GSDII, was previously described in GSDV by Martinuzzi and colleagues who demonstrated a strong correlation between the severe phenotype and the number of D alleles [Martinuzzi et al. 2003]. A double-blind, randomized, placebo-controlled trial involving eight patients treated with an ACE inhibitor (2.5 mg ramipril) demonstrated a significant change in disability score, even if it failed to show any treatment effect in the objective measures of exercise performance and muscle metabolism chosen as primary endpoints for the overall population [Martinuzzi et al. 2008]. The treatment seemed to modify exercise physiology in DD patients, raising the possibility of a differential haplotype-linked sensitivity to the treatment that still needs confirmation. The principal limitations of the study were the small study population and the outcome measures.

CPT2 deficiency

CPT2 deficiency is a rare autosomal disorder of mitochondrial fatty acid oxidation (FAO), clinically characterized by muscle stiffness, myalgia, exercise intolerance, and episodes of myoglobinuria, usually triggered by prolonged exercise, cold, or fasting [Liang and Nishino, 2010]. In a large series of patients, Fanin and colleagues divided molecularly defined patients into mild or severe categories according to the number of myoglobinuric episodes [Fanin et al. 2012]. Patients affected by CPT2 deficiency usually can control their symptoms by avoiding trigger factors and with an accurate dietary treatment using high-carbohydrates doses [Ørngreen et al. 2003]. Glucose infusion is of benefit in CPT2-deficient patients, but oral glucose did not achieve the same effects, probably because of the low glucose availability, and because of the insulin response that inhibits muscle glycogenolysis [Ørngreen et al. 2002]. In the past few years, new drugs have been tested to increase FAO. Fibrates are a class of hypolipidemic drugs that increase high-density lipoprotein levels by mRNA upregulation of many lipid-metabolism genes through interaction with the steroid/thyroid transcription factor PPARa. Recent studies demonstrated that bezafibrate increases CPT2 mRNA and normalizes enzyme activity in mild forms of CPT2-deficient cultured fibroblasts and myoblasts [Bonnefont et al. 2009]. A trial including six patients treated with bezafibrate was conducted in France, the primary outcome being the level of FAO in muscle evaluated in biopsies. The authors demonstrated normalization of the FAO level in all patients: a significant increase in palmitoyl-L-carnitine oxidation, increased CPT2 mRNA, and the translated protein. In addition, they observed a reduction of episodes of rhabdomyolysis, amelioration of quality of life measured by SF-36, and no AEs. There was also an increase in physical activity and a decline in muscular pain [Bonnefont et al. 2009, 2010]. Further collaborative studies and in vivo studies are needed to confirm the long-term efficacy and safety of these therapies.

ETF, ETF-dehydrogenase or RR-MADD

This disease is a rather unrecognized cause of myopathy with lipid storage [Wang et al. 2011]. RR-MADD is caused by defects in ETF or ETF-dehydrogenase, two mitochondrial enzymes that contain flavin adenine dinucleotide as prosthetic group. A defect of ETF and ETF dehydrogenase can affect all dehydrogenases in fatty-acid beta-oxidation. The disease is characterized by glutaric aciduria type 2 and lipid storage myopathy (Figure 2). The phenotype is heterogeneous: late-onset patients manifest proximal myopathy, high creatine kinase levels, vomiting, and hypoglycemia. The measurement of serum carnitine, the acylcarnitine profile, and urinary organic carnitine is useful for diagnosis [Liang and Nishino, 2011].

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Figure 2.

Muscle biopsy from a 22-year-old woman with lipid storage myopathy (riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency). Vacuolated and atrophic fibers, hematoxylin and eosin stain (a), and lipid droplets in many fibers stained with oil red O stain (b).

The disease appears very responsive to treatment, since riboflavin (100–400 mg/day) alone or in combination with carnitine supplementation can reverse the prominent weakness in a few months and normalize the deficiency [Cornelius et al. 2012].