Medical Hypotheses (2002) 59(6), 670–673 ª 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0306-9877(02)00223-2, available online at http://www.idealibrary.com
Mitochondrial 16S rRNA gene encodes a functional peptide, a potential drug for Alzheimer’s disease and target for cancer therapy V. Maximov,
1,2
A. Martynenko,
1,2
G. Hunsmann,
1,2
V. Tarantul
1,2
1
Institute of Molecular Genetics, Russian Academy of Science, Moscow, Russia; 2German Primate Center, Goettingen, Germany
Summary New functions of well-known genes have been revealed frequently. A new example is described in this report. Earlier we have detected an up-regulation of expression of the mitochondrial 16S rRNA gene in non-Hodgkin’s lymphomas. Here we demonstrate that the human mitochondrial 16S rRNA gene encodes a potential oncopeptide, Humanin described recently (1,2). This peptide suppresses neuronal cell death induced by mutant genes responsible for familial Alzheimer’s disease (AD). Analysis of the gene coding site structure showed that Humanin mRNA is translated most likely in the cytosol, but not in the mitochondrion in vivo. This led us to suppose that AD could be caused by a block of Humanin mRNA transport from mitochondria into the cytosol. Moreover, our data and reports by others an mitochondrial 16S rRNA transcription and characteristic of transcript structure suggests that Humanin is a potential oncopeptide. Thus, the use of Humanin for the treatment of AD may increase the risk for the development of malignant diseases. ª 2002 Elsevier Science Ltd. All rights reserved.
The broadening of our understanding of the molecular basis of biology reveals to us an increasing complexity of processes in living cells and often requires original concepts to be amended or exchanged. Examples of such postulates are: ‘one gene–one character’, ‘one gene–one protein,’ and ‘one protein–one function.’ If a polypeptide is a constituent of a protein with wellknown function, e.g., a ribosome or an enzyme subunit, then the function of the polypeptide is usually considered to be exclusively associated with the function of this complex. However, some subunits of such protein complexes have their own function (Table 1). Here we
Received 21 December 2001 Accepted 4 April 2002 Correspondence to: V. Tarantul, Department of Viral and Cellular Molecular Genetics, IMG RAS, Kurchatov sq., 2, Moscow, 123182, Russia. Phone: +7-095-196-00-02; Fax: +7-095-196-02-21; E-mail:
[email protected]
670
provide evidence that the mitochondrial gene for 16S rRNA not only encodes the 16S rRNA but also a functional peptide. In searching for genes specifically expressed in HIV-1associated diffuse large B-cell lymphomas (DLBCLs) we cloned transcripts with similarity to the 16S rRNA gene (6). A similar 16S rRNA-liked transcript from SIVmac -associated DLBCL of Macaca mulatta was also cloned (unpublished). An increased expression of the 16S rRNA gene has also been detected in other malignencies (15). So far, this finding was of little interest since it was thought to just confirm a more active metabolism of cancer cells. Puzzeling, however, the transcript similar to 16S rRNA contained 30 -poly(A)-sequence not corresponding to the 30 -end of 16S rRNA gene. Thus, the polyadenylated transcripts of the 16S rRNA gene differ from the 16S rRNA. This fact was already pointed out by Baserga et al. in 1985 (16). However, the role of these transcripts and their potential translational products remained unclear.
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Table 1 Polyfunction of polypeptides Polypeptides
Properties and functions
Cytochrome c oxidase subunit 2 Ribosomal protein S3a
Oxidative phosphorylation pathway, interaction with cyclin G1 (3) involved in the signaling pathway of Raf1 (4), expression selectively enhanced in lymphomas (5,6) Ribosome component (7) involved in apoptosis (8), effector of v-fos transformation (9,10), expression selectively enhanced in lymphomas (unpublished) Ribosomal component (11), apurinic/apyrimidinic endonuclease activity (12), probably, DNA repair (12,13), expression selectively enhanced in lymphomas (unpublished) RNase P subunit, ATPase activity (14)
Acidic ribosomal protein P0 RNase P protein subunit p20 (RPP20)
Recently, a 24 aa peptide named Humanin was discovered. Humanin protects neuronal cell of the F11 line from death induced by the expression of mutanted genes, causing early-onset familial Alzheimer’s disease (AD). Humanin protected CN-procaspase-3 from APPinduced cleavage thereby probably preventing apoptosis (1,2). However, these reports do not reveal the nature of the gene encoding Humanin. Our analysis demonstrates that Humanin encoding mRNA can be transcribed from the mitochondrial 16S rRNA gene but not from nuclear DNA (Table 2). The differing distinct sizes of Humanin mRNA of nuclear and mitochondrial origin as well as sequence differences unabiguously lead to this conclusion. The size of the Humanin mRNA-liked site on mitochondrial DNA is equivalent to the size of Humanin mRNA. However, Humanin mRNA-liked sites on the nuclear genome are significantly smaller. In addition, the Humanin mRNAliked site of the mitochondrial genome has two substitutions explained by mitochondrial DNA polymorphism (see footnotes of Table 2). Humanin mRNAliked sites of the nuclear genome have 51 and more substitutions. Therefore only the mitochondrial 16S rRNA gene can encode for the Humanin mRNA. A further independent argument strengthens this conclusion. Human nuclear DNA contains other open reading frames (ORFs) encoding peptides similar to
Humanin but not identical. These ORFs are located within the 16S rRNA-liked sites on chromosome 11 and 13. These hypothetical peptides have one important substitution discriminating them from Humanin, leucine in position 12 is substituted by the more hydrophilic serine (Fig. 1). Such types of substitutions into the hydrophobic core of peptides significantly affect and sometimes block the peptide’s transport across the membrane of the endoplasmic reticulum (18).This substitution is incompatible with sequence of known signal peptides (www.cbs.dtu.dk/services/SignalP, www.cbs. dtu.dk/services/SignalP-2.0). Hashimoto et al. (1) have shown that cell death is only suppressed by the secreted peptide, and peptides remaining intracellularly cannot mediate such an effect. Thus, peptides potentially expressed from nuclear ORFs should not have the biological effect of Humanin. The Humanin mRNA is transcribed in the mitochondrion, but the peptide is probably translated in the cytosol. If translation would take place in the mitochondrion, codon 22 of the Humanin ORF encoding arginin, would be a stop-codon and thus a shorter peptide would be obtained (Fig. 1). This peptide would even correspond less than Humanin to conditons required for signal peptides (www.cbs.dtu.dk/services/SignalP-2.0). Thus it could not have the biological effect of Humanin.
