Determination of acid alpha-glucosidase protein: evaluation as a screening marker for Pompe disease and other lysosomal storage disorders.

BACKGROUND: In recent years, there have been significant advances in the development of enzyme replacement and other therapies for lysosomal storage disorders (LSDs). Early diagnosis, before the onset of irreversible pathology, has been demonstrated 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. One approach to the development of such a program is the identification of suitable screening markers. In this study, the acid alpha-glucosidase protein was evaluated as a marker protein for Pompe disease and potentially for other LSDs. METHODS: Two sensitive immunoquantification assays for the measurement of total (precursor and mature) and mature forms of acid alpha-glucosidase protein were used to determine the concentrations in plasma and dried blood spots from control and LSD-affected individuals. RESULTS: In the majority of LSDs, no significant increases above control values were observed. However, individuals with Pompe disease showed a marked decrease in acid alpha-glucosidase protein in both plasma and whole blood compared with unaffected controls. For plasma samples, this assay gave a sensitivity of 95% with a specificity of 100%. For blood spot samples, the sensitivity was 82% with a specificity of 100%. CONCLUSIONS: This study demonstrates that it is possible to screen for Pompe disease by screening the concentration of total acid alpha-glucosidase in plasma or dried blood spots.

Altered trafficking and turnover of LAMP-1 in Pompe disease-affected cells.

The lysosome-associated membrane protein (LAMP-1) is elevated in the cells and plasma from lysosomal storage disorder-affected individuals; however, the mechanism of this elevation is not well defined. In this study we have investigated the synthesis, glycoprocessing, trafficking, and turnover of LAMP-1 in human skin fibroblasts from Pompe disease patients and control individuals. There were similar levels of LAMP-1 synthesis in both cell types, but glycoprocessing was retarded in Pompe (T1/2 = 25 min) compared to control (T1/2 = 17 min) fibroblasts. There was also a marked delay in trafficking of LAMP-1 to lysosomes of Pompe (T1/2 = 200 min) compared to control (T1/2 = 100 min) cells. A proportion of newly synthesized LAMP-1 (5.4% in Pompe and 8.5% in controls) was trafficked out of the cell (T1/2 = 3.5 h in controls) and, although significantly smaller than the lysosomal form, still had a transmembrane domain and cytoplasmic tail. In contrast, a soluble lysosomal pool of LAMP-1 had no tail sequence, suggesting that it had been clipped from the membrane. In turnover studies, LAMP-1 was more stable in Pompe (T1/2 = 4.9 days) compared to control (T1/2 = 1. 6 days) cells, implying either reduced proteolysis or lysosomal function, in Pompe cells. These results indicate altered traffic and turnover of LAMP-1 in storage disorders and identify different intracellular and extracellular pools of soluble LAMP-1, suggesting alternative trafficking pathways.

Newborn screening for lysosomal storage disorders.

Lysosomal storage disorders (LSD) represent a group of over 40 distinct genetic diseases with a total incidence of approximately 1:7,000 births. Bone marrow transplantation and enzyme replacement therapy are currently in use for the treatment of some disorders and new forms of enzyme and gene replacement therapy are actively being researched. The effectiveness of these therapies, particularly for the LSD involving the central nervous system and bone pathology, will rely heavily upon the early diagnosis and treatment of the disorder, before the onset of irreversible pathology. In the absence of a family history the only practical way to detect these disorders will be by a newborn screening program. One common feature of these disorders is an increase in the number and size of lysosomes within the cell from approximately 1% to as much as 50% of total cellular volume. Associated with this, is a corresponding increase in some lysosomal proteins. We propose that the measurement of one or more of these proteins in blood spots taken from Guthrie cards, will form the basis of a newborn screening program, for the detection of all LSD. We have identified a number of lysosomal proteins as potential markers for LSD. The level of these proteins has been determined in blood spots taken from Guthrie cards and in plasma samples from over 300 LSD affected individuals representing 25 disorders. Based on these results we have proposed a strategy for a newborn screening program involving a two tier system, utilizing time resolved fluorescence immunoquantification of the protein markers in the first tier, followed by tandem mass spectrometry for the determination of stored substrates in the second tier assays.

