alpha-glucosidase activity in human leucocytes: choice of lymphocytes for the diagnosis of Pompe's disease and the carrier state

Lysosomal storage disorders affecting the heart: a review

Lysosomal storage disorders (LSD) comprise a group of diseases caused by a deficiency of lysosomal enzymes, membrane transporters or other proteins involved in lysosomal biology. Lysosomal storage disorders result from an accumulation of specific substrates, due to the inability to break them down. The diseases are classified according to the type of material that is accumulated; for example, lipid storage disorders, mucopolysaccharidoses and glycoproteinoses. Cardiac disease is particularly important in lysosomal glycogen storage diseases (Pompe and Danon disease), mucopolysaccharidoses and in glycosphingolipidoses (Anderson-Fabry disease). Various disease manifestations may be observed including hypertrophic and dilated cardiomyopathy, coronary artery disease and valvular diseases. Endomyocardial biopsies can play an important role in the diagnosis of these diseases. Microscopic features along with ancillary tests like special stains and ultrastructural studies help in the diagnosis of these disorders. Diagnosis is further confirmed based upon enzymatic and molecular genetic analysis. Emerging evidence suggests that Enzyme replacement therapy (ERT) substantially improves many of the features of the disease, including some aspects of cardiac involvement. The identification of these disorders is important due to the availability of ERT, the need for family screening, as well as appropriate patient management and counseling.

Pompe disease: early diagnosis and early treatment make a difference

Pompe disease (glycogen storage disease type II or acid maltase deficiency) is a lysosomal disorder in which acid α-glucosidase (GAA) deficiencies lead to intralysosomal accumulation of glycogen in all tissues; most notably in skeletal muscles. Both the patient's age at the onset of Pompe disease symptoms and the rate of deterioration caused by the disease can vary considerably. In classical infant-onset Pompe disease (IOPD), symptoms start very early in life, and death occurs soon afterward if the disease remains untreated. In later-onset Pompe disease, symptoms are slower to appear, and patients often progress to wheelchair confinement and eventual respiratory failure. A diagnosis can be made by screening for GAA in dried blood samples, followed either by GAA assessment in lymphocytes or in fibroblasts or by the genetic analysis of mutations. Treatment by enzyme replacement therapy (ERT) with alglucosidase alfa was approved for human use in 2006. In classical IOPD, treatment significantly lengthens survival and improves motor development and cardiac function. The sooner ERT begins, the better are the results. Newborn screening aims to take advantage of different technologies for diagnosing and treating newborns early on and it yields better outcomes. However, newborns diagnosed early and other long-term survivors may encounter fresh problems, making up a new phenotype of IOPD patients. Further modifications of the treatment, such as a decrease in immune responses to ERT, a higher dosage, a better uptake formulation, and gene therapy delivered locally or systemically are being explored.

Optimization and validation of two miniaturized glucocerebrosidase enzyme assays for high throughput screening

Glucocerebrosidase (GC) catalyzes the hydrolysis of beta-glucocerebroside to glucose and ceramide in lysosomes. Mutations in the glucocerebrosidase gene (GBA) result in Gaucher disease, an autosomal recessive lysosomal storage disorder. Many of the mutations encountered in patients with Gaucher disease are missense alterations that may cause misfolding, decreased stability and/or mistrafficking of this lysosomal protein. Some inhibitors of GC have been shown to act as chemical chaperones, stabilizing the conformation of mutant proteins and thus restoring their function. High throughput screening (HTS) of small molecule libraries for such compounds with potential for chaperone therapy requires an accurate, reproducible and sensitive assay method. We have adapted and optimized two fluorogenic GC enzyme assays and miniaturized them into the 1536-well plate format for HTS. The two substrates, 4-methylumbelliferyl beta-D-glucopyranoside and resorufin beta-D-glucopyranoside, have K(m) values of 768 microM and 33 microM, respectively, and different emission spectra. Paired screening with the two assays helps to eliminate false inference of activity due to autofluorescence or fluorescence quenching by the screened compounds. Test screens with the LOPAC library indicated that both assays were robust for HTS, and gave comparable results for GC inhibitor activities. These two assays can be used to identify both GC activators and inhibitors with potential therapeutic value.

