1 Introduction
1.1 MicroRNAs as
Key Posttranscriptional
Regulators of Gene
Expression
MicroRNAs (miRNAs) are a family of small (19–25 nucleotides in
length) noncoding RNAs that have been reported to finely tune
gene expression following sequence-selective interaction with
mRNAs targets [1]. These interactions occur at the RISC
(RNA-induced silencing complex) and cause translational repression or mRNA degradation, depending on the extent of complementarity between miRNAs and mRNA target sequences
[2]. While miRNAs usually interact with the 3
0 UTR region of
mRNAs, functional binding to coding sequences and 5
0 UTR have
also been described [1–3]. Since a single miRNA is able to interact
with several mRNAs and a single mRNA may contain several signals
for miRNA recognition, it is calculated that at least 20–40% of
human mRNAs are targets of microRNAs [3]. With respect to
effects on gene expression it is proposed that, in general, a low
expression of a given miRNA is potentially associated with accumulation of the target mRNAs; conversely, a high expression of miRNAs is expected to be the cause of a low expression of the target
mRNAs [4–6].
1.2 MicroRNA
Therapeutics
Since the involvement of microRNAs in human pathologies is a
firmly established fact, the pharmacological modulation of their
activity appears to be a very interesting approach in the development of new types of drugs (miRNA therapeutics) [7–10]. For
instance, miRNAs are involved in cancer; several oncomiRNAs
and metastamiRNAs have been demonstrated to promote cancer
cell growth and tumor invasion; among the mRNA targets of these
miRNA, several tumor-suppressor mRNAs have been proposed,
including PTEN, Rb1, SOCS1, p27
Kip1
, PUMA, RB [11–
14].
Conversely,
tumor-suppressor
miRNAs
targeting
oncoprotein-coding mRNAs have been reported [13–15]. Accordingly, these tumor-specific miRNAs can be considered molecular
markers important for tumor diagnosis and prognosis, as well as
important targets of therapeutic interventions [11–15].
1.3 Peptide Nucleic
Acids and microRNA
Therapeutics
With respect to miRNA therapeutics, peptide nucleic acid (PNA)based molecules are appealing [16]. In PNAs, the pseudo-peptide
backbone is composed of unnatural N-(2-aminoethyl)glycine units
[17]; thus, they are resistant to both nucleases and proteases
[18, 19] and, more importantly, hybridize with high affinity to
complementary sequences of single-stranded RNA and DNA,
forming Watson-Crick double helices [20]. For these reasons,
PNAs were found to be excellent candidates for antisense and
antigene therapies [21–24]. Recently, PNAs have been shown to
be able of altering biological functions of microRNAs, both in vitro
and in vivo [25–33]. With respect to PNA-based targeting of
124
Alessia Finotti et al.
1.1 MicroRNAs as
Key Posttranscriptional
Regulators of Gene
Expression
MicroRNAs (miRNAs) are a family of small (19–25 nucleotides in
length) noncoding RNAs that have been reported to finely tune
gene expression following sequence-selective interaction with
mRNAs targets [1]. These interactions occur at the RISC
(RNA-induced silencing complex) and cause translational repression or mRNA degradation, depending on the extent of complementarity between miRNAs and mRNA target sequences
[2]. While miRNAs usually interact with the 3
0 UTR region of
mRNAs, functional binding to coding sequences and 5
0 UTR have
also been described [1–3]. Since a single miRNA is able to interact
with several mRNAs and a single mRNA may contain several signals
for miRNA recognition, it is calculated that at least 20–40% of
human mRNAs are targets of microRNAs [3]. With respect to
effects on gene expression it is proposed that, in general, a low
expression of a given miRNA is potentially associated with accumulation of the target mRNAs; conversely, a high expression of miRNAs is expected to be the cause of a low expression of the target
mRNAs [4–6].
1.2 MicroRNA
Therapeutics
Since the involvement of microRNAs in human pathologies is a
firmly established fact, the pharmacological modulation of their
activity appears to be a very interesting approach in the development of new types of drugs (miRNA therapeutics) [7–10]. For
instance, miRNAs are involved in cancer; several oncomiRNAs
and metastamiRNAs have been demonstrated to promote cancer
cell growth and tumor invasion; among the mRNA targets of these
miRNA, several tumor-suppressor mRNAs have been proposed,
including PTEN, Rb1, SOCS1, p27
Kip1
, PUMA, RB [11–
14].
Conversely,
tumor-suppressor
miRNAs
targeting
oncoprotein-coding mRNAs have been reported [13–15]. Accordingly, these tumor-specific miRNAs can be considered molecular
markers important for tumor diagnosis and prognosis, as well as
important targets of therapeutic interventions [11–15].
1.3 Peptide Nucleic
Acids and microRNA
Therapeutics
With respect to miRNA therapeutics, peptide nucleic acid (PNA)based molecules are appealing [16]. In PNAs, the pseudo-peptide
backbone is composed of unnatural N-(2-aminoethyl)glycine units
[17]; thus, they are resistant to both nucleases and proteases
[18, 19] and, more importantly, hybridize with high affinity to
complementary sequences of single-stranded RNA and DNA,
forming Watson-Crick double helices [20]. For these reasons,
PNAs were found to be excellent candidates for antisense and
antigene therapies [21–24]. Recently, PNAs have been shown to
be able of altering biological functions of microRNAs, both in vitro
and in vivo [25–33]. With respect to PNA-based targeting of
124
Alessia Finotti et al.