Twelve-year experience with a rapid and simple fluorometric tripeptidyl peptidase 1 (TPP1) assay using dried blood specimens to diagnose CLN2 disease

Abstract

CLN2 disease is a lysosomal storage disorder that belongs to the neuronal ceroid lipofuscinoses (NCL) and progressively leads to dementia, blindness and early death. It is caused by a deficiency of the lysosomal enzyme tripeptidyl peptidase 1 (TPP1). The disease has recently gained interest as an enzyme replacement therapy has become available. For this therapy to be effective, the diagnosis must be made early. We have investigated the diagnostic reliability of a test for TPP1 deficiency in dried blood specimens (DBS), previously developed by us, to detect CLN2 disease. During a 12-year period we have received 3882 DBS for testing TPP1 activity. To check for quality of samples, we measured activities of two additional lysosomal enzymes as controls, palmitoyl peptidase 1 and ß-galactosidase. Of all samples, 1.7% were excluded from the study because of subnormal activity of more than one enzyme. For all samples with subnormal TPP1 activity and good sample quality, we reviewed clinical information. Consequently, we obtained adequate clinical and molecular genetic data for 51 patients. All of those had doubtless evidence of CLN2 disease (including seven atypical patients) as shown by symptoms of a progressive brain disease and presence of known pathogenic CLN2variants. The sensitivity of the test could not be evaluated directly. However, as our NCL clinic is a major reference center for these disorders and we have never received feedback information that a patient with normal TPP1 activity in our DBS test was later diagnosed with CLN2 disease, this constitutes convincing circumstantial evidence of a high sensitivity. The TPP1 test on DBS as used in this study was shown to be a reliable, convenient and inexpensive tool for a first diagnostic step in a suspected case of CLN2 disease and, can be applied with minor modifications for mass throughput, like newborn screening programs.