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Introduction

Essential hypertension (EH) is a highly heterogeneous disorder that influenced by genetic and environmental factors and their interactions (1). It is reported that 27.2% of the adult population aged 35-74 years suffer from EH (2). Genetic factors are estimated to account for ~30-50% of variation in blood pressure (BP) levels. Great efforts have been made to identify the genes and chromosomal loci associated with blood pressure traits or hypertension over the past decades. EH is often accompanied by metabolic abnormalities such as insulin resistance (IR) and ~50% of hypertensive patients have abnormal glucose tolerance or non-insulin-dependent diabetes (3). IR and secondary hyperinsulinemia may increase blood pressure and participate in the occurrence and development of EH (4). However, the mechanism of EH concomitant IR remains to be elucidated.

Insulin must be mediated by insulin receptor (INSR) to perform its function. INSR gene mutation is often detected in IR patients with moderate to high blood pressure, suggesting that INSR gene may be one of the candidate genes for EH (5). Hanis and Bertin (6) have found that nucleotide at 6244 in human INSR exon 8 was mutated from G to A, resulting in a NsiI polymorphism. Schrader et al (7) found that the restriction fragment polymorphism of INSR NsiI was associated with EH in an Australian population. Similar results were detected in a Korean population (8). However, inconsistent results were obtained in Chinese populations (9,10).

In addition, insulin receptor substrate 1 (IRS-1) is a signaling protein widely distributed in the cytoplasm of insulin-sensitive tissues and serves a key role in signal transduction (11). The IRS-1 gene is a key factor for insulin signaling, which is transmitted radially in different directions from IRS-1(12). IRS-1 G972R (rs1801278) polymorphism was shown to be associated with insulin resistance and may be a candidate gene for EH (13). While Wang et al (14) and Xu et al (15) report no association of IRS-1 G972R polymorphism with EH risk.

Considering the limited sample sizes and inconclusive results in individual studies, it is necessary to evaluate more precise results on the genetic association between the INSR and ISR-1 gene polymorphisms and EH risk.

Materials and methods

Literature strategy

The present meta-analysis followed the Cochrane Collaboration definition and PRISMA 2009 guidelines for meta-analysis and systematic review. A systematically search was carried out in the databases including PubMed, EMBASE, Web of Science and China National Knowledge Infrastructure. All the related publications up to January 20th, 2023 were screened by using the following search strategy: ‘insulin receptor’ or ‘INSR’ and ‘insulin receptor substrate-1’ or ‘ISR-1’ and ‘polymorphism’ or ‘single nucleotide polymorphism’ or ‘SNP’ or ‘variant’ and ‘hypertension’ or ‘essential hypertension’ or ‘EH’ without restrictions on language. In addition, references from the relevant literature were screened manually.

Selection criteria

Inclusion criteria: i) Papers were case-control designed; ii) studies referred to the genetic association between INSR and ISR-1 polymorphisms and EH risk; and iii) publications had available genotype frequencies for estimating an odds ratio (OR) and their 95% confidence interval (CI).

Exclusion criteria: i) Papers were duplicates; ii) papers were abstracts, letters, short communication, review or case report; iii) studies had unavailable data in case or control group.

Data abstraction

The following information was extracted: First author's name, year of publication, ethnicity, mean ages, sex ratio (male%), body mass index, systolic pressure, diastolic blood pressure, methods of genotyping, number of cases and controls, number of genotypes in cases and controls. Authors Yan Wang and Qin Xiang conducted the data extraction independently. When there were discrepancies between the two authors, discussion was applied to resolve the inconsistencies. The quality assessment was performed by using Newcastle-Ottawa Scale (NOS). Studies with a score of ≥6 was included in the present meta-analysis study.

Statistical analysis

Combined ORs and 95% CIs were used to evaluate the relationship of the INSR Nsil, RsaI and ISR-1 G972R polymorphisms with EH susceptibility under different genetic models (allele, dominant and recessive models). The significance of the combined ORs were determined via a Z-test. A chi-squared-based Cochran's Q test and I2 statistics were applied for evaluating the heterogeneity between included articles. The random-effects model was used. Subgroup analyses were conducted stratified by ethnicity in the present meta-analysis. Sensitivity analysis by systematically removing one study at a time was performed to estimate the stability of the results. The possible publication bias was evaluated by the Begg's funnel plot and the Egger linear regression test. The statistical tests were performed using the STATA 12.0 software (Stata Corp) and RevMan 5.2 (Cochrane Collaboration). P<0.05 was considered to indicate a statistically significant difference.

