Methods for a prompt and reliable laboratory diagnosis of Pompe disease: report from an international consensus meeting

Efficacy and safety of enzyme replacement therapy with alglucosidase alfa for the treatment of patients with infantile-onset Pompe disease: a systematic review and metanalysis

Introduction

Pompe disease (PD) is a glycogen disorder caused by the deficient activity of acid alpha-glucosidase (GAA). We sought to review the latest available evidence on the safety and efficacy of recombinant human GAA enzyme replacement therapy (ERT) for infantile-onset PD (IOPD).

Methods

We systematically searched the MEDLINE (via PubMed) and Embase databases for prospective clinical studies evaluating ERT for IOPD on pre-specified outcomes. Meta-analysis was also performed.

Results

Of 1,722 articles identified, 16 were included, evaluating 316 patients. Studies were heterogeneous and with very low certainty of evidence for most outcomes. A moderate/high risk of bias was present for most included articles. The following outcomes showed improvements associated with alglucosidase alfa, over natural history of PD/placebo, for a mean follow-up of 48.3 months: left ventricular (LV) mass {mean change 131.3 g/m2 [95% confidence interval (CI) 81.02, 181.59]}, time to start ventilation (TSV) [HR 0.21 (95% CI: 0.12, 0.36)], and survival [HR 0.10 (95% CI: 0.05, 0.19)]. There were no differences between the pre- and post-ERT period for myocardial function and psychomotor development. Adverse events (AEs) after ERT were mild in most cases.

Conclusion

Our data suggest that alglucosidase alfa potentially improves LV mass, TSV, and survival in IOPD patients, with no important safety issues.

Systematic Review Registration

PROSPERO identifier (CRD42019123700).

[Diagnosis and Management of Late-Onset Pompe Disease]

Pompe disease is a lysosomal storage disorder, with onset between the first weeks after birth and adulthood, depending on its phenotype. It can affect multiple organ systems and presents itself with a wide variety of symptoms. Thus, recognizing Pompe disease is difficult. Especially since enzyme replacement therapy for Pompe disease was introduced (in Germany in 2006), early diagnosis by means of enzyme activity determination from dried blood spot analysis and genetic verification has become important for outcome and quality of life. When facing an obscure muscular disorder, it is crucial to consider Pompe disease. This article provides an overview about Pompe disease and focuses on the diagnosis of the late onset type. The most important aspects of interdiciplinary care for patients with Pompe disease are presented. Additionally, it contains a section focusing on psychosocial challenges for children with Pompe disease and their families, which may include mental disorders and social retreat, and gives advice on how to support parents of affected children.

Variant Classification for Pompe disease; ACMG/AMP specifications from the ClinGen Lysosomal Diseases Variant Curation Expert Panel

Accurate determination of the clinical significance of genetic variants is critical to the integration of genomics in medicine. To facilitate this process, the NIH-funded Clinical Genome Resource (ClinGen) has assembled Variant Curation Expert Panels (VCEPs), groups of experts and biocurators which provide gene- and disease- specifications to the American College of Medical Genetics & Genomics and Association for Molecular Pathology's (ACMG/AMP) variation classification guidelines. With the goal of classifying the clinical significance of GAA variants in Pompe disease (Glycogen storage disease, type II), the ClinGen Lysosomal Diseases (LD) VCEP has specified the ACMG/AMP criteria for GAA. Variant classification can play an important role in confirming the diagnosis of Pompe disease as well as in the identification of carriers. Furthermore, since the inclusion of Pompe disease on the Recommended Uniform Screening Panel (RUSP) for newborns in the USA in 2015, the addition of molecular genetic testing has become an important component in the interpretation of newborn screening results, particularly for asymptomatic individuals. To date, the LD VCEP has submitted classifications and supporting data on 243 GAA variants to public databases, specifically ClinVar and the ClinGen Evidence Repository. Here, we describe the ACMG/AMP criteria specification process for GAA, an update of the GAA-specific variant classification guidelines, and comparison of the ClinGen LD VCEP's GAA variant classifications with variant classifications submitted to ClinVar. The LD VCEP has added to the publicly available knowledge on the pathogenicity of variants in GAA by increasing the number of expert-curated GAA variants present in ClinVar, and aids in resolving conflicting classifications and variants of uncertain clinical significance.

A Comprehensive Update on Late-Onset Pompe Disease

Pompe disease (PD) is an autosomal recessive disorder caused by mutations in the GAA gene that lead to a deficiency in the acid alpha-glucosidase enzyme. Two clinical presentations are usually considered, named infantile-onset Pompe disease (IOPD) and late-onset Pompe disease (LOPD), which differ in age of onset, organ involvement, and severity of disease. Assessment of acid alpha-glucosidase activity on a dried blood spot is the first-line screening test, which needs to be confirmed by genetic analysis in case of suspected deficiency. LOPD is a multi-system disease, thus requiring a multidisciplinary approach for efficacious management. Enzyme replacement therapy (ERT), which was introduced over 15 years ago, changes the natural progression of the disease. However, it has limitations, including a reduction in efficacy over time and heterogeneous therapeutic responses among patients. Novel therapeutic approaches, such as gene therapy, are currently under study. We provide a comprehensive review of diagnostic advances in LOPD and a critical discussion about the advantages and limitations of current and future treatments.

Expert opinion on the diagnostic odyssey and management of late-onset Pompe disease: a neurologist's perspective

This consensus statement by a panel of neurology experts aimed to provide a practical and implementable guidance document to assist clinicians with the best clinical practice in terms of diagnosis, treatment, and monitoring of late-onset Pompe disease (LOPD). The participating experts consider the clinical suspicion of LOPD by the physician to be of utmost importance in the prevention of diagnostic and therapeutic delay in LOPD patients. A diagnostic algorithm is proposed to facilitate the diagnosis of LOPD in patients presenting with unexplained proximal/axial weakness (with or without respiratory symptoms) or restrictive respiratory insufficiency with hyperCKemia and/or exercise intolerance as the red flag symptoms/signs that raise the index of suspicion for LOPD diagnosis. The diagnosis is based on the subsequent use of dried blood spot (DBS) assay, and the DBS assay can be confirmed by acid alpha-glucosidase (GAA) tissue analysis in leukocytes, fibroblasts, or muscle fibers and/or genetic mutation analysis. Accordingly, experts consider increased awareness among physicians about potential presenting characteristics with a high index of suspicion for LOPD to be crucial to suspect and consider LOPD in the differential diagnosis, while strongly suggesting the use of a diagnostic algorithm combined with DBS assay and confirmatory tests in the timely diagnosis of LOPD and implementation of best practice patterns.

Enzyme Replacement Therapy (ERT) on Heart Function Changes the Outcome in Patients with Infantile-Onset Pompe Disease: A Familial History

Background

Lysosomal acid alpha-glucosidase (GAA) deficiency, also known as Pompe disease, is an autosomal recessive disorder that leads to the accumulation of glycogen in lysosomes and cytoplasm, resulting in tissue destruction. Infantile-onset GAA deficiency is characterized by cardiomyopathy and severe generalized hypotonia. Without treatment, most patients die within the first two years of life. The demonstration of reduced GAA activity, followed by sequencing of the GAA gene, confirms the disease. GAA deficiency is currently treated with enzyme replacement therapy (ERT) with improved clinical outcomes and survival. Case Presentation. We describe the case of DGAA in two siblings, in which the diagnostic time point, treatment, and outcomes were completely different. The girl was diagnosed with DGAA at the age of 6 months during investigations for poor weight gain and excessive sleepiness. The finding of severe cardiomyopathy through EKG and echocardiography led to the suspicion of storage disease, and the GAA deficiency was later confirmed by genetic analysis. The girl died of complications due to the clinical picture before starting ERT. Conversely, her younger brother had the opportunity to receive an early diagnosis and the rapid onset of ERT. He is showing a regression of cardiac hypertrophy.

