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.