Results

Studies selection and characteristics

A total of 433 studies were originally obtained. Among which, 121 studies were excluded for being duplicates, 185 studies were excluded for being unrelated to the association of INSR and ISR-1 gene polymorphisms and hypertension risk, 107 studies were excluded for not being original articles. Among the excluded studies, Zee et al (16) found that INSR intron 9 polymorphism was related to EH. Qiu et al (17), found that the polymorphism of INSR exon 17 was related to EH in a Chinese population. Finally, 10 eligible publications were chosen for data extraction and quality assessment (7-10,14,15,18-21; Fig. 1). All the original published articles had detected the polymorphisms for human blood. Among the eligible studies, six articles were related to the relationship of INSR Nsil and EH risk and two were related to the association between INSR Rsal and EH risk. A total of two studies were conducted on the association between ISR-1 G972R and EH risk. In addition, seven studies were conducted in Asian populations and three in Caucasian populations. The number of the included studies reporting the genetic associations between the INSR (Nsil and Rsal) and ISR-1 (G972R) polymorphisms and EH risk was enough to perform a meta-analysis. Thus, the present study chose the current polymorphisms for the analysis. The relevant information for eligible studies is listed in Table I.

Figure 1 Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow chart of studies inclusion and exclusion.

Table I Characters of eligible studies in the present study.

Table I

Characters of eligible studies in the present study.

First author, year	Ethnicity	Age	Sex (M%) (case/control)	BMI (case/control)	SBP (case/control)	DBP (case/control)	Genotyping methods	Case	Control	NOS	(Refs.)
Ao, 2006	Chinese	52.40±12.34/ 43.54±9.79	38.1/39.7	26.65±4.37/ 25.40±3.61	NA	NA	PCR-RFLP	84	199	6	(10)
Kang, 2000	Korean	NA/NA	NA	NA	NA	NA	PCR-RFLP	86	134	6	(8)
Lin, 2000	Chinese	53. 36±10.12/ 49. 47±9.59	55.0/62.8	24.91±3.22/ 25.08±2.74	169.15±28.36/ 170.48±10.36	7.98±16.15/ 8.76±8.44	PCR-RFLP	120	86	8	(9)
Schrader, 1996	Australian	52±12/46±10	46.3/62.7	26.2±4.5/ 25.4±4.3	176±24/ 116±9	111±18/ 73±8	PCR-RFLP	134	126	8	(7)
Yu, 2007	Chinese	53.36±10.12/ 49.47±9.59	36.7/44.6	23.00±3.54/ 24.88±4.25	177.01±17.25/ 114.00±11.53	98.31±13.54/ 75.23±9.14	PCR-RFLP	221	204	8	(21)
Zhu, 2009	Chinese	63.52±4.60/ 63.94±4.84	55.8/60.6	25.37±2.69/ 24.34±2.60	162±12/ 123±11	95±9/77±7	PCR-RFLP	190	94	8	(18)
Morris, 1993	Australia	50±14/46±19	54.0/61.0	25±5/NA	176±22/ 113±7	114±22/ 71±5	PCR-RFLP	85	100	8	(19)
Ying, 1991	Australia	51±14/40±11	NA	NA	NA	NA	PCR-RFLP	67	75	6	(20)
Wang, 2007	Chinese	51.22±10.86/ 53±11.85	55.8/58.0	26.48±4.09/ 24. 25±3.56	NA	NA	PCR-RFLP	120	100	7	(14)
Xu, 2006	Chinese	52.71±12.16/ 44.03±10.55	37.6/47.3	26.88±4.31/ 25.55±3.97	NA	NA	PCR-RFLP	182	375	7	(15)

[i] M, male; BMI, Body Mass Index; SBP, Systolic pressure; DBP, Diastolic blood pressure; NOS, Newcastle-Ottawa Scale; NA, Not available.

Meta-analyses for INSR and ISR-1 polymorphisms and EH risk

The combined ORs for the allele [P=0.94, OR=1.02, (95% CI)=(0.68, 1.53)], dominant [P=0.91, OR=0.98, (95% CI)=(0.65, 1.46)] and recessive models [P=0.68, OR=1.25, (95% CI)=(0.44, 3.52)] of INSR Nsil polymorphism failed to detect an association. Subgroup analysis according to ethnicity showed no association between the allele, dominant and recessive models of INSR Nsil polymorphism and EH risk either in Asian or in Caucasian populations (P>0.05; Table II; Fig. 2).