Conclusion

The advent of ERT improved clinical outcomes and survival in infantile-onset PD. Its impact on cardiac function is still under study, but different reports in the literature have shown encouraging data. Early recognition of DGAA and prompt initiation of ERT is therefore crucial to prevent the progression of the disease and improve the outcomes.

The Clinical Management of Pompe Disease: A Pediatric Perspective

Pompe disease (PD) is an inherited metabolic disorder caused by a deficiency of acid α-glucosidase (GAA), leading to lysosomal accumulation of glycogen, mainly in skeletal and cardiac muscles as well as the nervous system. Patients with PD develop cellular dysfunction and muscle damage. PD can be classified into two classic forms, namely infantile-onset PD (IOPD) and late-onset PD (LOPD). Delayed treatment, particularly in IOPD, would result in significant organ damage and early death. Nonetheless, early diagnosis and timely treatment are often hampered by the rarity of PD and its wide variety of, but overlapping, symptoms. This article reviews the common clinical presentations of PD and outlines the essentials of PD management. In particular, the implications of newborn screening (NBS) and clinical performance of enzyme replacement therapy (ERT) are highlighted.

Interpretation and actionability of genetic variants in cardiomyopathies: a position statement from the European Society of Cardiology Council on cardiovascular genomics

This document describes the contribution of clinical criteria to the interpretation of genetic variants using heritable Mendelian cardiomyopathies as an example. The aim is to assist cardiologists in defining the clinical contribution to a genetic diagnosis and the interpretation of molecular genetic reports. The identification of a genetic variant of unknown or uncertain significance is a limitation of genetic testing, but current guidelines for the interpretation of genetic variants include essential contributions from clinical family screening that can establish a de novo assignment of the variant or its segregation with the phenotype in the family. A partnership between clinicians and patients helps to solve major uncertainties and provides reliable and clinically actionable information.

A Systematic Review and Meta-Analysis of Enzyme Replacement Therapy in Late-Onset Pompe Disease

Pompe disease (PD) is a glycogen storage disorder caused by deficient activity of acid alpha-glucosidase (GAA). We sought to review the latest available evidence on the safety and efficacy of recombinant human GAA enzyme replacement therapy (ERT) for late-onset PD (LOPD).

METHODS

We systematically searched the MEDLINE (via PubMed), Embase, and Cochrane databases for prospective clinical studies evaluating ERT for LOPD on pre-specified outcomes. A meta-analysis was also performed.

RESULTS

Of 1601 articles identified, 22 were included. Studies were heterogeneous and with very low certainty of evidence for most outcomes. The following outcomes showed improvements associated with GAA ERT, over a mean follow-up of 32.5 months: distance walked in the 6-min walking test (6MWT) (mean change 35.7 m (95% confidence interval [CI] 7.78, 63.75)), physical domain of the SF-36 quality of life (QOL) questionnaire (mean change 1.96 (95% CI 0.33, 3.59)), and time on ventilation (TOV) (mean change -2.64 h (95% CI -5.28, 0.00)). There were no differences between the pre- and post-ERT period for functional vital capacity (FVC), Walton and Gardner-Medwin Scale score, upper-limb strength, or total SF-36 QOL score. Adverse events (AEs) after ERT were mild in most cases.

CONCLUSION

Considering the limitations imposed by the rarity of PD, our data suggest that GAA ERT improves 6MWT, physical QOL, and TOV in LOPD patients. ERT was safe in the studied population. PROSPERO register: 135102.

Treatable lysosomal storage diseases in the advent of disease-specific therapy

Lysosomal storage diseases (LSD) comprise a rare and heterogeneous group of nearly 50 heritable metabolic disorders caused by mutations in proteins critical for cellular lysosomal function. Defects in the activity of these proteins in multiple organs leads to progressive intra-lysosomal accumulation of specific substrates, resulting in disruption of cellular functions, extracellular inflammatory responses, tissue damage and organ dysfunction. The classification and clinical presentation of different LSD are dependent on the type of accumulated substrate. Some clinical signs and symptoms are common across multiple LSD, while others are more specific to a particular syndrome. Due to the rarity and wide clinical diversity of LSD, identification and diagnosis can be challenging, and in many cases diagnosis is delayed for months or years. Treatments, such as enzyme replacement therapy, haemopoietic stem cell transplantation and substrate reduction therapy, are now available for some of the LSD. For maximum effect, therapy must be initiated prior to the occurrence of irreversible tissue damage, highlighting the importance of prompt diagnosis. Herein, we discuss the clinical presentation, diagnosis and treatment of four of the treatable LSD: Gaucher disease, Fabry disease, Pompe disease, and two of the mucopolysaccharidoses (I and II). For each disease, we present illustrative case studies to help increase awareness of their clinical presentation and possible treatment outcomes.

Respiratory failure and sleep-disordered breathing in late-onset Pompe disease: a narrative review

Late-onset Pompe disease (LOPD) is a rare autosomal recessive glycogen storage disease that results in accumulation of glycogen in muscle cells causing muscular weakness. It causes a progressive proximal myopathy, accompanied by respiratory muscle weakness, which can lead to ventilatory failure. In untreated LOPD, the most common cause of death is respiratory failure. Patients suffering from respiratory compromise may present with symptoms of sleep-disordered breathing (SDB) before overt signs of respiratory failure. Diaphragm weakness leads to nocturnal hypoventilation, which can result in sleep disruption. Both subjective and objective sleep quality can be impaired with associated excessive daytime sleepiness (EDS). Health-related quality of life worsens as sleep disturbance increases. The mainstay of treatment for SDB and respiratory failure in LOPD is non-invasive ventilation (NIV), which aims to ensure adequate ventilation, particularly during sleep, and prevent acute hypercapnic failure. These patients are at risk of acute deterioration due to lower respiratory tract infections; effective secretion clearance and vaccination against common pathogens is an important facet of care. Whilst disease-modifying enzyme replacement therapy (ERT) delays progression of locomotor dysfunction and prolongs life, its effect on respiratory function and SDB remains unclear. There are no data demonstrating the impact of ERT on sleep quality or SDB.

Establishing Pompe Disease Newborn Screening: The Role of Industry

When clinical trials for enzyme replacement therapy for Pompe disease commenced, a need for newborn screening (NBS) for Pompe disease was recognized. Two methods for NBS for Pompe disease by measuring acid α-glucosidase in dried blood spots on filter paper were developed in an international collaborative research effort led by Genzyme. Both methods were used successfully in NBS pilot programs to demonstrate the feasibility of NBS for Pompe disease. Since 2009, all babies born in Taiwan have been screened for Pompe disease. Pompe disease was added to the Recommended Uniform (Newborn) Screening Panel in the United States in 2015. NBS for Pompe disease is possible because of the unprecedented and selfless collaborations of countless international experts who shared their thoughts and data freely with the common goal of establishing NBS for Pompe disease expeditiously.

