Functional consequence and clinical importance of distinct mutations in CLN2, the gene associated with classical late infantile ceroid lipofuscinosis

The neuronal ceroid lipofuscinoses (NCL) are lysosomal diseases characterized by progressive neurodegeneration resulting in loss of vision, seizures, mental regression, behavioral changes, movement disorders and premature death. One of the most frequent of these diseases is classical late infantile NCL caused by mutations in the CLN2 gene and is related to defects of lysosomal tripeptidyl peptidase I (TPP-I).

We have analyzed more than 30 families with suspected classical late infantile NCL for CLN2 mutations and could identify both pathogenic alleles in about 80%. Further, we have developed a scoring system to quantify the clinical course of NCL patients over long periods of time, covering the disease specific deficits of motor, visual and verbal performance. Using this clinical score we described the natural progression of the disease in genetically defined late infantile NCL patients. Finally, we analyzed the functional consequences of selected CLN2 mutations with regard to enzymatic activity, stability, posttranslational processing, and intracellular localisation of TPP-I. We found that four missense mutations, N286S, I287N, T353P, and Q422H, located in conserved protein regions of TPP-I, decreased dramatically enzymatic activity and led to rapid degradation in non-lysosomal compartments. We conclude that these amino acid substitutions induce major misfolding of the precursor peptidase. In contrast, the amino acid substitution R127Q within a non-conserved region did not significantly affect enzymatic activity, stability, and lysosomal targeting of TPP-I. The patient compound heterozygous for the R127Q mutation and for the R208X mutation showed a protracted clinical course and was originally diagnosed to have atypical juvenile NCL. Our molecular genetic results and functional studies point to various disease mechanisms and to a greater clinical variety in classical late infantile NCL than previously expected.