Rapid diagnosis of late-onset Pompe disease by fluorometric assay of alpha-glucosidase activities in dried blood spots

The enzymatic defect in Pompe disease is insufficient lysosomal acid alpha-glucosidase (GAA) activity which leads to lysosomal glycogen accumulation. We recently introduced a simple and reliable method to measure GAA activity in dried blood spots using Acarbose, a highly selective alpha-glucosidase inhibitor, to eliminate isoenzyme interference. Here we demonstrate that this method efficiently detects late-onset Pompe patients who are frequently misdiagnosed by conventional methods due to residual GAA activity in other tissue types.

A new diagnostic assay for glycogen storage disease type II in mixed leukocytes

We have established a new method for the enzymatic diagnosis of glycogen storage disease type II (Pompe disease or acid maltase deficiency) using mixed leukocytes. The method employs glycogen and 4-methylumbelliferyl-alpha-D-glucopyranoside (4MU-alphaGlc) as substrates for measuring the lysosomal acid alpha-glucosidase (acid alphaGlu) activity, and incorporates acarbose to eliminate the interference of unrelated alpha-glucosidases (predominantly maltase-glucoamylase). It is shown that 3.0 micromol/L acarbose completely inhibits the maltase-glucoamylase activity at pH 4.0, but the lysosomal acid alphaGlu activity by less than 5%. With this method, we determined the acid alphaGlu activity in mixed leukocytes from 25 patients with glycogen storage disease type II (2 infantile and 23 late-onset cases), one GAA2/GAA2 homozygote and 30 healthy subjects. In the assay with glycogen as substrate, the addition of acarbose created a clear separation between the patient and the control ranges. In the assay with 4MU-alphaGlc as substrate, the two ranges were fully separated but remained very close despite the use of acarbose. The separation of the patient and normal ranges was improved considerably by taking the ratio of acarbose-inhibited over uninhibited activity. A GAA2/GAA2 homozygote was correctly diagnosed with 4MU-alphaGlc but misdiagnosed as patient when glycogen was used as substrate. We conclude that the inclusion of 3.0 micromol/L acarbose in the assays with glycogen and 4MU-alphaGlc substrates at pH 4.0 allows for the specific measurement of lysosomal acid alphaGlu activity in mixed leukocytes, thus enabling a reliable diagnosis of glycogen storage disease type II in this specimen.

The use of acarbose inhibition in the measurement of acid alpha-glucosidase activity in blood lymphocytes for the diagnosis of Pompe disease

PURPOSE

Acid alpha-glucosidase is present in various tissues, including blood cells. Historically, enzyme measurement in cultured fibroblasts, or muscle, has been the gold standard to confirm a diagnosis of Pompe disease, due to the possibility of alternate isoenzyme activity masking disease in white cell assays. Enzyme measurement in an isolated lymphocyte population with acarbose, an inhibitor of neutral alpha-glucosidase, has greatly improved the sensitivity and specificity of the test in blood cells allowing for more rapid laboratory testing for Pompe disease.

METHODS

An assay for acid alpha-glucosidase was performed with and without inhibitor in lymphocytes from 14 patients with a clinical suspicion of infantile Pompe disease. Concurrent testing was performed in fibroblasts in an independent laboratory.

RESULTS

Thirteen of 14 patients demonstrated a clear deficiency in lymphocytes with acarbose inhibition. One patient was indeterminate, although below normal activity, suggesting the need for confirmatory testing. Tissue enzyme activity in all was consistent with infantile Pompe disease, and corroborated enzyme activity seen in lymphocytes. There were no false positives for disease, making the positive predictive value of lymphocyte enzyme testing 100%.

CONCLUSION

Enzyme assay using acarbose as an inhibitor, can be performed in isolated lymphocytes for rapid diagnosis of infantile Pompe disease.