Pompe disease: pathogenesis, molecular genetics and diagnosis

Pompe disease (PD) is a rare autosomal recessive disorder caused by mutations in the GAA gene, localized on chromosome 17 and encoding for acid alpha-1,4-glucosidase (GAA). Currently, more than 560 mutations spread throughout GAA gene have been reported. GAA catalyzes the hydrolysis of α-1,4 and α-1,6-glucosidic bonds of glycogen and its deficiency leads to lysosomal storage of glycogen in several tissues, particularly in muscle. PD is a chronic and progressive pathology usually characterized by limb-girdle muscle weakness and respiratory failure. PD is classified as infantile and childhood/adult forms. PD patients exhibit a multisystemic manifestation that depends on age of onset.

Early diagnosis is essential to prevent or reduce the irreversible organ damage associated with PD progression. Here, we make an overview of PD focusing on pathogenesis, clinical phenotypes, molecular genetics, diagnosis, therapies, autophagy and the role of miRNAs as potential biomarkers for PD.

Is it Pompe Disease? Australian diagnostic considerations

Pompe Disease is a spectrum disorder with an evolving phenotype in which diagnostic delay is common. Contributing factors include the rarity of the disorder, its wide clinical spectrum, signs and symptoms that overlap with those of other neuromuscular disorders, variable diagnostic approaches, lack of awareness of the clinical manifestations and difficulties in completing the diagnostic inventory. International updates and recommendations have been published providing diagnostic guidelines and management criteria. However, questions remain in the Australian setting. A panel (two neurologists, one clinical geneticist) reviewed the literature, examined clinical questions of relevance to the Australian setting, and developed a framework for the guidance. A wider panel, comprising the initial panel plus eight additional members, critiqued the framework and contributed clinical guidance within the scope of their respective areas of clinical expertise. The resultant expert consensus recommendations build on currently available data to propose an appropriate management framework incorporating the diagnosis, classification, therapeutic approach, multidisciplinary care, and on-going monitoring of patients with Pompe Disease in the Australian setting. It is hoped that diagnostic delay can be reduced with appropriate recourse to evidence-based insights and practical advice on diagnosis and management tailored to the Australian setting.

Screening for late-onset Pompe disease in Poland

OBJECTIVES

We aimed to screen for late-onset Pompe disease using the dried blood spot (DBS) test in a cohort of patients with limb-girdle muscle weakness or persistent hyperCKemia.

MATERIALS AND METHODS

Patients with limb-girdle muscle weakness, persistently elevated CK, rigid spine syndrome, dyspnoea, myalgia or sibling of the patient diagnosed with LOPD were included in the study. Acid α-glucosidase (GAA) activity was measured on DBS by tandem mass spectrometry and followed by genetic testing when required. Study was conducted between June 2014 and May 2017.

RESULTS

A total of 337 patients aged 32.2 years (range 2-80) were included in the study. Late-onset Pompe disease was diagnosed in 10 patients (3.0% of tested cohort). All were compound heterozygotes with common c.32-13T>G mutation on one allele and missense or frameshift mutation on the other. Two of the mutations (c.1951delG and c.397T>G) were not reported previously. Seven of the patients started enzyme replacement therapy.

CONCLUSIONS

DBS test is a reliable method for screening for late-onset Pompe disease.

Systematic review of oral and craniofacial findings in patients with Fabry disease or Pompe disease

Fabry disease and Pompe disease are rare lysosomal storage disorders that belong to a heterogeneous group of more than 200 distinct inborn metabolic diseases. Mutations followed by loss of function of enzymes or transporters that are localised in the acidic environment of the lysosome may result in degradation of many substrates, such as glycosaminoglycans, glycosphingolipids, glycogen, cholesterol, oligosaccharides, glycoproteins, and peptides, or the excretion of the products degraded by the lysosome. Our aim was to identify the oral signs and symptoms of Fabry disease and Pompe disease from a systematic review made using MEDLINE/PubMed, and a hand search for relevant articles, following the PRISMA guidelines. Both diseases show various craniofacial and oral changes, including supernumerary teeth, dental agenesis, angiokeratoma, and telangiectases in Fabry disease; and macroglossia, teeth fusion, and taurodontism in Pompe disease. Common clinical signs of Fabry disease include hyposalivation, hypohidrosis, and xerophthalmia, and a generally reduced physical resilience was apparent in patients with Pompe disease. Oral and craniofacial changes in patients with both diseases extend over their entire lifetime and can be detected even in an infant. Lysosomal storage diseases should be taken into consideration in the differential diagnosis of relevant diverse symptoms, because treatment, when available, is most effective when started early. The main therapeutic concepts are enzymatic replacement for Pompe disease, whereas patients with Fabry disease require additional oral chaperone treatment or enzyme replacement.

Diagnostic tools in late onset Pompe disease (LOPD)

Pompe disease is a rare metabolic disorder due to deficiency of the lysosomal acid alpha-glucosidase (GAA) that causes glycogen accumulation in all tissues with a predominant involvement of skeletal muscle. The late onset form of Pompe disease (LOPD) is characterized by a progressive weakness of proximal and axial muscles, often mimicking limb-girdle muscular dystrophies or inflammatory myopathies, with respiratory distress mainly due to a diaphragmatic weakness. Diagnostic delay is still common, and clinicians need a high index of suspicion to recognize this condition because the disorder is quite rare, the clinical spectrum is wide, and signs and symptoms are not distinguishable from those in other neuromuscular disorders that present in a similar fashion. Diagnostic laboratory tests are quite fast and reliable to detect the enzymatic deficiency. Enzyme replacement therapy has been available for several years, and other new therapeutic strategies such as gene therapy are underway. Here, we discuss the main diagnostic tools currently used for the evaluation of patients with suspected LOPD.

Late-onset Pompe disease in France: molecular features and epidemiology from a nationwide study

Pompe disease (PD) is caused by a deficiency of lysosomal acid α-glucosidase resulting from mutations in the GAA gene. The clinical spectrum ranges from a rapidly fatal multisystemic disorder (classic PD, onset < 1 year) to a milder adult onset myopathy. The aims of this study were to characterize the GAA mutations, to establish the disease epidemiology, and to identify potential genotype-phenotype correlations in French late-onset PD patients (onset ≥ 2 years) diagnosed since the 1970s. Data were collected from the two main laboratories involved in PD diagnosis and from the French Pompe registry. Two hundred forty-six patients (130 females and 116 males) were included, with a mean age at diagnosis of 43 years. Eighty-three different mutations were identified in the GAA gene, among which 28 were novel. These variants were spread all over the sequence and included 42 missense (one affecting start codon), 8 nonsense, 15 frameshift, 14 splice mutations, 3 small in-frame deletions, and one large deletion. The common c.-32-13T>G mutation was detected in 151/170 index cases. Other frequent mutations included the exon 18 deletion, the c.525del, and the missense mutations c.1927G>A (p.Gly643Arg) and c.655G>A (p.Gly219Arg). Patients carrying the c.-32-13T>G mutation had an older mean age at onset than patients non-exhibiting this mutation (36 versus 25 years). Patients with the same genotype had a highly variable age at onset. We estimated the frequency of late-onset PD in France around 1/69,927 newborns. In conclusion, we characterized the French cohort of late-onset PD patients through a nationwide study covering more than 40 years.

