Matrix metalloproteinase 9 (MMP9) level and MMP9 -1562C>T in patients with obstructive sleep apnea: a systematic review and meta-analysis of case-control studies.BACKGROUND
 The peripheral level of matrix metalloproteinase (MMP)-9 and polymorphism of MMP9 -1562C>T in patients with obstructive sleep apnea (OSA) remains controversial.  Therefore, the aims of this systemic review and meta-analysis are to assess the MMP9 level in OSA patients and identify the relationship between MMP9 -1562C>T and OSA susceptibility.


 This systematic review was performed following the PRISMA guideline.  We searched for studies in major databases, identifying those indexed from inception to July 3, 2019 which related to MMP9 level, MMP9 -1562C>T and OSA.  The pooled standardized mean differences (SMDs) and 95% confidence interval (CI) of MMP9 levels were calculated.  In addition, the relationship between MMP9 -1562C>T and OSA susceptibility was assessed by three genetic models.  The heterogeneity analysis and calculation of the pooled odds ratio (OR) were also performed, followed by quality assessment using the Newcastle-Ottawa Scale (NOS).


 In sum, our review included 15 eligible studies regarding MMP9 level and three regarding MMP9 -1562C>T.  The pooled results showed that peripheral level of MMP9 was increased in OSA patients (SMD = 1.37; 95% CI = 1.15-1.59).  Furthermore, significant difference of MMP9 level can be found between severe and mild-to-moderate OSA patients (SMD = 28.17; 95% CI = 4.23-52.11) or between moderate-severe and mild OSA (SMD = 36.62; 95% CI = 12.19-61.04).  However, no relationship was observed between MMP9 -1562C>T and OSA susceptibility in three genetic models (Homozygote model, OR = 1.37; 95% CI = 0.87-2.18); (Recessive model, OR = 1.42; 95% CI = 0.83-2.42); (Allele model, OR = 1.07; 95% CI = 0.96-1.18).


 CONCLUSIONS
 This systemic review and meta-analysis indicated that the level of MMP9 was increased in patients with OSA and this increase is relevant to OSA severity.  Moreover, the relationship between MMP9 -1562 C>T and OSA susceptibility has currently not been proven by current merging values.  Further analyses with larger sample size are required to verify these associations.
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