Drosha is a Class 2
ribonuclease IIIenzyme[5] that in humans is encoded by the DROSHA (formerly RNASEN)
gene.[6][7][8] It is the primary nuclease that executes the initiation step of miRNA processing in the nucleus. It works closely with DGCR8 and in correlation with
Dicer. It has been found significant in clinical knowledge for cancer prognosis[9] and HIV-1 replication.[10]
History
Human Drosha was cloned in 2000 when it was identified as a nuclear dsRNA ribonuclease involved in the processing of
ribosomal RNA precursors.[11] The other two human enzymes that participate in the processing and activity of miRNA are the
Dicer and
Argonaute proteins. Recently, proteins like Drosha have been found significant in cancer prognosis[9] and HIV-1 replication.[10]
The microRNAs thus generated are short
RNA molecules that regulate a wide variety of other genes by interacting with the
RNA-induced silencing complex (RISC) to induce cleavage of
complementarymessenger RNA (mRNA) as part of the
RNA interference pathway. MicroRNA molecules are synthesized as long RNA
primary transcripts known as a pri-miRNAs, which are cleaved by Drosha to produce a characteristic
stem-loop structure of about 70
base pairs long, known as a pre-miRNA.[11] Pre-miRNAs, when associated with EXP5, are stabilized due to removal of the 5' cap and 3' poly(A) tail.[13] Drosha exists as part of a
protein complex called the
Microprocessor complex, which also contains the double-stranded RNA binding protein
DGCR8 (called
Pasha in D. melanogaster and C. elegans).[14] DGCR8 is essential for Drosha activity and is capable of binding single-stranded fragments of the pri-miRNA that are required for proper processing.[15] The Drosha complex also contains several auxiliary factors such as
EWSR1, FUS,
hnRNPs, p68, and p72.[16]
Both Drosha and DGCR8 are localized to the
cell nucleus, where processing of pri-miRNA to pre-miRNA occurs. These two proteins homeostatically control miRNA biogenesis by an auto-feedback loop.[16] A 2nt 3' overhang is generated by Drosha in the nucleus recognized by
Dicer in the cytoplasm, which couples the upstream and downstream processing events. Pre-miRNA is then further processed by the RNase
Dicer into mature miRNAs in the cell
cytoplasm.[11][16] There also exists an isoform of Drosha that does not contain a nuclear localization signal, which results in the generation of c-Drosha.[17][18] This variant has been shown to localize to the cell
cytoplasm rather than the nucleus, but the effects on pri-miRNA processing are yet unclear.
Certain miRNAs have been found to deviate from conventional biogenesis pathways and do not necessarily require Drosha or
Dicer, which is because they do not require the processing of pri-miRNA to pre-miRNA.[16] Drosha-independent miRNAs derive from
mirtrons, which are genes that encode for miRNAs in their introns and make use of splicing to bypass Drosha cleavage. Simtrons are mirtron-like, splicing-independent, and do require Drosha mediated cleavage, although they do not require most proteins in the canonical pathway such as
DGCR8 or
Dicer.[10]
Clinical significance
Drosha and other miRNA processing enzymes may be important in cancer prognosis.[20] Both Drosha and
Dicer can function as
master regulators of miRNA processing and have been observed to be down-regulated in some types of
breast cancer.[21] The alternative splicing patterns of Drosha in
The Cancer Genome Atlas have also indicated that c-drosha appears to be enriched in various types of breast cancer,
colon cancer, and
esophageal cancer.[18] However, the exact nature of the association between microRNA processing and
tumorigenesis is unclear,[22] but its function can be effectively examined by siRNA knockdown based on an independent validation.[23]
Drosha and other miRNA processing enzymes may also be important in HIV-1 replication. miRNAs contribute to the innate antiviral defense. This can be shown by the knockdown of two important miRNA processing proteins, Drosha and Dicer, which leads to a significant enhancement of viral replication in PBMCs from HIV-1-infected patients. Thus, Drosha, in conjunction with Dicer, seem to have a role in controlling HIV-1 replication.[10]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Filippov V, Solovyev V, Filippova M, Gill SS (March 2000). "A novel type of RNase III family proteins in eukaryotes". Gene. 245 (1): 213–21.
doi:
10.1016/s0378-1119(99)00571-5.
PMID10713462.
^Filippov V, Solovyev V, Filippova M, Gill SS (March 2000). "A novel type of RNase III family proteins in eukaryotes". Gene. 245 (1): 213–21.
doi:
10.1016/S0378-1119(99)00571-5.
PMID10713462.
^Sloan, K. E., Gleizes, P. E., & Bohnsack, M. T. (2016). Nucleocytoplasmic transport of RNAs and RNA–protein complexes. Journal of molecular biology, 428(10), 2040-2059.
Gunther M, Laithier M, Brison O (July 2000). "A set of proteins interacting with transcription factor Sp1 identified in a two-hybrid screening". Molecular and Cellular Biochemistry. 210 (1–2): 131–42.
doi:
10.1023/A:1007177623283.
PMID10976766.
S2CID1339642.
Sugito N, Ishiguro H, Kuwabara Y, Kimura M, Mitsui A, Kurehara H, Ando T, Mori R, Takashima N, Ogawa R, Fujii Y (December 2006). "RNASEN regulates cell proliferation and affects survival in esophageal cancer patients". Clinical Cancer Research. 12 (24): 7322–8.
doi:
10.1158/1078-0432.CCR-06-0515.
PMID17121874.
S2CID7569257.