Figure 2 Forest plots of odds ratios for the association between INSR NsiI and hypertension. (A) Allelic model; (B) dominant model and (C) recessive model.

Table II Genetic association between the INSR (Nsil and Rsal) and ISR-1 (G972R) genes and hypertension risk.

Table II

Genetic association between the INSR (Nsil and Rsal) and ISR-1 (G972R) genes and hypertension risk.

					Test of association				Test of heterogeneity
Gene/polymorphism	Minor allele frequency	Genetic models	Ethnicity	Number of studies	OR	95% CI	P-value	Model	P-value	I2 (%)
INSR/Nsil	Unavailable	Allele	Total	6	1.02	(0.68, 1.53)	0.94	R	0.003	73
			Asian	5	1.11	(0.69, 1.78)	0.66	R	0.007	71
			Caucasian	1	0.72	(0.48, 1.07)	0.11	-	-	-
		Dominant	Total	6	0.98	(0.65, 1.46)	0.91	R	0.02	63
			Asian	5	1.06	(0.67, 1.69)	0.80	R	0.03	63
			Caucasian	1	0.70	(0.43, 1.14)	0.15	-	-	-
		Recessive	Total	6	1.25	(0.44, 3.52)	0.68	R	0.08	53
			Asian	5	1.97	(0.75, 5.22)	0.17	R	0.25	27
			Caucasian	1	0.45	(0.13, 1.55)	0.21	-	-	-
INSR/Rsal	Unavailable	Allele	Total	2	0.58	(0.42, 0.80)	0.0008	R	0.96	0
			Asian	0	-	-	-	-	-	-
			Caucasian	2	0.58	(0.42, 0.80)	0.0008	R	0.96	0
		Dominant	Total	2	0.59	(0.38, 0.92)	0.02	R	0.87	0
			Asian	0	-	-	-	-	-	-
			Caucasian	2	0.59	(0.38, 0.92)	0.02	R	0.87	0
		Recessive	Total	2	0.38	(0.20, 0.72)	0.003	R	0.93	0
			Asian	0	-	-	-	-	-	-
			Caucasian	2	0.38	(0.20, 0.72)	0.003	R	0.93	0
ISR-1/G972R	T=0.062104/	Allele	Total	2	1.66	(0.52, 5.28)	0.39	F	0.99	0
	11306 (ALFA)		Asian	2	1.66	(0.52, 5.28)	0.39	F	0.99	0
			Caucasian	0	-	-	-	-	-	-
		Dominant	total	2	1.67	(0.52, 5.33)	0.39	F	0.99	0
			Asian	2	1.67	(0.52, 5.33)	0.39	F	0.99	0
			Caucasian	0	-	-	-	-	-	-
		Recessive	Total	2	1.66	(0.52, 5.28)	0.39	F	0.99	0
			Asian	2	1.66	(0.52, 5.28)	0.39	F	0.99	0
			Caucasian	0	-	-	-	-	-	-

[i] ALFA, Allele Frequency Aggregator; INSR, insulin receptor; IRS-1, insulin receptor substrate-1; OR, odds ratios; CI, confidence interval; F, fixed model; R, Random model.

In addition, significant associations were found for the allele [P=0.0008, OR=0.58, (95% CI)=(0.42, 0.80)], dominant [P=0.02, OR=0.59, (95% CI)=(0.38, 0.92)] and recessive models [P=0.003, OR=0.38, (95% CI)=(0.20, 0.72)] of INSR Rsal polymorphism and EH susceptibility. Subgroup analysis according to ethnicity showed that the significant associations between the allele, dominant and recessive models of INSR Rsal polymorphism and EH risk were observed in Caucasian, but not in Asian populations (P>0.05; Table II; Fig. 3).

Figure 3 Forest plots of odds ratios for the association between INSR RsaI and hypertension. (A) Allelic model; (B) dominant model and (C) recessive model.

Furthermore, the pooled ORs for all the genetic models of ISR-1 G972R polymorphism failed to show an association with EH risk (P>0.05). Similar results were detected in the subgroup analysis stratified by ethnicity (Table II; Fig. 4).

Figure 4 Forest plots of odds ratios for the association between ISR-1 G972R and hypertension. (A) Allelic model; (B) dominant model and (C) recessive model.

