Cathepsin D Deficiency Is Associated with a Human Neurodegenerative
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Cathepsin D Deficiency Is Associated with a Human Neurodegenerative Disorder Robert Steinfeld,1 Konstanze Reinhardt,1 Kathrin Schreiber,1 Merle Hillebrand,1 Ralph Kraetzner,2 Wolfgang Bru¨ck,3 Paul Saftig,4 and Jutta Ga¨rtner1 Departments of 1Pediatrics and Pediatric Neurology, 2Molecular Pharmacology, and 3Neuropathology, University of Go¨ttingen, Go¨ttingen, Germany; and 4Biochemical Institute, University of Kiel, Kiel, Germany Cathepsin D is a ubiquitously expressed lysosomal protease that is involved in proteolytic degradation, cell invasion, and apoptosis. In mice and sheep, cathepsin D deficiency is known to cause a fatal neurodegenerative disease. Here, we report a novel disorder in a child with early blindness and progressive psychomotor disability. Two missense mutations in the CTSD gene, F229I and W383C, were identified and were found to cause markedly reduced proteolytic activity and a diminished amount of cathepsin D in patient fibroblasts. Expression of cathepsin D mutants in cathepsin D5/5 mouse fibroblasts revealed disturbed posttranslational processing and intracellular tar- geting for W383C and diminished maximal enzyme velocity for F229I. The structural effects of cathepsin D mutants were estimated by computer modeling, which suggested larger structural alterations for W383C than for F229I. Our studies broaden the group of human neurodegenerative disorders and add new insight into the cellular functions of human cathepsin D. Cathespin D (CatD) belongs to the pepsin family of pro- age of an autofluorescent material, ceroid lipofuscin, is teases and is one of the most studied aspartic proteases. found in neurons.10 Inactivation of the Drosophila CatD It has been implicated in diverse biological processes. homologue causes age-dependent neurodegeneration CatD promotes invasion and proliferation of cancer and also progressive neuronal accumulation of autofluo- cells1 but is also involved in caspase-independent apo- rescent storage material.11 In sheep, a naturally occurring ptosis.2,3 Increased concentrations of CatD are found missense mutation in the active-site aspartate (D293N) during ischemic, inflammatory, and regenerative pro- of ovine Ctsd results in early neurodegeneration. Because cesses, such as coronary heart disease,4 inflammatory of the autofluorescent neuronal storage material, the dis- bowel disease,5 wound healing, and epidermal differ- ease was subsequently named “congenital ovine neuronal entiation.6 CatD is a major component of lysosomes and ceroid lipofuscinosis” (CONCL [MIM *116840]).12 functions as a highly active endopeptidase with an op- Recently, a second natural animal model of CatD in- timum pH between 3.0 and 5.0. CatD preferentially sufficiency was discovered in American bulldogs.13 Af- cleaves peptide bonds that are flanked by bulky hydro- fected dogs had 36% of the CatD-specific enzymatic phobic amino acids and is strongly inhibited by pepstatin activity found in control dogs, and they presented with p A (inhibition constantKi 1–4 nM). Human CatD is a milder phenotype than CatD-deficient mice and sheep. synthesized and translocated into the endoplasmic retic- Similar to these animal models, all currently known hu- ulum (ER) as an inactive precursor proenzyme (53 kDa), man neuronal ceroid lipofuscinosis (NCL) forms are processed into an enzymatically active, intermediate pro- characterized by developmental regression, visual loss, enzyme (48 kDa), and finally converted in the lysosomal and epilepsy in addition to the name-giving accumula- compartment into a noncovalently associated two-chain tion of autofluorescent lysosomal storage material. At form with an N-terminal 15-kDa light chain and a C- present, six different disease genes have been identified terminal 33-kDa heavy chain.7,8 for human NCL: CLN1, CLN2, CLN3, CLN5, CLN6, Targeted disruption of the Ctsd gene in mice leads to and CLN8.14,15 weight loss in the 3rd wk of life that is associated with We screened a group of 25 infants and children with progressive atrophy of the intestinal mucosa. Massive a nonidentified genetic cause of an NCL-like disorder intestinal necrosis and profound destruction of lympho- and found the first case of human CatD deficiency. We cytes in the spleen and thymus are found just before the determined the molecular mechanisms and functional mice die in a state of anorexia at age 4 wk.9 In addition, consequences of two missense mutations in the human CatD-deficient mice develop seizures and progressive ret- CTSD gene, and, in doing so, have extended our un- inal atrophy, which leads to blindness. Lysosomal stor- derstanding of the biological functions of human CatD. Received December 12, 2005; accepted for publication March 10, 2006; electronically published March 29, 2006. Address for correspondence and reprints: Dr. Robert Steinfeld, Pa¨diatrie II, Zentrum Kinderheilkunde und Jugendmedizin, Georg-August- Universita¨tGo¨ ttingen, Bereich Humanmedizin, Robert-Koch-Strasse 40, D-37075 Go¨ ttingen, Germany. E-mail: [email protected] Am. J. Hum. Genet. 