the latest definition of lpe includes 3 points : ( i ) ielt<1 minute ; ( ii ) the inability to delay ejaculation ; and ( iii ) negative personal consequences .
previous studies of twins and familial studies had proved the genetic predisposition in lpe [ 57 ] .
a twins study in finnish men indicated that 0.30 of the etiology was a result of hereditary effect .
however , twins and familial studies did not clarify which genetic variant might explain the genetic effects in lpe . to explore this problem , genetic association studies may be a possible approach .
the susceptibility to lpe was considered to be a polygenic pattern , such as those genes linked to the metabolism of serotonin ( 5-ht ) and those related to the expression of serotonin transporter ( 5-htt ) .
firstly , various selective serotonin reuptake inhibitors ( ssris ) were successfully used in the treatment of pe , and dapoxetine became the first approved drug for pe treatment .
secondly , animal experiments also demonstrated the role of 5-ht in ejaculation . decreased duration of intravaginal ejaculation latency time ( ielt )
furthermore , snps located in genes encoding 5-ht receptors were also reported to play a role in ielt of patients with pe [ 1316 ] .
three common polymorphisms were described : an ins / del in the promoter , called 5-httlpr ; a variable number of tandem repeat ( vntr ) in intron 2 , called stin2 ; and a snp polymorphism in the noncoding 3-utr .
in fact , 5-httlpr has been studied thoroughly , but the effect and function of stin2 vntr is not yet clear . it has been proved to include 10-repeat and 12-repeat alleles of a 16/17 bp element in all ethnicities , and the rare 9-repeat allele appears only in people of european or african descent .
stin2 vntr works as a transcriptional regulator , and the transcriptional activity is determined by the number of repeats , with the stin2.12 having the highest expression .
many studies had linked vntr with disorders such as depression , bipolar depression , and a predisposition to obsessive compulsive disorder .
although many studies have explored slc6a4 and lpe [ 2428 ] , only 1 of them studied stin2 vntr , and there have been no such studies in chinese populations .
considering the important role of the serotonergic system in ejaculatory function , we aimed to determine whether stin2 vntr is related to lpe in china .
each participant was informed of the purpose of the study and signed the informed consent .
the initial evaluation of each subject completed a detailed face - to - face interview by an andrologist , including a questionnaire and physical examination .
finally , 115 patients with lpe were recruited by referral from the andrology outpatient clinic . at the same time ,
subjects had to meet the following inclusion criteria : ( i ) heterosexual male patient aged 2060 years ; ( ii ) han descent and speaking chinese ; ( iii ) in a regular sexual relationship with 1 female partner for 6 months .
none of the participants had received antidepressants or phosphodiesterase type 5 inhibitors before enrolling in the trial .
patients with a urinary infection or disorders of the nervous system were also excluded from the study .
the study was approved by the ethics committee of the first affiliated hospital of anhui medical university ( no.20150047 ) .
genomic dna was extracted from peripheral blood using the puregene dna extraction kit ( qiagen , hilden , germany ) following the manufacturer s protocol .
oligonucleotide primers were as follows : forward 5-tggcgagatttgacttttctacc-3 and reverse 5-ctgagcttcatcaaggggaac-3 were used to amplify the stin2 vntr region .
a 20-l mixture was ready for pcr amplification , including hotstartaq buffer , 3.0 mm mg , 0.3 mm dntp , 0.1m of primers , 1 u hotstartaq polymerase ( qiagen , inc . ) , and 1 l template dna .
pcr conditions were as follows : 15 min of initial denaturation at 95c ; 11 cycles of 94c for 20 s and 620.5c per cycle for 40 s and 68c for 2 min ; followed by 24 cycles of 20 s of denaturation at 94c , 56c for 40 s and 2 min of extension at 68c , and a final extension of 60 min at 60c .
after digestion , the products were analyzed on a 3730 dna analyzer ( abi , california ) .
for the quantitative data , results are expressed as mean  standard deviation ( sd ) and a 2-tailed t - test was used .
after the normality and homoscedasticity test , analyses of variance ( anova ) using ielt as a variate were performed to test differences between different genotypes .
each participant was informed of the purpose of the study and signed the informed consent .
the initial evaluation of each subject completed a detailed face - to - face interview by an andrologist , including a questionnaire and physical examination .
finally , 115 patients with lpe were recruited by referral from the andrology outpatient clinic . at the same time ,
subjects had to meet the following inclusion criteria : ( i ) heterosexual male patient aged 2060 years ; ( ii ) han descent and speaking chinese ; ( iii ) in a regular sexual relationship with 1 female partner for 6 months .
none of the participants had received antidepressants or phosphodiesterase type 5 inhibitors before enrolling in the trial .
patients with a urinary infection or disorders of the nervous system were also excluded from the study .
the study was approved by the ethics committee of the first affiliated hospital of anhui medical university ( no.20150047 ) .
genomic dna was extracted from peripheral blood using the puregene dna extraction kit ( qiagen , hilden , germany ) following the manufacturer s protocol .
