N.A.M. van der Beek

Broad spectrum of Pompe disease in patients with the same c.-32-13T -> G haplotype

Background: Pompe disease (acid maltase deficiency, glycogen storage disease type II; OMIM 232300) is an autosomal recessive lysosomal storage disorder characterized by acid α-glucosidase deficiency due to mutations in the GAA gene. Progressive skeletal muscle weakness affects motor and respiratory functions and is typical for all forms of Pompe disease. Cardiac hypertrophy is an additional fatal symptom in the classic infantile subtype. c.-32-13T→G is the most common mutation in adults. Objective: To delineate the disease variation among patients with this mutation and to define the c.-32-13T→G haplotypes in search for genotype–phenotype correlations. Methods: We studied 98 compound heterozygotes with a fully deleterious mutation (11 novel mutations are described) and the common c.-32-13T→G mutation. Results: All patients were Caucasian. None had the classic infantile form of Pompe disease. The clinical course varied far more than anticipated (age at diagnosis <1 to 78 years; age at onset: <1 to 52 years). The acid α-glucosidase activities in a subset of patients ranged from 4 to 19.9 nmol/mg/h. Twelve different c.-32-13T→G haplotypes were identified based on 17 single-nucleotide polymorphisms located in the GAA gene. In 76% of the cases, c.-32-13T→G was encountered in the second most common GAA core haplotype (DHRGEVVT). In only one case was c.-32-13T→G encountered in the major GAA core haplotype (DRHGEIVT). Conclusion: Patients with the same c.-32-13T→G haplotype (c.q. GAA genotype) may manifest first symptoms at different ages, indicating that secondary factors may substantially influence the clinical course of patients with this mutation.

Effect of enzyme therapy in juvenile patients with Pompe disease: A three-year open-label study

Pompe disease is a rare neuromuscular disorder caused by deficiency of acid α-glucosidase. Treatment with recombinant human α-glucosidase recently received marketing approval based on prolonged survival of affected infants. The current open-label study was performed to evaluate the response in older children (age 5.9-15.2 years). The five patients that we studied had limb-girdle muscle weakness and three of them also had decreased pulmonary function in upright and supine position. They received 20-mg/kg recombinant human α-glucosidase every two weeks over a 3-year period. No infusion-associated reactions were observed. Pulmonary function remained stable (n = 4) or improved slightly (n = 1). Muscle strength increased. Only one patient approached the normal range. Patients obtained higher scores on the Quick Motor Function Test. None of the patients deteriorated. Follow-up data of two unmatched historical cohorts of adults and children with Pompe disease were used for comparison. They showed an average decline in pulmonary function of 1.6% and 5% per year. Data on muscle strength and function of untreated children were not available. Further studies are required.

The Rasch-built Pompe-specific Activity (R-PAct) scale

We constructed a patient-based interval scale using Rasch analysis, specifically suited to quantify the effects of Pompe disease on patients ability to carry out daily life activities and their social participation: Rasch-built Pompe-specific Activity scale. Between July 2005 and April 2011, 186 patients aged 16 or older, participated to develop this scale. External construct validity was determined through correlations with the MRC sumscore and Rotterdam Handicap Scale. Furthermore, test-retest reliability was determined in a subgroup of 44 patients. Finally, individual person-level responsiveness was used to determine the proportion of patients demonstrating significant improvement or deterioration during their natural disease course, or during treatment with enzyme replacement therapy. Of the original 49 items, 31 were removed after investigation of model fit, internal reliability, threshold examination, item bias, and local dependency. The remaining 18 items were ordered on a linearly weighted scale and demonstrated good discriminative ability (Person Separation Index 0.96), external construct validity (intraclass correlation coefficient (ICC) for MRC sumscore 0.82, and for the Rotterdam handicap scale 0.86), reliability of persons location (ability comparison: ICC 0.95), and responsiveness. We therefore conclude that the R-PAct scale enables us to accurately detect limitations in activities and social participation throughout the entire disease spectrum in patients with Pompe disease.

Enzyme analysis for Pompe disease in leukocytes; superior results with natural substrate compared with artificial substrates

SummaryEnzyme analysis for Pompe disease in leukocytes has been greatly improved by the introduction of acarbose, a powerful inhibitor of interfering α-glucosidases, which are present in granulocytes but not in lymphocytes. Here we show that the application of acarbose in the enzymatic assay employing the artificial substrate 4-methylumbelliferyl-α-d-glucoside (MU-αGlc) is insufficient to clearly distinguish patients from healthy individuals in all cases. Also, the ratios of the activities without/with acarbose only marginally discriminated Pompe patients and healthy individuals. By contrast, when the natural substrate glycogen is used, the activity in leukocytes from patients (n = 82) with Pompe disease is at most 17% of the lowest control value. The use of artificial substrate in an assay with isolated lymphocytes instead of total leukocytes is a poor alternative as blood samples older than one day invariably yield lymphocyte preparations that are contaminated with granulocytes. To diagnose Pompe disease in leukocytes we recommend the use of glycogen as substrate in the presence of acarbose. This assay unequivocally excludes Pompe disease. To also exclude pseudo-deficiency of acid α-glucosidase caused by the sequence change c.271G>A (p.D91N or GAA2; homozygosity in approximately 1:1000 caucasians), a second assay employing MU-αGlc substrate plus acarbose or DNA analysis is required.