Source of heterogeneity

Significant heterogeneities were found in the allele, dominant, and recessive models of INSR Nsil polymorphism. The studies conducted by Schrader et al (7) and Kang et al (8) contributed mainly to the significant heterogeneity. A 0% (P>0.05) heterogeneity was obtained after removing these two studies (Fig. 5).

Figure 5 Sensitivity analyses between INSR NsiI, RsaI and ISR-1 G972R and hypertension. (A) INSR NsiI; (B) INSR RsaI; (C) ISR-1 G972R.

Sensitive analysis and Publication bias

Publication bias in the included studies was assessed using the Begg's funnel plot and Egger's linear regression test. The shapes of the Begg's funnel plot did not reveal any evidence of obvious asymmetry for INSR Nsil polymorphism as shown in Fig. 6. The Egger's linear regression test also did not display a strong statistical evidence of publication bias (t=-0.04, P=0.971). As there were only two studies in analyzing the relationship of INSR Rsal and ISR-1 G972R and EH risk, the Egger's linear regression test were not performed.

Figure 6 Publication bias of literatures for allelic model of INSR NsiI was tested by Begg's funnel plot and Egger's test. The Begg's funnel plot and Egger's test of INSR RsaI and ISR-1 G972R were not performed for lack of data.

Discussion

INSR gene mutation can affect receptor function in several ways: i) Reduced receptor synthesis rate; ii) abnormal receptor embedding process; iii) decreased affinity between the receptor and insulin; iv) decreased tyrosine kinase activity; and v) accelerated receptor degradation (22). The mutation inhibits INSR recycling and the receptor degradation is relatively dominant, leading to a decrease in the number of INSRs on the cell membrane (23). Insulin receptor gene, as a candidate gene for many diseases, is involved in the occurrence of hypertension, obesity, atherosclerosis and other diseases (24). The INSR gene is located on chromosome 19, which contains 22 exons and 21 introns (25). Some studies have confirmed that INSR gene polymorphism is related to EH. For example, Zee et al (16) found that INSR 9 intron polymorphism is related to EH. Ying et al (20) found that hypertension is related to INSR gene RsaI polymorphism, but not to insulin gene polymorphism. Qiu et al (17), found that the polymorphism of INSR exon 17 is related to EH in Chinese populations.

The nucleotide at INSR exon 8 6244 loci is mutated from G to A, resulting in NsiI polymorphism (6). Schrader et al (7) found that INSR NsiI polymorphism is associated with EH in the study of 134 cases in a Caucasian population. The frequency of N1 allele in EH group is higher compared with that in the normal control group. Lin et al (9) report that INSR NsiI polymorphism is associated with EH risk in the Chinese population and the frequency of N2 allele is higher compared with that in the normal group. In addition, the frequency of the N2 allele was higher in male EH patients compared with that in male controls, but not in the female group. Thus, the N2 allele may be the susceptible factor for EH in the Chinese male group. Moreover, no association was found between the INSR NsiI polymorphism and EH risk in Xinjiang Mongol and Jiangsu Chinese Han populations (10,18). These inconsistencies may be due to the regional differences between north and south China. The present meta-analysis enrolled six studies consisting of 835 cases and 843 controls and found that the INSR NsiI polymorphism was not a susceptible factor for EH. Subgroup analysis stratified by ethnicity showed that the INSR NsiI polymorphism was not a susceptible factor for EH in the Asian or Caucasian populations. It may be concluded that the INSR NsiI polymorphism is not associated with EH risk. Notably, INSR NsiI polymorphism does not cause alteration in amino acid sequences. Therefore, this nucleotide conversion was not directly involved in the pathogenesis of EH, but may be due to the linkage disequilibrium between this site and the pathogenic gene locus of hypertension. The genes that play a role in the pathogenesis of EH may be in other exons, introns, or regulatory sequences of INSR, and may also be in other genes. Finding the key gene of primary hypertension is still the focus of current EH molecular epidemiology research, which will not only improve and clarify the molecular mechanism of EH pathogenesis, but also provide the basis for EH gene therapy research.

