The Let-7 microRNA precursor was identified from a study of developmental timing in C. elegans, and was later shown to be part of a much larger class of non-coding RNAs termed microRNAs. miR-98 microRNA precursor from human is a let-7 family member. Let-7 miRNAs have now been predicted or experimentally confirmed in a wide range of species (MIPF0000002). miRNAs are initially transcribed in long transcripts (up to several hundred nucleotides) called primary miRNAs (pri-miRNAs), which are processed in the nucleus by Drosha and Pasha to hairpin structures of about 70 nucleotide. These precursors (pre-miRNAs) are exported to the cytoplasm by exportin5, where they are subsequently processed by the enzyme Dicer to a ~22 nucleotide mature miRNA. The involvement of Dicer in miRNA processing demonstrates a relationship with the phenomenon of RNA interference.
Genomic Locations
In human genome, the cluster let-7a-1/let-7f-1/let-7d is inside the region B at 9q22.3, with the defining marker D9S280-D9S1809. One minimal LOH (loss of heterozygosity) region, between loci D11S1345-D11S1316, contains the cluster miR-125b1/let-7a-2/miR-100. The cluster miR-99a/let-7c/miR-125b-2 is in a 21p11.1 region of HD (homozygous deletions). The cluster let-7g/miR-135-1 is in region 3 at 3p21.1-p21.2.
The let-7 family
The lethal-7 (let-7) gene was first discovered in the nematode as a key developmental regulator and became one of the first two known microRNAs (the other one is lin-4). Soon, let-7 was found in fruit fly, and identified as the first known human miRNA by a BLAST (basic local alignment search tool) research. The mature form of let-7 family members is highly conserved across species.
In C.elegans
In C.elegans, the let-7 family consists of genes encoding nine miRNAs sharing the same seed sequence. Among them, let-7, mir-84, mir-48 and mir-241 are involved in C.elegans heterochronic pathway, sequentially controlling developmental timing of larva transitions. Most animals with loss-of-function let-7 mutation burst through their vulvas and die, and therefore the mutant is lethal (let). The mutants of other let-7 family members have a radio-resistant phenotype in vulval cells, which may be related to their ability to repress RAS.
In Drosophila
There is only one single let-7 gene in the Drosophila genome, which has the identical mature sequence to the one in C.elegans. The role of let-7 has been demonstrated in regulating the timing of neuromuscular junction formation in the abdomen and cell-cycle in the wing. Furthermore, the expression of pri-, pre- and mature let-7 have the same rhythmic pattern with the hormone pulse before each cuticular molt in Drosophila.
In vertebrates
The let-7 family has a lot more members in vertebrates than in C.elegans and Drosophila. The sequences, expression timing, as well as genomic clustering of these miRNAs members are all conserved across species. The direct role of let-7 family in vertebrate development has not been clearly shown as in less complex organisms, yet the expression pattern of let-7 family is indeed temporal during developmental processes. Given that the expression levels of let-7 members are significantly low in human cancers and cancer stem cells, the major function of let-7 genes may be to promote terminal differentiation in development and tumor suppression.
Regulation of expression
Although the levels of mature let-7 members are undetectable in undifferentiated cells, the primary transcripts and the hairpin precursors of let-7 are present in these cells. It indicates that the mature let-7 miRNAs may be regulated in a post-transcriptional manner.
By pluripotency promoting factor LIN28
As one of the genes involved in (but not essential for) induced pluripotent stem (iPS) cell reprogramming, LIN28 expression is reciprocal to that of mature let-7. LIN28 selectively binds the primary and precursor forms of let-7, and inhibits the processing of pri-let-7 to form the hairpin precursor. This binding is facilitated by the conserved loop sequence of primary let-7 family members and RNA-binding domains of LIN28 proteins. Lin-28 uses two zinc knuckle domains to recognize the NGNNG motif in the let-7 precursors, while the Cold-shock domain, connected by a flexible linker, binds to a closed loop in the precursors. On the other hand, let-7 miRNAs in mammals have been shown to regulate LIN28, which implies that let-7 might enhance its own level by repressing LIN28, its negative regulator.