Pompe Disease Could Mimic Exam Findings of Amyloidosis: Two Rare Diagnoses Bona Fide

A 70-year-old female presented with a three-year history of evolving macroglossia causing dysphagia and dysarthria, with proximal muscle weakness. Given the classic physical finding of macroglossia, the patient underwent extensive evaluation for amyloidosis which proved to be negative apart from a bone marrow biopsy which stained positive for transthyretin without amino acid sequence abnormality, thus giving wild-type transthyretin amyloidosis. Since the wild-type transthyretin amyloidosis could not entirely explain her clinical presentation and evaluation, further studies were conducted in a sequential manner, thus leading to a diagnosis of Pompe disease explaining her presenting signs and symptoms including her macroglossia. Through this fascinating case, we attempt to highlight the approach for the diagnoses of two rare diseases in a patient by emphasizing the importance of having a broad differential diagnosis when presented with findings which may have been thought as pathognomonic for certain diseases.

Acute respiratory failure as presentation of late-onset Pompe disease complicating the diagnostic process as a labyrinth: a case report

Background

Acute respiratory failure can be triggered by several causes, either of pulmonary or extra-pulmonary origin. Pompe disease, or type II glycogen storage disease, is a serious and often fatal disorder, due to a pathological accumulation of glycogen caused by a defective activiy of acid α-glucosidase (acid maltase), a lysosomal enzyme involved in glycogen degradation. The prevalence of the disease is estimated between 1 in 40,000 to 1 in 300,000 subjects.

Case presentation

This case report describes a difficult diagnosis of late-onset Pompe disease (LOPD) in a 52 year old Caucasian woman with acute respiratory failure requiring orotracheal intubation and subsequent tracheostomy for long-term mechanical ventilation 24 h/day. Despite a complex diagnostic process including several blood tests, bronchoscopy with BAL, chest CT, brain NMR, electromyographies, only a muscle biopsy allowed to reach the correct diagnosis.

Discussion

The most frequent presentation of myopathies, including LOPD, is proximal limb muscle weakness. Respiratory related symptoms (dyspnea on effort, reduced physical capacity, recurrent infections, etc.) and respiratory failure are often evident in the later stages of the diseases, but they have been rarely described as the onset symptoms in LOPD. In our case, a third stage LOPD, the cooperation between pulmonologists and neurologists was crucial in reaching a correct diagnosis despite a very complex clinical scenario due to different confounding co-morbidities as potential causes of respiratory failure and an atypical presentation. In this patient, enzyme replacement therapy with infusion of alglucosidase alfa was associated with progressive reduction of ventilatory support to night hours, and recovery of autonomous walking.

Newborn Screening for Lysosomal Storage Diseases: Methodologies, Screen Positive Rates, Normalization of Datasets, Second-Tier Tests, and Post-Analysis Tools

All of the worldwide newborn screening (NBS) for lysosomal storage diseases (LSDs) is done by measurement of lysosomal enzymatic activities in dried blood spots (DBS). Substrates used for these assays are discussed. While the positive predictive value (PPV) is the gold standard for evaluating medical tests, current PPVs for NBS of LSDs cannot be used as a performance metric due to statistical sampling errors and uncertainty in the onset of disease symptoms. Instead, we consider the rate of screen positives as the only currently reliable way to compare LSD NBS results across labs worldwide. It has been suggested that the expression of enzymatic activity data as multiple-of-the-mean is a way to normalize datasets obtained using different assay platforms, so that results can be compared, and universal cutoffs can be developed. We show that this is often not the case, and normalization is currently not feasible. We summarize the recent use of pattern matching statistical analysis together with measurement of an expanded group of enzymatic activities and biomarkers to greatly reduce the number of false positives for NBS of LSDs. We provide data to show that these post-enzymatic activity assay methods are more powerful than genotype analysis for the stratification of NBS for LSDs.

Elevated Creatine Kinase in a 6-Year-Old Boy

Paucisymptomatic or asymptomatic but persistently elevated serum creatine kinase is not an uncommon pediatric neurology referral question. The challenge is in promptly identifying etiologies with specific treatments, even if they are rare. The presenting features for a child or adolescent with juvenile-onset Pompe disease (JOPD) can be nonspecific and heterogeneous. Clinical manifestations can appear at any age after 2 years and before adulthood, with insidious onset of symptoms related to slowly progressive skeletal or respiratory muscle weakness. This reported case highlights the importance of screening for JOPD in children with "hyper-CK-emia." Dried blood spot measuring acid α-glucosidase enzyme activity is reliable, rapid, noninvasive, and inexpensive, allowing early diagnosis. Diagnosis of JOPD is important as enzyme replacement therapy with alglucosidase alpha, an intravenous recombinant α-glucosidase, is available, and early treatment improves muscle function, quality of life, and long-term survival.

Vacuolated PAS-positive lymphocytes as an hallmark of Pompe disease and other myopathies related to impaired autophagy

Autosomal recessive Pompe disease is a lysosomal disorder caused by mutations of the acid-α-glucosidase (GAA) gene. Deficiency of GAA enzyme leads to glycogen accumulation and autophagy impairment in cardiac and skeletal muscles, but also in lymphocytes. Since an effective therapy is available, a rapid, sensitive, and specific test is crucial to early identify affected subjects. Number of lymphocytes containing PAS-positive vacuoles was evaluated on blood films from 72 consecutive adult patients with hyperckemia and/or muscle weakness, 13 genetically confirmed late-onset-Pompe-disease (LOPD) and 13 of their offspring. GAA activity, measured on dried blood spot (DBS) in all patients inversely correlated with number of PAS-positive lymphocytes. More than 4 PAS-positive lymphocytes were found in 11 out of the 72 patients (6 new diagnosis of LOPD, 3 different glycogen storage myopathies, 1 glucose-6-phosphate dehydrogenase deficiency, 1 caveolinopathy), in all 13 LOPD patients and in the 13 LOPD offspring. These latter resulted to have all a single GAA mutation but low GAA levels. Immunostaining with the autophagy markers LC3 and p62 confirmed the autophagic nature of lymphocytes vacuoles. ROC curve assessment of PAS-positive lymphocytes disclosed 100% of sensitivity and 94% of specificity in recognizing both compound heterozygous and heterozygous GAA carriers. The other myopathies with more than 4 PAS-positive lymphocytes appeared to be all related to impaired autophagy, which seems to be responsible of PAS-positive vacuolated lymphocytes formation. Quantification of PAS-positive lymphocytes in blood films is useful to identify autophagic vacuolar myopathies and should be routinely used as first level test for Pompe disease.

