Investigating the suitability of high-resolution mass spectrometry for newborn screening: identification of hemoglobinopathies and ß-thalassemias in dried blood spots.

A fast and reliable method for the determination of hemoglobinopathies and thalassemias by high-resolution accurate mass spectrometry (HRAM/MS) is presented. The established method was verified in a prospective clinical study (HRAM/MS vs. high-pressure liquid chromatography [HPLC]) of 5335 de-identified newborn samples from the Hamburg area. The analytical method is based on a dual strategy using intact protein ratios for thalassemias and tryptic digest fragments for the diagnosis of hemoglobinopathies. Due to the minimal sample preparation and the use of flow injection, the assay can be considered as a high-throughput screening approach for newborn screening programs (2 min/sample). Using a simple dried blood spot (DBS) extraction (tryptic digest buffer), the following results were obtained: (1) a carrier incidence of 1:100 newborns (35 FAS, nine FAC, eight FAD and two FAE), and (2) no homozygous affected patient was detected. Using the HRAM/MS protocol, an unknown Hb mutation was identified and confirmed by genetic testing. In addition to greater specificity toward rare mutations and ß-thalassemia, the low price/sample (1-2€) as well as an automated data processing represent the major benefits of the described HRAM/MS method.

Diagnostic efficacy of the fluorometric determination of enzyme activity for Pompe disease from dried blood specimens compared with lymphocytes-possibility for newborn screening.

BACKGROUND: Pompe disease is a rare, autosomal-recessive disorder which results from a defect in the lysosomal enzyme acid alpha-glucosidase (GAA). The onset of this disease is highly variable, with infantile types being the most severe. Traditionally, lymphocytes, fibroblasts or muscle biopsies were necessary for enzyme activity measurement, because these materials do not express maltase-glucoamylase (MGA) that interferes with the assay. Recently, acarbose was found to inhibit MGA activity selectively, so that dried blood became accessible for GAA assessment. AIM: To evaluate the diagnostic efficacy of GAA measurement in dried blood specimens (DBSs) in comparison with lymphocytes. If DBSs provided reliable results, the diagnosis of Pompe disease could be facilitated, and high-throughput screening would become possible. METHODS AND RESULTS: GAA activity was measured in DBSs of known patients at pH 3.8 (with and without acarbose) and at pH 7.0. Additionally, lymphocytes were obtained from the same patients, and the enzyme activity was determined at pH 4 to pH 7. In total, seven infantile patients and 29 patients with late-onset variants were investigated. All patients were reliably identified by both methods. Furthermore, a simplified protocol was established for neonatal screening. CONCLUSION: The fluorometric technique for the assessment of GAA activity in DBS provides a reliable diagnosis for all variants of Pompe disease. The assay protocol could be simplified for neonatal screening, without increasing the false positive rate significantly or burdening the laboratory with time-consuming procedures.

At-Risk Testing for Pompe Disease Using Dried Blood Spots: Lessons Learned for Newborn Screening.

Pompe disease (GSD II) is an autosomal recessive disorder caused by deficiency of the lysosomal enzyme acid-α-glucosidase (GAA, EC 3.2.1.20), leading to generalized accumulation of lysosomal glycogen especially in the heart, skeletal, and smooth muscle, and the nervous system. It is generally classified based on the age of onset as infantile (IOPD) presenting during the first year of life, and late onset (LOPD) when it presents afterwards. In our study, a cohort of 13,627 samples were tested between January 2017 and December 2018 for acid-α-glucosidase (GAA, EC 3.2.1.20) deficiency either by fluorometry or tandem mass spectrometry (MS). Testing was performed for patients who displayed conditions of unknown etiology, e.g., CK elevations or cardiomyopathy, in the case of infantile patients. On average 8% of samples showed activity below the reference range and were further assessed by another enzyme activity measurement or molecular genetic analysis. Pre-analytical conditions, like proper drying, greatly affect enzyme activity, and should be assessed with measurement of reference enzyme(s). In conclusion, at-risk testing can provide a good first step for the future introduction of newborn screening for Pompe disease. It yields immediate benefits for the patients regarding the availability and timeliness of the diagnosis. In addition, the laboratory can introduce the required methodology and gain insights in the evaluation of results in a lower throughput environment. Finally, awareness of such a rare condition is increased tremendously among local physicians which can aid in the introduction newborn screening.

Validity of a rapid and simple fluorometric tripeptidyl peptidase 1 (TPP1) assay using dried blood specimens to diagnose CLN2 disease.

PURPOSE: CLN2 disease is a genetic disorder caused by dysfunction of the lysosomal enzyme tripeptidyl peptidase 1 (TPP1) that belongs to the neuronal ceroid lipofuscinoses (NCL) and leads to epilepsy, dementia, and death in young persons. CLN2 disease has recently become treatable by enzyme replacement, which can only be effective when the disease is diagnosed early. We have investigated the reliability of a test for TPP1 deficiency in dried blood specimens (DBS) to detect CLN2 disease. RESULTS: During a 12-year period we have received 3882 samples for testing TPP1. Quality of samples was checked by measuring two additional lysosomal enzyme activities. For 50 samples with subnormal TPP1 activity and good sample quality, we obtained adequate clinical and molecular genetic data. All 50 patients had doubtless evidence of CLN2 disease (including seven atypical patients) as shown by clinical findings and the presence of known pathogenic CLN2 variants. Our institution is a major reference center for NCL, and we have never received information that a patient with a normal DBS test was later diagnosed with CLN2 disease. CONCLUSIONS: We consider our TPP1 test on DBS to be a reliable, convenient and inexpensive tool for a first diagnostic step in suspected CLN2 disease.

Prevalence of Pompe disease in 3,076 patients with hyperCKemia and limb-girdle muscular weakness.

OBJECTIVE: We prospectively screened a large European cohort of patients presenting with hyperCKemia and/or limb-girdle muscular weakness (LGMW) for acid α-glucosidase (GAA) deficiency by dried blood spot (DBS) investigation. METHODS: DBS were collected from 3,076 consecutive adult patients from 7 German and British neuromuscular centers. All specimens were investigated for GAA deficiency by fluorometry. Samples with reduced enzyme activity were subsequently investigated for GAA gene mutations. RESULTS: Of 3,076 patients with DBS samples, 232 patients (7.6%) showed low GAA enzyme activity. Of these 232 patients, 55 (24%) presented with isolated hyperCKemia and 176 (76%) with hyperCKemia and LGMW. With both features present, 94% of the patients showed a low enzymatic activity. Mutational analysis found GAA gene mutations in 74 patients (2.4%); herein 70 patients were heterozygote for the common GAA gene splice-site mutation c.-32-13T>G. The most common clinical presentation in the confirmed Pompe cohort was a limb-girdle phenotype (85.3%) combined with ventilatory insufficiency (61%). Isolated hyperCKemia was found in 12%, while 2.7 had hyperCKemia and ventilatory insufficiency only. CONCLUSIONS: In a large cohort of unselected adult patients with hyperCKemia and/or LGMW, we found a prevalence of late-onset Pompe disease of 2.4%. Therefore, targeted screening of such a population should be encouraged in clinical practice.