TREX1 Suppresses L1 And Maintains Genome Integrity Through ORF1p Depletion Pathway

Maintaining genome integrity is important for cells,and damaged DNA triggers autoimmunity.Previous studies have reported that TREX1,an endogenous DNA exonuclease,prevents immune activation by depleting damaged DNA,thus preventing the development of certain autoimmune diseases.Consistently,mutations in TREX1 are linked with autoimmune diseases such as systemic lupus erythematosus,Aicardi-Goutières syndrome(AGS),and familial chilblain lupus.Therefore,it is significant to do depth research on the biological functions of TREX1 protein.Absed on all the above,researches in the field approve the suggestion that TREX1 prevents autoimmune activation by digesting DNA.However,TREX1 mutants competent for DNA exonuclease activity are also linked to AGS,which indicated that there are more to be known about the function of TREX1 on auto-immunology.L1,as the only auto-active retroelement,the over active of L1 will lead to some autoimmune diseases like AGS.Moreover,it was reported that TREX1 could suppress the activity of L1.Based on above,we are sure that there are more to be known on TREX1 suppressing L1 and TREX1 function on TREX1-AGS.Firstly,It is not certain whether TREX1 suppresses L1 activity through its DNase activity.Secondly,the mechanism of TREX1 suppressing L1 needs to be discovered.Thirdly,the relationship between TREX1 against L1 and TREX1 mutantions induced AGS is unknown.The above are the questions need to be discovered in this paper.We got the following achievement by researching on the biological function of TREX1 against L1: 1.We report a nuclease-independent involvement of TREX1 in suppressing L1L1 maintains about 17% of the human genome,which means a big percentage.However,the copies of active L1 are only about 100,maintaining a very low level.If the active level of L1 is elevated,there will be more L1 retrotranscripted cDNA fragments and more insertions to genome by L1,which induce great damage to the genome.Moreover,the DNA fragments produced will be recognized by DNA sensor,which activates the auto-immune activation.TREX1 protein has a strong inhibitory effect on L1 activity,in theory,it is likely that the TREX1 DNase function plays a role in the down-regulation of L1 activity.However,we tested the DNase activity of two AGS related TREX1 mutants(R114H,V201D),we found that even if they own strong the ability of hydrolysis of DNA,they still cannot inhibit the activity of L1.On the other hand,we tried the TREX1 DNase mutants(D18A,E20 A,D130A,H195 A,D200A),and found that they own the ability to inhibit L1 activity,although they do not own the ability of DNA hydrolysis.From the two aspects above,we can prove that: TREX1 inhibition of L1 function does not require the DNase enzyme activity of TREX1 itself.2.Inhibition of TREX1 to L1 can protect the genome from L1 induced damage to the genomeL1 is a kind of self reverse transcription element,which can be used to reverse the cDNA through the expression of its proteins and insert the fragments into the genome,which can cause great damage to the human genome.The comet assay is designed to detect genome damage level,we found when L1 was transfected into cells,the genome has been destroyed with the comet tails,but when TREX1 added to the system,we found that the comet tail disappeared,indicating that TREX1 inhibits L1 activity to protect the genome from damage by L1.3.TREX1 suppresses L1 by down-regulating ORF1 p through the proteasome pathway.L1 has two open reading frames that expressing the protein: ORF1 p and ORF2 p,which are necessary to complete its function.We verified that TREX1 had a strong effect on the degradation of ORF1 p,and we determined that the degradation of TREX1 to ORF1 p was accomplished through proteasome hydrolysis by proteasome inhibitor MG132.Thus,we completed the study of the molecular mechanism of TREX1 inhibiting L1 activity.4.The AGS related mutants of TREX1 lack the ability to degrade ORF1 p and can not prevent L1 mediated destruction of the genome,leading to the occurrence of autoimmune diseases such as AGS.TREX1,the first identified protein associated with AGS disease,was named as AGS1,and the TREX1 mutants were also reported to be the direct cause of AGS disease.Our study showed that AGS related TREX1 mutants lack the ability of degradation ORF1 p,which can not inhibit the activity of L1.When the activity of L1 is too high,on one hand,excess DNA fragments will produce,on the other hand,there will be more insertions into the genome to destabilize the genome,these two factors will lead to cell immune activation,resulting in the occurrence of AGS disease.The results of our research provide more detailed evidence,at the same time,we have done a better explanation.Therefore,our findings not only reveal a new mechanism for TREX1-mediated L1 suppression and uncover a new function for TREX1 in protein destabilization,but they also suggest a novel mechanism for TREX1-mediated suppression of innate immune activation through maintaining genome integrity.