Muscle ultrasound: A useful tool in newborn screening for infantile onset pompe disease

Our study aimed to evaluate the utility of muscle ultrasound in newborn screening of infantile-onset Pompe disease (IOPD) and to establish a system of severity grading. We retrospectively selected 35 patients with initial low acid alpha-glucosidase (GAA) activity and collected data including muscle ultrasound features, GAA gene mutation, activity/performance, and pathological and laboratory findings. The echogenicity of 6 muscles (the bilateral vastus intermedius, rectus femoris, and sartorius muscles) was compared to that of epimysium on ultrasound and rated either 1 (normal), 2 (mildly increased), or 3 (obviously increased). These grades were used to divide patients into 3 groups. IOPD was present in none of the grade-1 patients, 5 of 9 grade-2 patients, and 5 of 5 grade-3 patients (P < .001). Comparing grade-2 plus grade-3 patients to grade-1 patients, muscle ultrasound detected IOPD with a sensitivity and specificity of 100.0% (95% confidence interval [CI]: 69.2%-100%) and 84.0% (95% CI: 63.9%-95.5%), respectively. The mean number of affected muscles was larger in grade-3 patients than in grade-2 patients (4.2 vs. 2.0, P = .005). Mean alanine transaminase (ALT), aspartate transaminase (AST), creatine kinase (CK), and lactate dehydrogenase (LDH) levels were differed significantly different between grade-3 and grade-1 patients (P < .001). Because it permits direct visualization of injured muscles, muscle ultrasound can be used to screen for IOPD. Our echogenicity grades of muscle injury also correlate well with serum levels of muscle-injury biochemical markers.

A molecular analysis of the GAA gene and clinical spectrum in 38 patients with Pompe disease in Japan

Pompe disease is an autosomal recessive disorder caused by acid α-glucosidase (GAA) deficiency, which results in the accumulation of glycogen in lysosomes in multiple tissues, including cardiac, skeletal, and smooth muscle cells. Thus far, 558 sequence variants of the GAA gene have been published in the Pompe Disease Mutation Database, and some mutations appear with considerable frequency in particular ethnic groups, such as Caucasians, Taiwanese, Chinese, and Koreans. However, the GAA mutation pattern in Japanese patients remains poorly understood. We analyzed the relationship between the genetic and clinical features of 38 mostly Japanese patients with Pompe disease from 35 unrelated families. We identified 28 different GAA gene mutations, including 7 novel mutations, by a GAA gene analysis. c.546G > T (22.9%) and c.1857C > G (14.3%) were the most common mutations and accounted for 37.1% of the total mutant alleles. In the six patients with infantile-onset Pompe disease (IOPD), c.1857C > G was also the most common mutation. In addition, there were 13 homozygotes (5 with the c.546G > T) among the 35 families, which is the highest frequency reported thus far. Regarding the initial symptoms, cardiomegaly was the most common (3/6 = 50%) in IOPD patients, while muscle weakness was observed the most frequently in patients with late-onset Pompe disease (LOPD) (15/30 = 50%). Notably, all IOPD patients who showed respiratory distress at the time of onset require respiratory assistance at present (4/4 = 100%). Regarding the presenting symptoms, cardiomegaly (6/6 = 100%) and hepatomegaly (4/6 = 66.7%) were more commonly seen in IOPD, and muscle weakness (24/29 = 82.7%) was observed more frequently in LOPD. Respiratory assistance is required at present in 33.3% of IOPD patients and 50% of LOPD patients, and 20% of IOPD patients and 29.6% of LOPD patients are wheelchair users. These individual clinical courses may be influenced by the timing of the diagnosis and treatment; for example, in 2007, an ERT orphan drug for treatment of Pompe disease, Alglucosidase alfa, was made available in Japan, and there were 5 (5/6 = 83.3%) wheelchair users diagnosed from 2008 to 2009 (cases 32–38) and 4 (4/27 = 14.8%) from 2010 to 2015 (cases 1–31). These findings underscore the importance of the early diagnosis and treatment.

Evaluation prevalence of Pompe disease in Iranian patients with myopathies of unknown etiology

Background

Pompe disease is a rare but potentially treatable metabolic disorder having an estimated worldwide incidence of one in forty thousand live births. While the introduction of enzyme replacement therapy (ERT) has considerably increased the awareness of the disease, the delay in diagnosis is still consistent and most patients go undetected.

Objective

This study aimed to determine the prevalence of late-onset Pompe disease (LOPD) in a high-risk population, using dried blood spot (DBS) as a main screening tool.

Methods

This cross-sectional study was performed on the 93 patients who attended to the neuromuscular center of Bu-ali hospital in Tehran, Iran, during 2014–2015. Inclusion criteria were: 1) age ≥1 years, 2) proximal myopathies of unknown etiology in lower limbs or symptoms of limb girdle muscle weakness (LGMW), and 3) unexplained elevated CPK (>174). Acid α-glucosidase (GAA) activity was measured separately on DBS by fluorometric method. For the final diagnosis, GAA deficiency was confirmed by a biochemical assay in skeletal muscle, whereas genotype was assessed by GAA molecular analysis. All statistical tests were performed using the SPSS version 16. Results are presented as mean (SD) or median (IQR), as appropriate.

Results

In a 12-month period, we studied 93 cases: 5 positive samples (5.3%) were detected by DBS screening, biochemical and molecular genetic studies finally confirmed LOPD diagnosis in 3 cases (3.22%). Among the 93 patients, 100% showed hyperCKemia, 89 patients (95.7%) showed LGMW and 4 patients had symptoms of proximal myopathies in the lower limb.

Conclusions

Results from the LOPED study suggest that GAA activity requires accurate screening by DBS in all patients referred for hyperCKemia and/or LGMW.

Mass Spectrometry but Not Fluorimetry Distinguishes Affected and Pseudodeficiency Patients in Newborn Screening for Pompe Disease

BACKGROUND

Deficiency of the lysosomal enzyme acid α-glucosidase (GAA) causes Pompe disease. Newborn screening for Pompe disease is ongoing, and improved methods for distinguishing affected patients from those with pseudodeficiency, especially in the Asian population, would substantially reduce the number of patient referrals for clinical follow-up.

METHODS

We measured the enzymatic activity of GAA in dried blood spots on newborn screening cards (DBS) using a tandem mass spectrometry (MS/MS) assay. The assay displayed a relatively large analytical range compared to the fluorimetric assay with 4-methylumbelliferyl-α-glucoside. DBS from newborns confirmed to have infantile-onset Pompe disease (IOPD, n = 11) or late-onset Pompe disease (LOPD) (n = 12) and those from patients bearing pseudodeficiency alleles with or without Pompe mutations, or Pompe disease carriers (n = 230) were studied.

RESULTS

With use of the MS/MS GAA assay in DBS, 96% of the pseudodeficiency newborns and all of the Pompe disease carriers were well separated from the IOPD and LOPD newborns. The fluorimetric assay separated <10% of the pseudodeficiencies from the IOPD/LOPD group.

CONCLUSIONS

The relatively large analytical range MS/MS GAA assay but not the fluorimetric assay in DBS provides a robust approach to reduce the number of referrals and should dramatically facilitate newborn screening of Pompe disease.