Pilot neonatal screening program for lysosomal storage disorders, using lamp-1.

We have demonstrated that the lysosome associated membrane protein (LAMP-1) is elevated in plasma from approximately 70% of lysosomal storage disorder patients. As part of the development of a newborn screening program for lysosomal storage disorders we have developed a first tier screening assay based upon the level of LAMP-I in blood spots taken from newborn Guthrie cards. To determine the effectiveness of the first-tier marker a prospective pilot Guthrie neonatal screening program for the identification of LSD was commenced in April 1998. Prior to commencement of the pilot program ethical approval was obtained and information leaflets regarding the neonatal screening of LSD were distributed to parents at the time of their infant's Guthrie collection. The LAMP-1 assay utilizes a chicken polyclonal and a mouse monoclonal in a sandwich time resolved fluorescent immunoassay. LAMP-1 blood-spot calibrators and quality control specimens were developed and shown to be stable and reproducible. To date 11,183 infants have been screened using LAMP-1. The population distribution is described with a median and 98th percentile of 220pg/l whole blood and 483microg/l whole blood respectively. Acceptable CV% for intra and inter assay of 8.9% and 10% respectively were obtained.

Diagnosis of lysosomal storage disorders: evaluation of lysosome-associated membrane protein LAMP-1 as a diagnostic marker.

Early diagnosis of lysosomal storage disorders (LSDs), before the onset of irreversible pathologies, will be a key factor in the development of effective therapies for many of these disorders. Newborn screening offers a potential mechanism for the early detection of these disorders. From studies of both normal and LSD-affected human skin fibroblasts we identified the lysosome-associated membrane protein LAMP-1 as a potential diagnostic marker. We have developed a sensitive method for the quantification of this protein with a time-resolved fluorescence immunoassay. A soluble form of LAMP-1 was observed in plasma samples, and determination of 152 unaffected individuals gave a median value of 303 micrograms/L with the 5th and 95th percentile at 175 and 448 micrograms/L respectively. Plasma samples from 320 LSD-affected individuals representing 25 different disorders were assayed. We observed that 17 of the 25 disorder groups tested had > 88% of individuals above the 95th percentile of the control population, with 12 groups having 100% above the 95th percentile. Overall, 72% of patients had LAMP-1 concentrations above the 95th percentile of the unpartitioned control population. We suggest that LAMP-1 may be a useful marker in newborn screening for LSDs.

Lysosomal biogenesis in lysosomal storage disorders.

Lysosomal biogenesis is an orchestration of the structural and functional elements of the lysosome to form an integrated organelle and involves the synthesis, targeting, functional residence, and turnover of the proteins that comprise the lysosome. We have investigated lysosomal biogenesis during the formation and dissipation of storage vacuoles in two model systems. One involves the formation of sucrosomes in normal skin fibroblasts and the other utilizes storage disorder-affected skin fibroblasts; both of these systems result in an increase in the size and the number of lysosomal vacuoles. Lysosomal proteins, beta-hexosaminidase, alpha-mannosidase, N-acetylgalactosamine-4-sulfatase, acid phosphatase, and the lysosome-associated membrane protein, LAMP-1, were shown to be elevated between 2- and 28-fold above normal during lysosomal storage. Levels of mRNA for the lysosome-associated membrane proteins LAMP-1 and LAMP-2, N-acetylgalactosamine-4-sulfatase, and the 46- and 300-kDa mannose-6-phosphate receptors were also elevated 2- to 8-fold. The up-regulation of protein and mRNA lagged 2-4 days behind the formation of lysosomal storage vacuoles. Correction of storage, in both systems, resulted in the rapid decline of the mRNA to basal levels, with a slower decrease in the levels of lysosomal proteins. Lysosomal biogenesis in storage disorders is shown to be a regulated process which is partially controlled at, or prior to, the level of mRNA. Although lysosomal proteins were differentially regulated, the coordination of these events in lysosomal biogenesis would suggest that a common mechanism(s) may be in operation.