2006;78:988–998. ᭧ 2006 by The American Society of Human Genetics. All rights reserved. 0002-9297/2006/7806-0009$15.00 988 The American Journal of Human Genetics Volume 78 June 2006 www.ajhg.org Material and Methods was clearly reduced. Sequencing the coding regions of CTSD resulted in the identification of the missense mutations g.6517TrA and g.10267GrC, whereas no pathogenic base Material alterations were revealed in the CLN3, CLN5, CLN6, and Cell culture medium (Dulbecco’s modified Eagle medium CLN8 genes. [DMEM]), zeocin, hygromycin, Lipofectamin 2000, and Taq After a normal early psychomotor development, the patient DNA polymerase were purchased from Invitrogen. Oligonu- first showed neurodegenerative symptoms—in particular, cleotides were synthesized by MWG-Biotech. For amplification ataxia and visual disturbances—at early-school age. The ocular of genomic DNA fragments of CTSD, the primer combination fundus was found to show retinitis pigmentosa, and cranial hCDgDNA6960F (5-GTGTAAACCGAGCCCTGATGACTT-3) magnetic resonance imaging (MRI) scans revealed cerebral and and hCDgDNA7525R (5-CAGCAGCAGGGAGGGGGCAGC- cerebellar atrophy. In the course of disease, she developed pro- ACT-3) and the primer combination hCDgDNA10788F (5-GGG- gressive cognitive decline, loss of speech, retinal atrophy, and GAGCCCCAAGGCCACCACTA-3) and hCDgDNA11224R loss of motor functions. Now, at age 17 years, she is wheel- (5-CTCGGCGAAGCCCACCCTGTTGTT-3) were used. For chair bound and severely mentally retarded. Until now, there amplification of the CTSD cDNA fragments, the primer com- has been no evidence of hematological disease, immunodefi- bination hCDcDNA526F (5-CGTGAAGAATGGTACCTCG- ciency, intestinal abnormalities, or dystrophia. In fact, because TTTGA-3) and hCDcDNA1319R (5-CAGTGTAGTAGCG- of immobilization, the patient has become overweight during GCCGATGAAGAC-3) was used. [a-32P]dCTP, [35S]methio- the past few years. nine, and molecular-weight protein markers were obtained from Amersham Biosciences. Restriction enzymes were sup- Expression of Wild-Type CatD and Mutants in CatDϪ/Ϫ plied by New England BioLabs. Goat anti-human CatD an- Mouse Fibroblasts and Flp-In 3T3 Cells tibody and rabbit anti-mouse CatD antibody were a generous gift from Professor Kurt von Figura (University of Go¨ ttingen). CTSD cDNA was received from the IMAGE consortium Rat anti-mouse lysosomal-associated membrane protein 1 (RZPD clone ID IRAUp969A1048D6) and was cloned into (LAMP-1) antibody was acquired from the Developmental the expression vectors pcDNA3.1/Zeo and pEF5/FRT/V5-D- Studies Hybridoma Bank (University of Iowa, Iowa City). TOPO (Invitrogen). Site-directed mutagenesis was performed Biotin SP–conjugated donkey anti-goat antibody, peroxidase- using a QuickChange site-directed mutagenesis kit (Strata- conjugated donkey anti-goat antibody, and peroxidase-con- gene), in accordance with the supplier’s protocol. All final con- jugated streptavidin were purchased from Jackson Immuno- structs were verified by semiautomated sequencing. Ϫ Ϫ Research Lab. For western blotting, peroxidase staining was CatD / mouse fibroblasts and Flp-In 3T3 cells were seeded done with tetramethyl benzidine (TMB) substrate (Seramun). on 35-mm plates at a density of2 # 105 cells/well in DMEM Alexa Fluor 488 donkey anti-goat antibody and Alexa Fluor supplemented with fetal calf serum (FCS) at 37ЊC and 5% 546 goat anti-mouse and goat anti-rat antibodies were ob- CO2. After 24 h of culturing, the cells were transfected with tained from Molecular Probes. The BigDye terminator kit was 2 mg of plasmid and 5 ml of Lipofectamin 2000 in 0.5 ml of used for semiautomated sequencing, in accordance with the Opti-MEM. The medium was replaced with DMEM supple- recommendations of the manufacturer (Perkin Elmer Applied mented with FCS after 6 h of incubation. Selection with zeocin Biosystems). RNA was isolated from patient fibroblasts with or hygromycin was started 48 h after transfection, and stable the RNeasy kit as described by the manufacturer (Qiagen). clones were achieved after 14 d of selection. Cellular homog- Total RNA was fractionated by agarose-formaldehyde gel elec- enates from individual clones were assayed for CatD activity trophoresis, the gel was blotted onto a Hybond-XL membrane by use of the substrate MOCAc-GKPIIFFRLK(Dnp)-R-NH2, (Amersham Biosciences), and the northern blot was hybridized as described elsewhere.16 Alternatively, cell lysates were ana- in accordance with standard procedures by use of the Nick lyzed by western blotting