The Initial Evaluation of Patients After Positive Newborn Screening: Recommended Algorithms Leading to a Confirmed Diagnosis of Pompe Disease

Newborn screening (NBS) for Pompe disease is done through analysis of acid α-glucosidase (GAA) activity in dried blood spots. When GAA levels are below established cutoff values, then second-tier testing is required to confirm or refute a diagnosis of Pompe disease. This article in the "Newborn Screening, Diagnosis, and Treatment for Pompe Disease" guidance supplement provides recommendations for confirmatory testing after a positive NBS result indicative of Pompe disease is obtained. Two algorithms were developed by the Pompe Disease Newborn Screening Working Group, a group of international experts on both NBS and Pompe disease, based on whether DNA sequencing is performed as part of the screening method. Using the recommendations in either algorithm will lead to 1 of 3 diagnoses: classic infantile-onset Pompe disease, late-onset Pompe disease, or no disease/not affected/carrier. Mutation analysis of the GAA gene is essential for confirming the biochemical diagnosis of Pompe disease. For NBS laboratories that do not have DNA sequencing capabilities, the responsibility of obtaining sequencing of the GAA gene will fall on the referral center. The recommendations for confirmatory testing and the initial evaluation are intended for a broad global audience. However, the Working Group recognizes that clinical practices, standards of care, and resource capabilities vary not only regionally, but also by testing centers. Individual patient needs and health status as well as local/regional insurance reimbursement programs and regulations also must be considered.

An 18-Month-Old Child with Infantile Pompe Disease: Oral Signs

We aim to create an information platform by contributing orodental findings of Pompe disease to literature. An 18-month-old male patient with Pompe disease was referred to our clinic due to swelling of the gums. In first dental examination, a nonfluctuant, normal gingiva colored swelling at the right anterior region of maxilla was detected. His parents were recommended to perform finger massage to the region. Six months later, 51, 52, 62, and 74 numbered teeth had erupted, there was a fusion between 51 and 52 numbered teeth, 84 numbered tooth was seen to be erupted, and a swelling at the site of this tooth, similar to previous one, was present. Finger massage was recommended for this area as well, and the swelling was found to have decreased at the follow-up, one week later. Tooth eruption problems and developmental dental abnormalities should be included in the signs for Pompe disease.

Clinical Analysis of Algerian Patients with Pompe Disease

Pompe's disease is a metabolic myopathy caused by a deficiency of acid alpha-glucosidase (GAA), also called acid maltase, an enzyme that degrades lysosomal glycogen. The clinical presentation of Pompe's disease is variable with respect to the age of onset and rate of disease progression. Patients with onset of symptoms in early infancy (infantile-onset Pompe disease (IOPD)) typically exhibit rapidly progressive hypertrophic cardiomyopathy and marked muscle weakness. Most of them die within the first year of life from cardiac and/or respiratory failure. In the majority of cases of Pompe's disease, onset of symptoms occurs after infancy, ranging widely from the first to sixth decade of life (late-onset Pompe's disease or LOPD). Progression of the disease is relentless and patients eventually progress to loss of ambulation and death due to respiratory failure. The objective of this study was to characterize the clinical presentation of 6 patients (3 with EOPD and the other 3 with LOPD) of 5 families from the East of Algeria. All our patients were diagnosed as having Pompe's disease based on biochemical confirmations of GAA deficiency by dried blood spots (DBS) and GAA gene mutations were analyzed in all patients who consented (n = 4). Our results are similar to other ethnic groups.

Alglucosidase alfa enzyme replacement therapy as a therapeutic approach for a patient presenting with a PRKAG2 mutation

OBJECTIVE

PRKAG2 syndrome, an autosomal dominant disorder, is characterized by severe infantile hypertrophic cardiomyopathy and heart rhythm disturbances to cases with a later presentation and a spectrum of manifestations including cardiac manifestations, myopathy and seizures. The cardiac features of PRKAG2 resemble the cardiac manifestations of Pompe disease. We present a patient who was initially diagnosed with Pompe disease and treated with alglucosidase-alfa enzyme replacement therapy (ERT); however, he was eventually diagnosed to carrying a PRKAG2 pathogenic gene mutation; he did not have Pompe disease instead he was a carrier for the common adult leaky splice site mutation in the GAA gene.

CASE REPORT

At 2.5months, the patient had hypotonia/generalized muscle weakness, a diagnosis of non-classic infantile Pompe disease was made based on low acid alpha-glucosidase activity and the patient started on ERT at 11months. However, 1month later, the patient began to have seizures. As the patient's medical history was somewhat unusual for infantile Pompe disease, further evaluation was initiated and included a glycogen storage disease sequencing panel which showed that the patient had a pathogenic mutation in PRKAG2 which had been reported previously. ERT was discontinued and patient had a progression of motor deficits. ERT was reinitiated by the treating physician, and a clinical benefit was noted.

CONCLUSION

This report outlines the benefits of ERT with alglucosidase alfa in a patient with PRKAG2 syndrome, the decline in his condition when the ERT infusions were discontinued, and the significant positive response when ERT was reinitiated.

Inherited Metabolic Disorders: Efficacy of Enzyme Assays on Dried Blood Spots for the Diagnosis of Lysosomal Storage Disorders

High consanguinity rates, poor access to accurate diagnostic tests, and costly therapies are the main causes of increased burden of lysosomal storage disorders (LSDs) in developing countries. Therefore, there is a major unmet need for accurate and economical diagnostic tests to facilitate diagnosis and consideration of therapies before irreversible complications occur. In cross-country study, we utilized dried blood spots (DBS) of 1,033 patients clinically suspected to harbor LSDs for enzymatic diagnosis using modified fluorometric assays from March 2013 through May 2015. Results were validated by demonstrating reproducibility, testing in different sample types (leukocytes/plasma/skin fibroblast), mutation study, or measuring specific biomarkers. Thirty percent (307/1,033) were confirmed to have one of the LSDs tested. Reference intervals established unambiguously identified affected patients. Correlation of DBS results with other biological samples (n = 172) and mutation studies (n = 74) demonstrated 100% concordance in Gaucher, Fabry, Tay Sachs, Sandhoff, Niemann-Pick, GM1, Neuronal ceroid lipofuscinosis (NCL), Fucosidosis, Mannosidosis, Mucopolysaccharidosis (MPS) II, IIIb, IVa, VI, VII, and I-Cell diseases, and 91.4% and 88% concordance in Pompe and MPS-I, respectively. Gaucher and Pompe are the most common LSDs in India and Pakistan, followed by MPS-I in both India and Sri Lanka. Study demonstrates utility of DBS for reliable diagnosis of LSDs. Diagnostic accuracy (97.6%) confirms veracity of enzyme assays. Adoption of DBS will overcome significant hurdles in blood sample transportation from remote regions. DBS enzymatic and molecular diagnosis should become the standard of care for LSDs to make timely diagnosis, develop personalized treatment/monitoring plan, and facilitate genetic counseling.

Search for Pompe disease among patients with undetermined myopathies

OBJECTIVE

Pompe disease is a rare treatable glycogen storage disease with in adults - a limb-girdle muscle weakness. Muscle biopsy may fail to show the typical vacuolar myopathy. We asked if we had un-diagnosed patients with Pompe disease in western Sweden.

MATERIAL AND METHODS

We searched the muscle biopsy registry during the time period 1986 until 2006 including 3665 biopsies and included patients at our Neuromuscular Center with unspecified myopathy or limb-girdle muscular dystrophy. The dry blood spot test was used to identify patients with Pompe disease.

RESULTS

A total of 82 patients (46 from the biopsy register and 36 from our center) were seen and dry blood spot test was obtained. No patient with Pompe disease was found. The dry blood spot test was low in three cases (11, 16, and 18% of normal) but a second blood sample showed a normal result based on GAA enzyme activity in lymphocytes in all three patients. In one patient with low normal result of the analysis in lymphocytes a genetic test showed no pathogenic mutations. Further investigation gave a definite diagnose of another myopathy in 12 patients.

