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Developmental genetics and embyronal cancer > microRNAs > Flashcards

microRNAs Flashcards

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1
Q

What are microRNAs?

A

single-stranded, non-coding and RNA silencing + post-transcriptional regulation

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2
Q

What is the basic overall process of miRNA biogenesis?

A
  • pri-miRNA transcribed in the nucleus
  • processed by microprocessor into pre-miRNA
  • shuttled into the cytoplasm
  • made into miRNA by DICER
  • associates with AGO proteins
  • active strand leads this complex to target mRNA and inhibits its translation
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3
Q

What 3 ways can pri-miRNA be transcribe in the nucleus?

A
  • independent promoter
  • between two exons as part of a ‘host’ gene
  • as its own exon on part of a ‘host’ gene
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4
Q

How is pri-miRNA processed into pre-mRNA in the nucleus?

A
  • micoprocessor = DROSHA + DGCR8 co factor
  • Drosha cleaves the lower stem and leaves 3’ overhands
  • DGCR8 allows RNA binding
  • F1 + 3 are degraded
  • dsF2 (pre-miRNA) is shuttled out of the nucleus
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5
Q

How is pre-miRNA processed into miRNA in the cytoplasm?

A
  • DICER cleaves the terminal loop
  • PAZ binding pocket anchors and stabilises the 3’ overhang
  • miRNA duplex is formed
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6
Q

How does miRNA go to carry out its function?

A
  • Dicer binds with argonaute proteins
  • one strand of miRNA also binds AGO proteins
  • scaffolding proteins bridge with others to form mRISC (miRNA-induced silencing complex)
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7
Q

How and where do miRNAs act with complete complementarity?

A
  • mostly in plants
  • perfect binding between mRNA and miRNA
  • places RNAse domain of AGOs in the middle of the RNA
  • slices between the 10th and 11th nucleotide from the 5’ end
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8
Q

How and where do miRNAs act with incomplete complimentarity?

A
  • in mammals
  • binds seed residues with almost perfect binding
  • anchors the miRNA to the mRNA and represses its transcription
  • hard to predict where miRNAs will bind as secondary RNA structures are also involved
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9
Q

What are 3 ways miRNAs can lead to transcriptional repression?

A
  • repress transcription initiation
  • repress post-initiation
  • target mRNA destabilisation
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10
Q

How can miRNAs repress translation initiation? (2)

A
  • inhibit cap recognition
  • prevent binding of 40S or 60S of the ribosome
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11
Q

How can miRNAs inhibit translation post-initiation?

A

when the ribosomes encounter the bound miRNA they drop off due to steric hindrance

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12
Q

How can miRNAs cause mRNA destabilisation?

A
  • recruits RNA decapping proteins to digest the 5’ cap
  • recruits deadenylating enzymes to digest the poly A tail
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13
Q

How can miRNA be altered in cancer? (4)

A
  • altered transcriptino of pri-miRNA
  • altered processing of pri-mRNA
  • altered transport of pre-mRNA
  • altered processing of pre-mRNA
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14
Q

How could deregulation in pri-mRNA transcription lead to cancer?

A
  • increased transcription leads to decreased expression of its target and vice versa
  • overexpression of oncogenic miRNAs
  • underexpression of TSmiRNAs
  • e/g/ decreased miR-143+145 in colorectal cancer that normally inhibit EMT, proliferation and survival
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15
Q

How can defective pri-miRNA processing lead to cancer?

A
  • mutations in cofactors such as DGCR8
  • 70% of wilms tumours have mutations that alter metal binding reducing the function of Drosha
  • incorrect processing leads to differential expression of miRNAs and mRNAs
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16
Q

How can defective pre-miRNA export lead to cancer

A
  • mutations in XPO5 cause a truncated protein that reduces export seen in colorectal cancer
  • changes in XPO5 expression is seen in prostate adenocarcinoma
17
Q

How can defective pre-miRNA processing lead to cancer?

A
  • mutations of DICER1 increase risk of childhood lung cancer
  • mutations in metal binding residues can lead to incomplete cleavage where the loop remains in tact
18
Q

How can miRNA be useful in research?

A
  • pre-miRNA or miRNA can be integrated into genomes to inhibit protein translation
  • complete complimentary can minimise off-target effects
19
Q

How can miRNA be used for RNA-based therapies?

A
  • RNA as the drug i.e. restoring tumour suppressor miRNA
  • RNA drugged i.e. inhibiting oncogenic miRNA or altering their biogenesis
  • example of an oncogenic miRNA is miR-155 that inhibits TSGs and SOCS proteins

Developmental genetics and embyronal cancer (17 decks)

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