Table 2 Comparison of the Humanin mRNA sequence (AY029066.1) with sequences of human mitochondrial and nuclear genomes Genome
Accesion number
Similarity site of genome sequence
Similarity site of Humanin mRNA sequencea
Similarity %
Similarity n.i.b./c.n.b.b
Mitochondrion Nuclear, chromosome 11 Nuclear, chromosome 3
AF346981.1 NT_009243.5
1685–3237c 407832–406543
1–1552 1–1290
99.9 92
1550/1552d 1198/1290
NT_022475.4
511532–510509
12–1036
95
974/1025
a
Humanin mRNA has 1567 bases. The first 1552 bases are transcribed and the last 15 bases represent the poly(A)-tail. n.i.b./c.n.b., Number of identical bases per complete number of bases on similar site. c 16S rRNA gene is located on the mitochodrial genome sequence from 1673 to 3230 bp. d Humanin mRNA sequence is identical to the mitochondrial genome sequence except for two substitutions. Thymine 711 of Humanin mRNA is substituted by cytosine in the mitochondrial sequence (AF346981.1), and thymine 935 of the Humanin mRNA is substituted by adenine. These substitutions are situated outside the Humanin ORF. They can be completely explained by human mitochondrial DNA polymorphism. It was cloned the cDNA containing Humanin ORF which has adenine in position 935 (1). In addition, the human mitochondrial genome can contains thymine in position 711 of Humanin mRNA, as verified by sequencing (17). b
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Medical Hypotheses (2002) 59(6), 670–673
672 Maximov et al.
Fig. 1 Comparison of mitochondrial and nuclear ORF potentially coding for Humanin. The mitochondrial code for the translation of mitochondrial ORF was used as a standard. ORF A is the human mitochondrial ORF (AF346981.1). ORF B is the human nuclear ORF on chromosome 11 (NT_009243.6) and chromosome 3 (NT_022475.4). Peptide Am is a transcript deduced from ORF A (mitochondrial code). Peptide Ac is a transcript deduced from ORF A (standard code). Peptide B is a transcript deduced ORF B (standard code). R is G or A. Indicates a stop-codon.
Fig. 2 Transcripts of the human mitochondrial 16S rRNA gene. T1–T4 represent different transcripts described earlier (1,16).
One can suppose that the block of Humanin transport is one of the causes of AD. Polyadenylated transcripts of mitochondrial DNA cloned by Hashimoto et al. (1) and Baserga et al. (16) can be allocated to four groups depending on the position of their 30 -end (Fig. 2). Therefore there are at least four different transcripts of the 16S rRNA gene. This grouping is supported by Northern blot analysis of Hashimoto et al. (1). Sequences of two of the groups do not contain the Humanin ORF. In addition, the 16S rRNA gene has several additional ORFs (Table 3). Some of them may be translated to peptides playing a role as significant as that of Humanin. Thus, in addition to the 16S rRNA the mitochondrial 16S rRNA gene encodes at least one antiapoptotic pep-
Medical Hypotheses (2002) 59(6), 670–673
tide – Humanin. Therefore, the up-regulation of the 16S rRNA gene transcription in cancer cells in particular in lymphomas, obtains functional significance. It probably Table 3 Potential ORFs of the human 16S rRNA genea; b 16S rRNA gene site (base positions)
Size of ORF (aa)
36–152 421–501 775–855 818–892 963–1037 1113–1187
38 26 26 24 24c 24
a
ORF size of more then 20 aa. Standard code for translation. c Humanin ORF. b
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enhances Humanin expression interfering with apoptosis and thereby sustains cancer development. According to this hypothesis, Humanin is an oncopeptide. Hashimoto and coworkers (1,2) have suggested to apply Humanin as a treatment for AD. However, the oncogenic potential of this peptide should first be evaluated. ACKNOWLEDGMENT
8.
9.
The work was supported by a grant from the Volkswagen Foundation. 10.
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