oligonucleotide primers were as follows : forward 5-tggcgagatttgacttttctacc-3 and reverse 5-ctgagcttcatcaaggggaac-3 were used to amplify the stin2 vntr region .
a 20-l mixture was ready for pcr amplification , including hotstartaq buffer , 3.0 mm mg , 0.3 mm dntp , 0.1m of primers , 1 u hotstartaq polymerase ( qiagen , inc . ) , and 1 l template dna .
pcr conditions were as follows : 15 min of initial denaturation at 95c ; 11 cycles of 94c for 20 s and 620.5c per cycle for 40 s and 68c for 2 min ; followed by 24 cycles of 20 s of denaturation at 94c , 56c for 40 s and 2 min of extension at 68c , and a final extension of 60 min at 60c .
after digestion , the products were analyzed on a 3730 dna analyzer ( abi , california ) .
results are expressed as mean  standard deviation ( sd ) and a 2-tailed t - test was used .
after the normality and homoscedasticity test , analyses of variance ( anova ) using ielt as a variate were performed to test differences between different genotypes .
among 216 subjects who met the inclusion criteria , only 1 patient was excluded from the present study because of sample coagulation .
the patients and controls showed no significant difference in terms of any characteristic except age ( 35.37.8 vs. 31.78.1 , p<0.001 ) .
however , because lpe is assumed be lifelong , the difference did not affect the comparability of the 2 groups .
the most common allele was stin2.12 ( patients : 82.5% , controls : 75.2% ) , and no significant difference was found between the 2 groups ( p=0.182 ) .
the stin2.9/12 and stin2.9/9 genotypes were infrequent , so they were excluded from further analyses .
the results showed a significantly higher frequency of stin2.12/12 genotype in lpe patients than in controls ( p=0.026 ) .
interestingly , the less common stin2.9 allele was present in our study , which was found in people of european or african descent in previous studies .
table 3 shows results of analyses under the 3 models : ( i ) dominant stin2 .
12 model ( stin2.12/12+stin2.12/10 vs. stin2.10/10 ) , ( ii ) dominant stin2.10 model ( stin2.10/10+stin2.12/10 vs. stin2.12/12 ) , and ( iii ) codominant model ( stin2.12/12 vs. stin2.12/10 vs. stin2.10/10 ) . to investigate the association between the 3 models and the risk of pe
, binary logistic regression models were fitted . in the codominant model , we found a significantly lower frequency of stin2.12/10 in the lpe patients ( 23.0% vs. 35.7% , or=0.52 , 95% ci=0.280.96 ) , although the difference was not statistically significant ( p=0.108 ) .
a statistically significant association of the dominant stin2.10 model of stin2 was observed . in this model ,
the combined frequency of stin2.10 carriers was significantly lower in patients with lpe when compared with controls ( 28.3% vs. 41.8% , or=0.55 , 95% ci=0.310.97 , p=0.040 ) .
the mean ielts were 22.6811.41 s , 33.629.13 s , and 33.179.26 s for stin2.12/12 , stin2.12/10 , and stin2.10/10 , respectively .
the results showed significant difference in patients with stin2.12/12 when compared to stin2.12/10 and stin2.10/10 genotypes ( stin2.12/12 vs. stin2.12/10 ; p=0.001 ; stin2.12/12 vs. stin2.10/10 ; p=0.024 ) .
according to the stin2.10 dominant model , the ielt results of stin2.12/12 also showed a significant difference compared to stin2.12/10 + stin2.10/10 genotypes ( p=0.001 ) .
the fold - increase of the mean ielt in the stin2.12/10 and stin2.10/10 genotypes compared to stin2.12/12 genotype was 1.48 .
these data indicate that stin2.10 carriers on average had nearly a 50% longer intravaginal ejaculation time compared to stin2.12 homozygous in this study .
among 216 subjects who met the inclusion criteria , only 1 patient was excluded from the present study because of sample coagulation .
the patients and controls showed no significant difference in terms of any characteristic except age ( 35.37.8 vs. 31.78.1 , p<0.001 ) .
however , because lpe is assumed be lifelong , the difference did not affect the comparability of the 2 groups .
the most common allele was stin2.12 ( patients : 82.5% , controls : 75.2% ) , and no significant difference was found between the 2 groups ( p=0.182 ) .
the stin2.9/12 and stin2.9/9 genotypes were infrequent , so they were excluded from further analyses .
the results showed a significantly higher frequency of stin2.12/12 genotype in lpe patients than in controls ( p=0.026 ) .
interestingly , the less common stin2.9 allele was present in our study , which was found in people of european or african descent in previous studies .
table 3 shows results of analyses under the 3 models : ( i ) dominant stin2 .