CONCLUSIONS

The prevalence of Pompe disease in western Sweden (3 in 1.27 million or 0.24 per 100.000 inhabitants) is lower than in the Netherlands and New York. Re-evaluation of patients with myopathies but without definite diagnosis is rewarding since 12 of 82 patients in our study had a definite molecular diagnosis after workup.

Review and evaluation of the methodological quality of the existing guidelines and recommendations for inherited neurometabolic disorders

Background

Inherited neurometabolic disorders (iNMDs) represent a group of almost seven hundred rare diseases whose common manifestations are clinical neurologic or cognitive symptoms that can appear at any time, in the first months/years of age or even later in adulthood. Early diagnosis and timely treatments are often pivotal for the favorable course of the disease. Thus, the elaboration of new evidence-based recommendations for iNMD diagnosis and management is increasingly requested by health care professionals and patients, even though the methodological quality of existing guidelines is largely unclear. InNerMeD-I-Network is the first European network on iNMDs that was created with the aim of sharing and increasing validated information about diagnosis and management of neurometabolic disorders. One of the goals of the project was to determine the number and the methodological quality of existing guidelines and recommendations for iNMDs.

Methods

We performed a systematic search on PubMed, the National Guideline Clearinghouse (NGC), the Guidelines International Network (G-I-N), the Scottish Intercollegiate Guideline Network (SIGN) and the National Institute for Health and Care Excellence (NICE) to identify all the published guidelines and recommendations for iNMDs from January 2000 to June 2015. The methodological quality of the selected documents was determined using the AGREE II instrument, an appraisal tool composed of 6 domains covering 23 key items.

Results

A total of 55 records met the inclusion criteria, 11 % were about groups of disorders, whereas the majority encompassed only one disorder. Lysosomal disorders, and in particular Fabry, Gaucher disease and mucopolysaccharidoses where the most studied. The overall methodological quality of the recommendation was acceptable and increased over time, with 25 % of the identified guidelines strongly recommended by the appraisers, 64 % recommended, and 11 % not recommended. However, heterogeneity in the obtained scores for each domain was observed among documents covering different groups of disorders and some domains like 'stakeholder involvement' and 'applicability' were generally scarcely addressed.

Conclusions

Greater efforts should be devoted to improve the methodological quality of guidelines and recommendations for iNMDs and AGREE II instrument seems advisable for new guideline development. The elaboration of new guidelines encompassing still uncovered disorders is badly needed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-015-0376-9) contains supplementary material, which is available to authorized users.

Guidelines for the diagnosis, treatment and clinical monitoring of patients with juvenile and adult Pompe disease

ABSTRACT Pompe disease (PD) is a potentially lethal illness involving irreversible muscle damage resulting from glycogen storage in muscle fiber and activation of autophagic pathways. A promising therapeutic perspective for PD is enzyme replacement therapy (ERT) with the human recombinant enzyme acid alpha-glucosidase (Myozyme®). The need to organize a diagnostic flowchart, systematize clinical follow-up, and establish new therapeutic recommendations has become vital, as ERT ensures greater patient longevity. A task force of experienced clinicians outlined a protocol for diagnosis, monitoring, treatment, genetic counseling, and rehabilitation for PD patients. The study was conducted under the coordination of REBREPOM, the Brazilian Network for Studies of PD. The meeting of these experts took place in October 2013, at L’Hotel Port Bay in São Paulo, Brazil. In August 2014, the text was reassessed and updated. Given the rarity of PD and limited high-impact publications, experts submitted their views.

Diagnosis and treatment of late-onset Pompe disease in the Middle East and North Africa region: consensus recommendations from an expert group

Background

Pompe disease is a rare autosomal recessive disorder caused by a deficiency of the lysosomal enzyme alpha-glucosidase responsible for degrading glycogen. Late-onset Pompe disease has a complex multisystem phenotype characterized by a range of symptoms.

Methods

An expert panel from the Middle East and North Africa (MENA) region met to create consensus-based guidelines for the diagnosis and treatment of late-onset Pompe disease for the MENA region, where the relative prevalence of Pompe disease is thought to be high but there is a lack of awareness and diagnostic facilities.

Results

These guidelines set out practical recommendations and include algorithms for the diagnosis and treatment of late-onset Pompe disease. They detail the ideal diagnostic workup, indicate the patients in whom enzyme replacement therapy should be initiated, and provide guidance on appropriate patient monitoring.

Conclusions

These guidelines will serve to increase awareness of the condition, optimize patient diagnosis and treatment, reduce disease burden, and improve patient outcomes.

Delayed diagnosis of late-onset Pompe disease in patients with myopathies of unknown origin and/or hyperCKemia

Pompe disease is a rare metabolic myopathy whose diagnosis is sometimes delayed despite being essential for improving clinical outcomes. We aimed to investigate the prevalence of late-onset Pompe disease among patients with a myopathy of unknown etiology, including polymyositis, or with idiopathic rise of creatine kinase (CK) levels, in a department of internal medicine. A cohort study was conducted in 241 subjects: 140 patients with myopathies of unknown origin or increased CK levels, 30 with polymyositis and 71 who constituted the control group of other myopathies. Acid α-glucosidase (GAA) activity was tested in dried blood spots. If a positive result was obtained, GAA activity in isolated lymphocytes and/or genetic testing was performed as a confirmatory diagnosis. Out of the 140 investigated patients, 2 patients with myopathies of unknown origin were confirmed to be positive for Pompe disease. Thus, late-onset Pompe disease should be considered among adult patients with myopathy of unknown origin.

Novel GAA mutations in patients with Pompe disease

Pompe disease is an autosomal recessive disorder linked to GAA gene that leads to a multi-system intralysosomal accumulation of glycogen. Mutation identification in the GAA gene can be very important for early diagnosis, correlation between genotype-phenotype and therapeutic intervention. For this purpose, peripheral blood from 57 individuals susceptible to Pompe disease was collected and all exons of GAA gene were amplified; the sequences and the mutations were analyzed in silico to predict possible impact on the structure and function of the human protein. In this study, 46 individuals presented 33 alterations in the GAA gene sequence, among which five (c.547-67C>G, c.547-39T>G, p.R437H, p.L641V and p.L705P) have not been previously described in the literature. The alterations in the coding region included 15 missense mutations, three nonsense mutations and one deletion. One insertion and other 13 single base changes were found in the non-coding region. The mutation p.G611D was found in homozygosis in a one-year-old child, who presented low levels of GAA activity, hypotonia and hypertrophic cardiomyopathy. Two patients presented the new mutation p.L705P in association with c.-32-13T>G. They had low levels of GAA activity and developed late onset Pompe disease. In our study, we observed alterations in the GAA gene originating from Asians, African-Americans and Caucasians, highlighting the high heterogeneity of the Brazilian population. Considering that Pompe disease studies are not very common in Brazil, this study will help to better understand the potential pathogenic role of each change in the GAA gene. Furthermore, a precise and early molecular analysis improves genetic counseling besides allowing for a more efficient treatment in potential candidates.