Imaging of respiratory muscles in neuromuscular disease: A review

Respiratory muscle weakness frequently occurs in patients with neuromuscular disease. Measuring respiratory function with standard pulmonary function tests provides information about the contribution of all respiratory muscles, the lungs and airways. Imaging potentially enables the study of different respiratory muscles, including the diaphragm, separately. In this review, we provide an overview of imaging techniques used to study respiratory muscles in neuromuscular disease. We identified 26 studies which included a total of 573 patients with neuromuscular disease. Imaging of respiratory muscles was divided into static and dynamic techniques. Static techniques comprise chest radiography, B-mode (brightness mode) ultrasound, CT and MRI, and are used to assess the position and thickness of the diaphragm and the other respiratory muscles. Dynamic techniques include fluoroscopy, M-mode (motion mode) ultrasound and MRI, used to assess diaphragm motion in one or more directions. We discuss how these imaging techniques relate with spirometric values and whether these can be used to study the contribution of the different respiratory muscles in patients with neuromuscular disease.

G.P.129

Severe pulmonary dysfunction is a serious threat to patients with Pompe disease, a treatable metabolic neuromuscular disorder caused by lysosomal acid α-glucosidase deficiency. This pulmonary dysfunction - which is particularly severe in the supine position – is mainly caused by diaphragmatic weakness. Standard pulmonary function tests provide only indirect information about diaphragmatic function, and they do not supply information about chest mechanics in detail. We therefore used cine-MRI to examine the dynamic performance of respiratory muscles, and compared these data with the results of simultaneously performed pulmonary function testing. Ten adult Pompe patients and six healthy volunteers participated. We performed two static scans at end-inspiration and end-expiration to evaluate lung anatomy and lung volumes. Three dynamic 3D acquisitions were performed to investigate overall respiratory dynamics. Using manual segmentation of the acquired images, three length ratios were calculated. Diaphragmatic displacement manifests itself by motion in cranio-caudal direction, while movement in antero-posterior and left–right directions reflects chest wall displacement. Pompe patients have a significantly reduced cranio-caudal length ratio compared to healthy volunteers (p

P.17.1 Phenotypic variation within 22 families with Pompe disease

Pompe disease is an autosomal recessive disorder caused by a deficiency of acid α -glucosidase (GAA) leading to glycogen accumulation in different tissues. Pompe disease has a broad clinical spectrum, in which the phenotype can vary widely, even in patients with a similar GAA genotype. The aim of this study was to describe phenotypic variation among siblings with non-classic Pompe disease in the Netherlands. We identified 22 families consisting of two or three siblings (50 patients: 42 adults and eight children). All carried the most common mutation c.-32-13T>G in combination with another pathogenic mutation. Siblings typically all had symptom onset either in childhood or in adulthood, however, there was a wide variation in age of symptom onset between siblings (median difference of nine years). Presenting symptoms were similar across siblings in 14 families and limb girdle weakness was most frequently reported. In certain families ptosis, bulbar weakness or scapular winging were present in all siblings. The majority of wheelchair and/or ventilator dependent patients had an ambulant or non-ventilated sibling; half of these less affected siblings had a longer disease duration. Gender, GAA activity and co-morbidity did not appear to explain differences in phenotype between siblings. Since the course of disease and its severity in some families varied to the same extent as seen in unrelated patients with an identical genotype, other factors like epigenetic and environmental effects are likely to influence the clinical presentation and disease course. Additional studies are needed to identify these factors, as possible prognostic factors for disease course and outcome on ERT of an individual patient.

P3.57 Clinical features, disease progression and prognostic factors of muscle weakness in adults with Pompe disease

35%. The auditory system assessment included vocal and pure tone audiometry, transient evoked oto-acoustic emissions (TEOAE), impedenziometry and auditory brainstem responses (ABR). A combined interpretation of those tests let us to define the origin of the hearing deficit (conductive, cochlear or retro-cochlear). Traumatic, inflammatory, iatrogenic and otosurgical causes have been excluded. From the clinical point of view, all but one, patients denied subjective hearing disturbances. On the other hand, audiological evaluation revealed that 12/20 patients (60%) had an hearing impairment. Among them, 5 pts showed a conductive hearing loss, while 7 pts had a sensorineural hearing deficit (5 pts with a cochlear dysfunction and 2 pts with a retro-cochlear pathology) and just one had a mixed pattern. Our observations revealed that, in this group of late onset Pompe patients, the auditory impairment is often present (60% of Pompe patients) Our data emphasize the importance of monitoring the auditory function since childhood in all patients with Pompe disease.