siRNA expression system

1. A recombinant vector for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA, comprising from 5′ to 3′:

a) an RNA polymerse II dependent promoter sequence derived from the U1 snRNA gene;

b) suitable restriction sites for cloning the sequence that transcribes a presiRNA or a pre-miRNA;

c) a sequence transcribing the pre-siRNA or pre-miRNA comprising: in position +1 an A or a G residue; a sequence from 21 to 23 nucleotides corresponding to a sense region of the mRNA transcribed by the gene to be silenced; a sequence selected to form a loop region; a sequence from 21 to 23 nucleotides corresponding to the antisense region of the mRNA transcribed by the gene to be silenced; wherein the residues at the 3′ end protrude in such a way that the following structure is obtained:

wherein Y is optionally present, and X and Y if Y is present are independently one or more nucleotides, wherein the nucleotides at the 3′ end are selected to result in asymmetry in siRNA or miRNA strand selection into an interference complex; and

d) termination sequences derived from the sequence at 3′ of the gene for U1 snRNA which are necessary and sufficient for the correct formation of the 3′ of the pre-siRNA or of the pre- miRNA, wherein the recombinant vector provides for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA.

2. The vector according to claim 1 , wherein the restriction site for cloning at the 5′ end of the sequence transcribing the pre-siRNA is BglII .

3. A recombinant vector for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA, comprising from 5′ to 3′:

a) an RNA polymerse II dependent promoter sequence derived from the U1 snRNA gene;

b) suitable restriction sites for cloning the sequence that transcribes a presiRNA or a pre- miRNA;

c) a sequence transcribing the pre-siRNA or the pre-miRNA comprising: in positions +1 an A or a G residue; a sequence from 21 to 23 nucleotides corresponding to a sense region of the mRNA transcribed by the gene to be silenced; a sequence selected to form a loop region; a sequence selected from 21 to 23 nucleotides corresponding to the antisense region of the mRNA transcribed by the gene to be silenced; wherein the residues at the 3′ end protrude in such a way that the following structure is obtained:

wherein each N is independently A, U, G or C and N′ is its complementary nucleotide, wherein Y is optionally present, wherein X and Y if Y is present are independently one or more nucleotides, wherein the nucleotides at the 3′ end are selected to result in asymmetry in siRNA or miRNA strand selection into an interference complex; and

d) termination sequences derived from the sequence at 3′ of the gene for U1 snRNA which are necessary and sufficient for the correct formation of the 3′ of the pre-siRNA or of the pre-miRNA, wherein the recombinant vector provides for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA.

4. A recombinant vector for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA, comprising from 5′ to 3′:

a) an RNA polymerse II dependent promoter sequence derived from the U1 snRNA gene;

b) suitable restriction sites for cloning the sequence that transcribes a presiRNA or a pre- miRNA;

c) a sequence transcribing the pre-siRNA or the pre-miRNA comprising: in position +1 an A or a G residue; a sequence from 21 to 23 nucleotides corresponding to a sense region of the mRNA transcribed by the gene to be silenced; a sequence selected to form a loop region; a sequence from 21 to 23 nucleotides corresponding to the antisense region of the mRNA transcribed by the gene to be silenced that constitutes the second segment of the stem of the pre-siRNA; wherein the residues at the 3′ end protrude in such a way that the following structure is obtained:

and

d) termination sequences derived from the sequence at 3′ of the gene for U1 snRNA which are necessary and sufficient for the correct formation of the 3′ of the pre-siRNA or of the pre-miRNA, wherein the recombinant vector provides for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA.

5. The vector according to claim 4 wherein the termination sequences derived from the sequence at 3′ of the gene for U1 snRNA are as follows:

CCCCTG/ACTTTCTGGAGTTTCAAAAGTAGAC.

(SEQ ID NO:18)

6. The vector according any of claims 1 to 5 further comprising suitable sequences to make inducible the RNA pol II promoter.

7. A recombinant vector for the correct, stable and effective expression in mammalian cells of a siRNA or a miRNA, comprising from 5′ to 3′:

a) an RNA polymerse II dependent promoter sequence derived from the U1 snRNA gene;

b) suitable restriction sites for cloning the sequence that transcribes a presiRNA or a pre- miRNA;

c) a sequence transcribing the pre-siRNA comprising: in position +1 an A or a G residue; a sequence from 21 to 23 nucleotides corresponding to a sense region of the mRNA transcribed by the gene to be silenced; a sequence selected to form a loop region; a sequence from 21 to 23 nucleotides corresponding to the antisense region of the mRNA transcribed by the gene to be silenced; wherein one of the following structures is obtained:

wherein each N is independently A, U, G or C and N′ is its complementary nucleotide, wherein Y is optionally present, wherein X and Y if Y is present are independently one or more nucleotides, wherein the nucleotides at the 3′ end are selected to result in asymmetry in siRNA strand selection into an interference complex, or

or alternatively a sequence transcribing the pre-miRNA; and

d) termination sequences derived from the sequence at 3′ of the gene for U1 snRNA which are necessary and sufficient for the correct formation of the 3′ of the pre-siRNA or of the pre-miRNA.

8. A composition for gene therapy comprising the vector according to claim 1 , 3 , 4 or 7 .