INSR RsaI R1 has been mapped to the 5.3 kb region of INSR cDNA, located between 1928 and 2478, that is, near the hinge region separating the receptor α and β strands, where rare mutations lead to severe insulin resistance. Defects in EH that lead to insulin resistance are likely to involve defects in the glucose transport pathway coupled with the insulin receptor and glucose transport system (26). However, neither of these substances is likely to cause EH. Insulin receptor subunits have complete tyrosine kinase activity and may participate in the second messenger generation (27). Genetic variation in this receptor region may lead to reduced glucose transport and insulin resistance. However, the mechanism by which EH occurs is unclear. The INSR RsaI polymorphism may itself alter the activity of the encoded chain tyrosine kinase or may affect other sites associated with changes in enzyme activity. In addition, INSR RsaI polymorphism may result in decreased affinity of insulin receptors, decreased number of receptors, or altered gene regulation (16). Therefore, the function of INSR RsaI polymorphism affecting the EH need to be studied further. The present meta-analysis found an association between the INSR RsaI polymorphism and EH, which agreed with previous result of Zee et al (19) and Ying et al (20). However, subgroup analysis in Asian population failed to find an association between INSR RsaI polymorphism and EH risk due to lack of data.

The IRS-1 gene is located at 2q36-37 and serves an important role in insulin signaling (28). Studies have shown significant increases in blood pressure and triglyceride levels in IRS-1 deficient mice, accompanied by impaired endothelium-dependent vasodilation (29). Perticone et al (30) found in a study of 100 hypertensive patients that IRS-1 gene polymorphisms may contribute to hypertension by causing endothelial dysfunction. Federici et al (31) found that IRS-1 G972R mutation damages the release of NO in endothelial cells, which may lead to endothelial dysfunction and cardiovascular disease. There are ethnic differences in the mutation rate of the IRS-1 G972R polymorphism. The mutation rate of IRS-1 G972R polymorphism in Asian population is lower compared with that in European and American populations (32-33). However, the present study found no link between G972R and EH risk. In the present study, no association was found between all the genetic models of IRS-1 G972R polymorphism, which was in contrast with previous work conducted by Wang et al (14) and Xu et al (15). Thus, we may conclude that the IRS-1 G972R polymorphism might not be a susceptible factor for EH. Due to the small number of samples in the present combined study, large sample studies are still needed for confirmation.

Limitations of this study should also be considered. First, the number of included studies and subjects in the present study were relatively small, which might partly reduce the calculation power for the association between the INSR RsaI and ISR-1 G972R polymorphisms and EH susceptibility. Second, a subgroup analysis based on ethnicity could not be performed due to a lack of data. It is necessary to conduct additional studies among multiple ethnicities in the future. Third, multiple gene changes are involved in the occurrence and development of EH and there are strong interactions among multiple mutant genotypes and environmental risk factors, which greatly increases the risk of EH susceptibility. However, the present study could not assess the effect of interaction of these two types of factors in the development of EH in this study. Fourth, only case-control studies were included in present study which may partly influence the accuracy of the genetic polymorphisms and EH. To confirm this results, other kinds of studies, such as cohort studies, would be necessary. Fifth, multiple factors were shown to serve a role in the pathology of EH, However, the present study could not assess the association between the INSR (Nsil and Rsal) and ISR-1 (G972R) polymorphisms and EH adjusting for traditional risk factors, such as gender, diet, stress, smoking, weight and drugs, for lack of sufficient data. Association studies with detailed definition of traditional risk factors would be necessary. Last, the NOS scores were not high in several studies, which may be due to the study design, Thus, more studies with higher NOS scores should be included.

The data of the present study suggested that the INSR Rsal polymorphism is highly probable to be a protective factor for EH and provided evidence the INSR Rsal polymorphism is associated with EH risk.

Acknowledgements

Not applicable.

Funding

Funding: The present study was supported by the Changsha Outstanding Innovative Young People Training Scheme (grant nos. kq2206058 and kq2206056), The Foundation of Project of Hunan Health and Family Planning Commission (grant no. 202202082739), The Foundation of the Education Department of Hunan Province (grant no. 21A0586), The Foundation of the Education Department of Guangxi Province (grant no. 2021KY1959); and The Hunan Key Laboratory Cultivation Base of the Research and Development of Novel Pharmaceutical Preparations (grant no. 2016TP1029).

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Authors' contributions

LT and JML participated in study design and data collection, carried out the initial analysis and drafted the article. YW and QX aided in data acquisition, data analysis and statistical analysis. LT and YW carried out literature search, data acquisition and manuscript editing. QX and JML confirm the authenticity of all the raw data. TL and JML performed manuscript review. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

References