Determination of Acid α-Glucosidase Activity in Blood Spots as a Diagnostic Test for Pompe Disease

Background: Pompe disease is an autosomal recessive disorder of glycogen metabolism that is characterized by a deficiency of the lysosomal acid α-glucosidase. Enzyme replacement therapy for the infantile and juvenile forms of Pompe disease currently is undergoing clinical trials. Early diagnosis before the onset of irreversible pathology is thought to be critical for maximum efficacy of current and proposed therapies. In the absence of a family history, the presymptomatic detection of these disorders ideally can be achieved through a newborn-screening program. Currently, the clinical diagnosis of Pompe disease is confirmed by the virtual absence, in infantile onset, or a marked reduction, in juvenile and adult onset, of acid α-glucosidase activity in muscle biopsies and cultured fibroblasts. These assays are invasive and not suited to large-scale screening.

Methods: A sensitive immune-capture enzyme activity assay for the measurement of acid α-glucosidase protein was developed and used to determine the activity of this enzyme in dried-blood spots from newborn and adult controls, Pompe-affected individuals, and obligate heterozygotes.

Results: Pompe-affected individuals showed an almost total absence of acid α-glucosidase activity in blood spots. The assay showed a sensitivity and specificity of 100% for the identification of Pompe-affected individuals.

Conclusions: The determination of acid α-glucosidase activity in dried-blood spots is a useful, noninvasive diagnostic assay for the identification of Pompe disease. With further validation, this procedure could be adapted for use with blood spots collected in newborn-screening programs.

Localization of lysosomal antigens in activated T-lymphocytes

The lysosomal compartment has been examined in activated T-lymphocytes by immunogold electron microscopy and subcellular fractionation. Immunoprecipitation and sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of radiolabelled extracts of the T-cells showed that they contained three antigens which are fundamental to normal lysosomal function: a representative lysosomal enzyme beta-glucuronidase, a lysosomal associated membrane protein (LAMP-1), and the cation-independent mannose 6-phosphate lysosomal enzyme targeting receptor (MPR). Immunogold labelling showed that beta-glucuronidase was present in the rough endoplasmic reticulum, the Golgi complex and Golgi-associated vesicles. The enzyme was also found to accumulate in distinct, non-Golgi organelles in which LAMP-1 was co-localized, probably lysosomes. LAMP-1 was also found in tubular elements of the Golgi and in a complex of vesicles clustered near the nucleus where MPR was also present at high density. Fractionation of homogenates from lymphocytes on Percoll gradients revealed that beta-glucuronidase was distributed throughout the low density region containing rough endoplasmic reticulum, Golgi and plasma membrane components, and the high density region which contained only lysosomal activity. Multiple immunogold electron microscopy of the latter fraction showed the presence of homogenous vesicles which had large amounts of beta-glucuronidase within the lumen, LAMP-1 at the periphery and no MPR. These vesicles were probably mature lysosomes, arising from pre-lysosomal organelles enriched for LAMP-1 and MPR.

The "muscular variant" of Pompe disease: clinical, biochemical and histologic characteristics

We report on a 2-yr-old boy with progressive muscular weakness and respiratory failure. There was no clinical evidence of heart muscle involvement. Autopsy showed marked intralysosomal glycogen deposition in skeletal muscle and liver with no histological evidence of glycogen deposition in cardiac muscle. The activity of the lysosomal enzyme alpha-1,4-glucosidase was deficient in skin fibroblasts, skeletal muscle, cardiac muscle, and liver; however, the enzymatic activity in peripheral blood leukocytes was in the low normal range. The child's mother had normal enzymatic activity in leukocytes but heterozygote levels in skin fibroblasts.

Diagnosis of Pompe's disease using leukocyte preparations. Kinetic and immunological studies of 1,4-alpha-glucosidase in human fetal and adult tissues and cultured cells

Kinetic and immunological studies of 1,4-alpha-glucosidase show that the distribution of acid, renal and neutral alpha-glucosidase at pH 4.0 and 6.5 is as follows: in liver and cultured fibroblasts and amniotic fluid cells the activity at pH 4.0 is mainly due to the acid enzyme. Even at pH 6.5, the activity is largely due to the residual activity of the acid enzyme. In kidney and leukocytes, however, the activity by acid enzyme at pH 4.0 represents only 30-60% of the total activity and the remaining activity is from renal enzyme. At pH 6.5, the activity is almost exclusively of renal enzyme. Renal alpha-glucosidase has a higher affinity for maltose (Km, 0.8 mmol/l) than acid enzyme, however; for glycogen acid enzyme shows the highest affinity (20.7 g/l). There is no significant difference in the kinetic characteristics of alpha-glucosidase between fetal and adult tissues. In kidney, however, a relative increase in renal enzyme to acid enzyme with age is found, i.e. in fetal kidney the alpha-glucosidase activity at pH 4.0 is more than twice that at pH 6.5, whereas in adult kidney, the activity ratio at pH 4.0-6.5 is approximately 1. Antibodies for human liver acid alpha-glucosidase decrease the alpha-glucosidase activity in normal leukocytes by 22-75% at pH 4.0 (0.54-3.8 nmol/min per mg protein). The decrease is significantly lower in patients with Pompe's disease (0-0.11 nmol/min per mg protein) as well as in their parents and some siblings (0.15-0.70).

