Autosomal-dominant striatal degeneration is caused by a mutation in the phosphodiesterase 8B gene

Abstract

Autosomal-dominant striatal degeneration (ADSD) is an autosomal-dominant movement disorder affecting the striatal part of the basal ganglia. ADSD is characterized by bradykinesia, dysarthria, and muscle rigidity. These symptoms resemble idiopathic Parkinson disease, but tremor is not present. Using genetic linkage analysis, we have mapped the causative genetic defect to a 3.25 megabase candidate region on chromosome 5q13.3-q14.1. A maximum LOD score of 4.1 (Theta = 0) was obtained at marker D5S1962. Here we show that ADSD is caused by a complex frameshift mutation (c.94G>C+c.95delT) in the phosphodiesterase 8B (PDE8B) gene, which results in a loss of enzymatic phosphodiesterase activity. We found that PDE8B is highly expressed in the brain, especially in the putamen, which is affected by ADSD. PDE8B degrades cyclic AMP, a second messenger implied in dopamine signaling. Dopamine is one of the main neurotransmitters involved in movement control and is deficient in Parkinson disease. We believe that the functional analysis of PDE8B will help to further elucidate the pathomechanism of ADSD as well as contribute to a better understanding of movement disorders.

Figure 1

MRI Features and Pedigree of the ADSD Family (A) T2-weighted images of a healthy control (left) and ADSD patient III:5 (right; encircled are regions of pathologic signal increase). (B) Pedigree of the ADSD family. Clear symbols represent unaffected individuals. Filled symbols represent individuals with ADSD. Individuals were considered affected on the basis of the characteristic symmetric MRI abnormalities of the basal ganglia. The index patient (IV:2, arrow) is shown in Movie S1. Segregation analysis showed perfect cosegregation of the PDE8B mutation (c.94G>C+c.95delT) with ADSD. +/m denotes heterozygous mutation carriers; +/+ denotes individuals homozygous for the wild-type allele.

Figure 2

ADSD Candidate Region, Genomic Organization of PDE8B, PDE8B Mutation Found in the ADSD Family, Structural and Functional Consequences for the Protein (A) The 3.25 Mb candidate region with locations of all 21 RefSeq annotated protein coding genes. (B) Genomic organization of the different PDE8B variants. The localization of the ADSD-causing mutation in exon 1 is indicated by an arrow. Accession numbers of PDE8B isoforms are given in Table S4. (C) Complex (c.94G>C+c.95delT) mutation found in the ADSD family. Shown is the reverse sequence. (D) Protein structure of PDE8B and position of the functional domains REC, PAS, and PDE in PDE8B isoform 1. (E) Structure of the truncated protein. The altered amino acid sequence starting at residue 32 is shaded. (F) Amino acid sequence of the truncated protein. Residues 1–31 (upper row, black) and altered residues 32–63 (lower row, red) are shown. (G) Effects of the PDE8B mutation on cAMP levels. All error bars indicate standard deviation (SD). Each sample was run in triplicate (n = 3) and the experiment was repeated three times. ^∗ indicates a significant difference between wild-type and mutant (p < 0.05) and wild-type and empty vector (p < 0.01). The difference between mutant and empty vector was not significant. Luminescence (RLU) is inversely proportional to cAMP levels. High PDE8B activity results in low cAMP concentration, leading to high luminescence.

Figure 3

Expression of PDE8B Relative quantification of PDE8B in various human tissues by real-time PCR and standard deviation. Quantification was performed with ACTB, GAPDH, and PPIA as reference genes. All error bars indicate SD. Each sample was run in triplicate (n = 3).