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A codon deletion correlates with early-onset dystonia
DYT1 codes for a torsin A protein
A codon (three base pair [GAG]) deletion in cDNA DQ2 was found to occur in, and thus may be responsible for, the vast majority of cases of early-onset dystonia. The cDNA for DQ2 consists of 2,072 bp, with a predicted open reading frame of 998 bp from nucleotide 43 to 1041. The 3'-untranslated portion contains two poly (A)+ addition sites, one at nucleotide 1390 and the other at 2054 (Fig. 4).
The 3 bp deletion, GAG, occurs at nucleotide 946-948 in the coding region and results in the deletion of one of a pair of adjacent glutamic-acid residues found in the gene from unaffected individuals (Fig. 5). This deletion was confirmed by many different methods such as SSCP, direct sequencing, and digestion with BseR. The results, collectively, showed that the GAG (glutamic acid) deletion was uniquely associated with typical cases of early-onset dystonia. Futhermore, this deletion mutation appears to have arisen independently on different haplotypes in a number of ethnic groups (Ozelius, et al, 1997).
Fig 4. A map of the cDNA DQ2 transcript in the critical region. Poly (A)+ addition sites are marked by the symbol *. The symbol (^) indicates that no stop codon 5' to the first predicted methionine (M) has been found (Ozelius, et al, 1997).
Fig 5:------Open Reading Frame of cDNA DQ2 on Chr 9 ------------------- 1 ATG AAG CTG GGC CGG GCC GTG CTG GGC CTG CTG CTG CTG GCG CCG 15
16 TCC GTG GTG CAG GCG GTG GAG CCC ATC AGC CTG GGA CTG GCC CTG 30
31 GCC GGC GTC CTC ACC GGC TAC ATC TAC CCG CGT CTC TAC TGC CTC 45
46 TTC GCC GAG TGC TGC GGG CAG AAG CGG AGC CTT AGC CGG GAG GCA 60
61 CTG CAG AAG GAT CTG GAC GAC AAC CTC TTT GGA CAG CAT CTT GCA 75
76 AAG AAA ATC ATC TTA AAT GCC GTG TTT GGT TTC ATA AAC AAC CCA 90
91 AAG CCC AAG AAA CCT CTC ACG CTC TCC CTG CAC GGG TGG ACA GGC 105
106 ACC GGC AAA AAT TTC GTC AGC AAG ATC ATC GCA GAG AAT ATT TAC 120
121 GAG GGT GGT CTG AAC AGT GAC TAT GTC CAC CTG TTT GTG GCC ACA 135
136 TTG CAC TTT CCA CAT GCT TCA AAC ATC ACC TTG TAC AAG GAT CAG 150
151 TTA CAG TTG TGG ATT CGA GGC AAC GTG AGT GCC TGT GCG AGG TCC 165
166 ATC TTC ATA TTT GAT GAA ATG GAT AAG ATG CAT GCA GGC CTC ATA 180
181 GAT GCC ATC AAG CCT TTC CTC GAC TAT TAT GAC CTG GTG GAT GGG 195
196 GTC TCC TAC CAG AAA GCC ATG TTC ATA TTT CTC AGC AAT GCT GGA 210
211 GCA GAA AGG ATC ACA GAT GTG GCT TTG GAT TTC TGG AGG AGT GGA 225
226 AAG CAG AGG GAA GAC ATC AAG CTC AAA GAC ATT GAA CAC GCG TTG 240
241 TCT GTG TCG GTT TTC AAT AAC AAG AAC AGT GGC TTC TGG CAC AGC 255
256 AGC TTA ATT GAC CGG AAC CTC ATT GAT TAT TTT GTT CCC TTC CTC 270
271 CCC CTG GAA TAC AAA CAC CTA AAA ATG TGT ATC CGA GTG GAA ATG 285
286 CAG TCC CGA GGC TAT GAA ATT GAT GAA GAC ATT GTA AGC AGA GTG 300
301 GCT GAG GAG ATG ACA TTT TTC CCC AAA GAG GAG AGA GTT TTC TCA 315
316 GAT AAA GGC TGC AAA ACG GTG TTC ACC AAG TTA GAT TAT TAC TAC 330
331 GAT GAT TGA
The sequence of this DYT1 gene reveals that it codes for a new class of protein called"torsin A". Torsin A is an ATP- binding protein and has high homology to three mammalian genes (human, mouse, and rat torsin A) as well as two other proteins related to Tosin A (torp1 and torp2) and a torsin-related protein in C. elegans (torpCel). The glutamic-acid pair is conserved in all human, rat and mouse torsin A transcripts, suggesting that it is part of a functional domain. Torsin A also has a distant relationship to the heat-shock protein/Clp protease family of proteins (Ozelius, et al, 1997). Heat shock proteins typically contain one or two highly conserved ATP-binding domains and display ATPase activity. These proteins act as thermo "protectors" to other proteins involved in cellular function and metabolism. They protect proteins from temperature fluctuations and help proteins maintain their shape. By maintaining the strength and resiliency of cellular proteins, heat-shock proteins protect cells from deadly environmental, biological, and chemical stress (Schirmer, 1996). The torsin A protein is comparable to two representative members of the heat-shock protein family:
The torsin and torps both have four domains (A, B, SN, and IV) which are very similar to the conserved domains in these heat-shock proteins. The most prominent feature shared between them is a conserved ATP/GTP-binding sequence comprising two motifs:
Domain 'A' is followed aproximately 60 amino acids by domain 'B'. Key residues of domains 'SN' and 'IV' are also conserved (Fig.6) (Ozelius, et al, 1997). This is the first time a human disease has been associated with this class of heat-shock proteins.
Fig. 6: Comparison of predicted amino-acid sequences of torsin 'A' and torps with two representative members of the heat-shock protein family: SKD3 & HSP101. Blue residues are identical to a consensus sequence. Here, the conserved motifs (A & B), which are present in all six proteins, are shown.
[ -----------------------ATP-binding domain---------------------------------------]
[A motif ] [B motif]
torsin A
LTLSL-HGWTGTGKNFVSKIIAENIYEGGLN-------SDYVHLFVATLHFPHASNITLYKDQLSLWIRGNVSACARSIFIFDEMDKMtorpCel
LVLSF-HGYTCSGKNYVAEIIANNTRRLGLR-------STFVQHIVATNDFPDKNKLEEYQVELRNRILTTVQKCQRSIFIFDEADKLtorp 1
LVLSL-HGWTCTGKSYVSSLLAQHLFRDCLR-------SPHVHHFSPIIHFPHPSRTEQYKKELKSWVQCNLTACERSLFLFDEMDKLtorp 2
-----------------------------------------------------------------------AAALHQILFIFDEAHKLSKD3
LV-FLFLGSSG-GKTELAKQTAKYMHKDAKKGFIRLDMSCFQERHCVAKFIGSPPGYIGHEEGGO--LTKKLKOCPNAVVLFDEVDKAHSP101
TGSFLFLGPIGVGKTELAKALAEQLF-DNENQLVRIDMSEYMEQHSVSRLIGAPPGYVGHEEGGO--LTEAVRRRPYSVVLFDEVEKA
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