Diagnosis of late-infantile neuronal ceroid lipofuscinosis using dried blood spot-based assay for TPPI enzyme activity: TPPI diagnostic assay from DBS.

Clin Chim Acta 2020 Aug 13;507:62-68. Epub 2020 Apr 13.

Department of Molecular Biology, New York State Institute for Basic Research in Developmental Disabilities, 1050 Forest Hill Road, Staten Island, NY 10314, United States. Electronic address:

Background: The neuronal ceroid lipofuscinosis 2 (NCL2) or classic late-infantile neuronal ceroid lipofuscinosis (LINCL) is a neurogenetic disorder caused by mutations in the TPPI gene, which codes for the lysosomal tripeptidyl peptidase 1 (TPPI) EC 3.4.14.9. Loss of functional TPPI activity results in progressive visual and neurological symptoms starting at around 1-2 years of age causing early death.

Methods: We report a DBS-based TPPI assay that cleaves a synthetic tetrapeptide substrate generating a product that is detected by HPLC. Probands and carriers were identified with 100% accuracy (7 probands, 30 carriers, 13 controls).

Results: The assay detected a single TPPI activity at a lower pH towards the substrate tested. TPPI activity measurable when extracted at lower pH while inactive at neutral pH showed steady increase for at least 8 h incubation. No loss in TPPI activity was observed when DBS were stored for at least 2 weeks either in freezer, refrigerator, room temperature or 42 °C.

Conclusion: A sequence variant causing Arg339Gln substitution in a proband had 12% TPPI. TPPI activity can be reliably measured in DBS, giving an opportunity to diagnose NCL2 at birth and refer patients for enzyme replacement or other therapies for earliest intervention, or alternatively offers a second-tier confirmatory test.

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http://dx.doi.org/10.1016/j.cca.2020.04.010DOI Listing
August 2020
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