12 model ( stin2.12/12+stin2.12/10 vs. stin2.10/10 ) , ( ii ) dominant stin2.10 model ( stin2.10/10+stin2.12/10 vs. stin2.12/12 ) , and ( iii ) codominant model ( stin2.12/12 vs. stin2.12/10 vs. stin2.10/10 ) . to investigate the association between the 3 models and the risk of pe
, binary logistic regression models were fitted . in the codominant model , we found a significantly lower frequency of stin2.12/10 in the lpe patients ( 23.0% vs. 35.7% , or=0.52 , 95% ci=0.280.96 ) , although the difference was not statistically significant ( p=0.108 ) .
a statistically significant association of the dominant stin2.10 model of stin2 was observed . in this model ,
the combined frequency of stin2.10 carriers was significantly lower in patients with lpe when compared with controls ( 28.3% vs. 41.8% , or=0.55 , 95% ci=0.310.97 , p=0.040 ) .
the mean ielts were 22.6811.41 s , 33.629.13 s , and 33.179.26 s for stin2.12/12 , stin2.12/10 , and stin2.10/10 , respectively .
the results showed significant difference in patients with stin2.12/12 when compared to stin2.12/10 and stin2.10/10 genotypes ( stin2.12/12 vs. stin2.12/10 ; p=0.001 ; stin2.12/12 vs. stin2.10/10 ; p=0.024 ) . according to the stin2.10 dominant model
, the ielt results of stin2.12/12 also showed a significant difference compared to stin2.12/10 + stin2.10/10 genotypes ( p=0.001 ) .
the fold - increase of the mean ielt in the stin2.12/10 and stin2.10/10 genotypes compared to stin2.12/12 genotype was 1.48 .
these data indicate that stin2.10 carriers on average had nearly a 50% longer intravaginal ejaculation time compared to stin2.12 homozygous in this study .
recently , polymorphisms in slc6a4 have been increasingly associated with lpe , especially 5-httlpr polymorphism .
our study evaluated 114 patients and 101 controls for the relationship between stin2 vntr and lpe .
the allelic frequency of the stin2.12 was 0.825 in patients and 0.752 in controls , which showed no significant difference ( p=0.067 ) .
there was a significant difference in stin2.12/12 genotype , which was more common in the lpe group ( p=0.026 ) .
however , our results contradicted a previous study by zuccarello et al . , which showed no difference between these 2 groups in genotypic and allelic frequency .
more studies and meta - analyses of stin2 vntr polymorphisms in this field are needed .
our data demonstrate that stin2.12/10 and stin2.10/10 genotypes , which carry the lower transcriptional activity stin2.10 allele , were protective genotypes when compared with the stin2.12/12 genotype .
a third major finding was that ielt in the patients group was associated with stin2 vntr .
therefore , the men carrying the higher expressing allele homozygote stin2.12/12 could have more functioning 5-htt , thus leading to lower 5-ht availability .
on the one hand , gene frequencies of stin2 vntr vary significantly among different races and ethnicities , and is more common in japanese ( 0.997 ) than in african americans ( 0.713 ) and white americans ( 0.523 ) .
our sample size of participants , as well as the relative homogeneity of demographic and clinical characteristics , make our study distinct . on the other hand , the function of stin2 vntr is still not clear .
it may have a synergistic effect together with other polymorphisms on ejaculation , such as 5-httlpr .
, only 1 study found that therapeutic effects were related to both stin2 vntr and 5-httlpr polymorphisms .
the findings indicated stin2.12/12 genotype responded better to sertraline ( p=0.001 ) . we will plan to replicate the present study in this interesting field in the future .
to the best of our knowledge , this is the first study to focus on the stin2 vntr polymorphism in chinese patients with lpe .
our present results indicate that stin2 vntr polymorphism is related to lpe . in summary ,
men with at least 1 copy of stin2.10 allele were at lower risk of pe than men with other variant at this locus .
the current study shows that the stin2 vntr polymorphism is associated with ielts of lpe .
 backgroundthe stin2 vntr polymorphism has a variable number of tandem repeats in intron 2 of the serotonin transporter gene . 
 we aimed to explore the relationship between stin2 vntr polymorphism and lifelong premature ejaculation ( lpe).material / methodswe recruited a total of 115 outpatients who complained of ejaculating prematurely and who were diagnosed as lpe , and 101 controls without pe complaint . 
 allelic variations of stin2 vntr were genotyped using pcr - based technology . 
 we evaluated the associations between stin2 vntr allelic and genotypic frequencies and lpe , as well as the intravaginal ejaculation latency time ( ielt ) of different stin2 vntr genotypes among lpe patients.resultsthe patients and controls did not differ significantly in terms of any characteristic except age . 
 a significantly higher frequency of stin2.12/12 genotype was found among lpe patients versus controls ( p=0.026 ) . 
 frequency of patients carrying at least 1 copy of the 10-repeat allele was significantly lower compared to the control group ( 28.3% vs. 41.8% , or=0.55 ; 95%ci=0.310.97 , p=0.040 ) . in the lpe group , the mean ielt showed significant difference in stin2.12/12 genotype when compared to those with stin2.12/10 and stin2.10/10 genotypes . 
 the mean ielt in10-repeat allele carriers was 50% longer compared to homozygous carriers of the stin2.12 allele.conclusionsour results indicate the presence of stin2.10 allele is a protective factor for lpe . 
 men carrying the higher expression genotype stin2 . 
 12/12 have shorter ielt than 10-repeat allele carriers . 