Newborn screening for lysosomal storage diseases

BACKGROUND

There is worldwide interest in newborn screening for lysosomal storage diseases because of the development of treatment options that give better results when carried out early in life. Screens with high differentiation between affected and nonaffected individuals are critical because of the large number of potential false positives.

CONTENT

This review summarizes 3 screening methods: (a) direct assay of enzymatic activities using tandem mass spectrometry or fluorometry, (b) immunocapture-based measurement of lysosomal enzyme abundance, and (c) measurement of biomarkers. Assay performance is compared on the basis of small-scale studies as well as on large-scale pilot studies of mass spectrometric and fluorometric screens.

SUMMARY

Tandem mass spectrometry and fluorometry techniques for direct assay of lysosomal enzymatic activity in dried blood spots have emerged as the most studied approaches. Comparative mass spectrometry vs fluorometry studies show that the former better differentiates between nonaffected vs affected individuals. This in turn leads to a manageable number of screen positives that can be further evaluated with second-tier methods.

Methods of diagnosis of patients with Pompe disease: Data from the Pompe Registry

Pompe disease is a rare, autosomal recessive disorder characterized by deficiency of lysosomal acid alpha-glucosidase and accumulation of lysosomal glycogen in many tissues. The variable clinical manifestations, broad phenotypic spectrum, and overlap of signs and symptoms with other neuromuscular diseases make diagnosis challenging. In the past, the diagnosis of Pompe disease was based on enzyme activity assay in skin fibroblasts or muscle tissue. In 2004, methods for measuring acid alpha-glucosidase activity in blood were published. To compare how diagnostic methods changed over time and whether they differed by geographic region and clinical phenotype, we examined diagnostic methods used for 1059 patients enrolled in the Pompe Registry in three onset categories (Group A: onset of signs/symptoms ≤ 12 months of age with cardiomyopathy; Group B: onset ≤ 12 months without cardiomyopathy and onset >1 year to ≤ 12 years; Group C: onset >12 years). Enzyme activity-based assays were used more frequently than other diagnostic methods. Measuring acid alpha-glucosidase activity in blood (leukocytes, lymphocytes, or dried-blood spot) increased over time; use of muscle biopsy decreased. The increased use of blood-based assays for diagnosis may result in a more timely diagnosis in patients across the clinical spectrum of Pompe disease.

Screening for late-onset Pompe disease in Finland

Pompe disease (glycogen storage disease type II) is caused by autosomal recessive mutations in GAA gene. The estimated frequency of late-onset Pompe disease is around 1:60,000. However, only two infantile and one late-onset Pompe patients have been reported in Finland with a population of 5 million. We screened for late-onset Pompe disease in a cohort of undetermined myopathy patients with proximal muscle weakness and/or elevated serum creatine kinase values. Acid α-glucosidase (GAA) activity in dried blood spots was measured and clinical data collected in 108 patients. Four patients had low normal GAA activity; all the others had activities well within the normal range. Re-analyses of these patients did not reveal new Pompe patients. Our findings suggest that Pompe disease is extremely rare in Finland. Finland is an example of an isolated population with enrichment of certain mutations for genetic disorders and low occurrence of some autosomal recessive diseases.

Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery

The Arf small GTP-binding (G) proteins regulate membrane traffic and organelle structure in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. The first function identified for Arf proteins was recruitment of cytosolic coat complexes to membranes to mediate vesicle formation. However, subsequent studies have uncovered additional functions, including roles in plasma membrane signalling pathways, cytoskeleton regulation, lipid droplet function, and non-vesicular lipid transport. In contrast to other families of G proteins, there are only a few Arf proteins in each organism, yet they function specifically at many different cellular locations. Part of this specificity is achieved by formation of complexes with their guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP binding and hydrolysis, respectively. Because these regulators outnumber their Arf substrates by at least 3-to-1, an important aspect of understanding Arf function is elucidating the mechanisms by which a single Arf protein is incorporated into different GEF, GAP, and effector complexes. New insights into these mechanisms have come from recent studies showing GEF–effector interactions, Arf activation cascades, and positive feedback loops. A unifying theme in the function of Arf proteins, carried out in conjunction with their regulators and effectors, is sensing and modulating the properties of the lipids that make up cellular membranes.

Glycogenosome accumulation in the arrector pili muscle in Pompe disease

BACKGROUND

Glycogenosis type II or Pompe disease is an autosomal-recessive lysosomal storage disease due to mutations in the gene encoding acid alpha-glucosidase (GAA), an enzyme required for lysosomal glycogen degradation. The disease predominantly affects the skeletal and respiratory muscles but there is growing evidence of the involvement of smooth muscle cells in blood vessel walls, suggesting a multi-system disorder. Moreover, the failure of autophagy in Pompe disease could contribute to muscular atrophy and disease progression and is thought to compromise the efficacy of enzyme replacement therapy (ERT).

METHODS

We investigated the light microscopical and ultrastructural pathology of the arrector pili muscle from punch skin biopsies from the calf of 6 adult Pompe disease patients and 6 age and gender matched healthy controls. Two patients had a follow-up biopsy after 19 and 20 month of ERT.

RESULTS

The electron microscopic investigation of patient biopsies revealed the widespread occurrence of glycogenosomes, membrane bound accumulations of granular glycogen, associated with autophagic vacuoles. In the controls we detected only muscle cells with non-membrane bound forms of glycogen. These morphological changes in smooth muscle cells are similar to those seen in skeletal muscle and smooth muscle cells of arterioles of Pompe patients. Furthermore, two patients with pre- and post-ERT skin biopsies showed a decrease in the number of cells with extensive autophagy after treatment.

CONCLUSIONS

Electron microscopic examination of the arrector pili muscles appears to be a surrogate marker for the involvement of smooth muscles reflecting disease severity. These findings suggest that the standardized and widely used skin biopsy could offer a minimally invasive way to screen for smooth muscle involvement and warrant further studies in larger cohorts of patients.

Timing of diagnosis of patients with Pompe disease: data from the Pompe registry

Diagnostic delays in Pompe disease are common. The diagnostic gap (the time from the onset of symptoms to the diagnosis of Pompe disease) and factors associated with diagnostic delays were examined among Pompe Registry patients in three onset categories: Group A, onset ≤12 months of age with cardiomyopathy; Group B, onset ≤12 months without cardiomyopathy and onset >12 months to ≤12 years; and Group C, onset >12 years. Of 1,003 patients, 647 were available for analysis. In all groups, musculoskeletal signs and symptoms were among the most frequent presenting signs and symptoms, in addition to cardiomyopathy in Group A, which was part of the group's definition. Diagnostic gaps existed in all three groups. Patients presenting with respiratory and musculoskeletal signs and symptoms concurrently had the shortest diagnostic gap, while those presenting with neither respiratory nor musculoskeletal signs and symptoms had the longest. Independent factors influencing the probability of a long diagnostic gap included presenting signs and symptoms (all three groups) and year of diagnosis and age at symptom onset (Groups B and C). Group B, which represents the infantile patients without cardiomyopathy and juvenile Pompe cases, had the longest median gap (12.6 years). Diagnostic testing methods used also were reviewed. Despite the availability of blood-based assays that can be used to quickly and accurately diagnose Pompe disease, diagnostic gaps in Pompe patients across the disease spectrum continue.