Effects of mitogenic stimulation of lymphocytes on lysosomal enzyme activity

Changes in the activities of several lysosomal enzymes were studied during transformation of mouse spleen cells in vitro. The activity of beta-glucuronidase increased during culture in the presence of T or B-cell mitogens, and lymphoblasts contained higher levels of activity than did small, non-transformed lymphocytes. Moreover, lymphoblasts in well-transformed cultures had higher activities than those in poorly-transformed cultures. The activities of other lysosomal enzymes (N-acetyl-beta-glucosaminidase, alpha-mannosidase, beta-glucosidase) also increased during mitogenic stimulation, but each at different rates, although aryl sulphatase was unaffected. Such differences may be of importance when lymphocytes are used for diagnosis of inherited lysosomal deficiency diseases.

Late-onset acid maltase deficiency. Biochemical studies of leukocytes

The activities of acid and neutral maltase were measured in lymphocytes, granulocytes, and platelets isolated from controls and from 5 patients with late-onset acid maltase deficiency (AMD). Lymphocytes from patients had markedly decreased activity of acid maltase and elevated neutral/acid ratios. In granulocytes, acid maltase was also lower than in controls, but significant activity was retained at pH 4: neutral/acid ratios were consistently elevated. Normal platelets had low acid and high neutral maltase activities: both enzyme activities varied within wide ranges and patients could not be distinguished from controls. The variable proportion of different cell types in unfractionated leukocyte preparations may yield unreliable values when used for detection of AMD. However, lymphocytes isolated from 20 ml of blood provide a readily accessible and reliable source of tissue for accurate diagnosis.

Identification of heterozygotes for glycogenosis 2 (acid maltase deficiency)

In 21 obligate and 9 possible heterozygotes for acid maltase deficiency (AMD) (glycogenosis 2, Pompe's disease), different methods of identifying heterozygotes have been studied. Heterozygosity could not be demonstrated by physical examination, serum CPK assays, morphological examination of muscle biopsy (including light-microscopy, histochemistry and electron-microscopy), or by ultrastructural examination of a skin biopsy. Heterozygotes could be identified to a large, but still limited extent, by measuring the acid alpha-glucosidase activity in urine, cultivated fibroblasts, leucocytes, or skeletal muscle. Heterozygotes for the generalized from of AMD could not be distinguished from those for the muscular form. The limitations of heterozygote identification by means of enzyme assays are discussed, and some practical aspects for genetic counselling are mentioned.

Lysosomal acid hydrolases in lymphocytes of I-cell disease

Several lysosomal acid hydrolases were assayed in peripheral blood leukocytes from a patient with I-cell disease by the method of Hindman, J. and Cotlier, E. ((1972) Clin. Chem. 18, 971--975). The activities of lysosomal hydrolases in polymorphonuclear cells showed no significant differences between the patient, the parents, and normal controls, while lymphocytes from the patient exhibited the reduced activities of alpha- and beta-galactosidases, beta-glucuronidase, and N-acetyl-beta-glucosaminidase. After phytohemagglutinin-stimulated culture, lymphocytes from the patient showed a more definite reduction in the activities of these lysosomal enzymes; the activities of beta-glucuronidase, alpha-mannosidase, and N-acetyl-beta-glucosaminidase were reduced to about 20--30% of the activity in the phytohemagglutinin-stimulated control cultures, and the activities of alpha- and beta-galactosidases to 10% or less. Lymphocytes from the parents showed no significant difference in the activities of these enzymes from the controls, whether stimulated or not.