Introduction
In East Asian population, the prevalence of early-onset diabetes has increased rapidly in recent years, posing serious challenges for healthcare systems [
1]. Patients with early-onset diabetes are more likely to have a family history and poor metabolic control compared with patients with late onset [
2], which is suggestive of the strong involvement of genetic factors underlying the pathophysiology of early-onset diabetes. Therefore, an insight into the genetic basis of early-onset diabetes may contribute to the discovery of novel pathogenic mutations and therapeutic targets [
3].
Studies have reported extensive discrepancies in diabetes between East Asians and Europeans [
4]. The prevalence of early-onset diabetes in East Asians is much higher than in Europeans [
5]. East Asian diabetic patients usually have a younger onset age and a lower body mass index (BMI) than those in Europe, and their insulin secretion tends to decline quickly [
4,
6,
7]. Of note,
PAX4 plays a critical role in the development, differentiation, expansion, and survival of islet insulin-producing β-cells [
8,
9]; the missense variant R192H impairs β-cell development and survival [
10,
11]. Besides,
PAX4 R192H and R192S were reported to be common in East Asians but rare in Europeans [
12]. Thus, it would be of interest to explore the association of variants in
PAX4 with early-onset diabetes in young Chinese, attempting to explain the discrepancy of diabetes between the East and the West from a genetic perspective.
Unlike the extensive genetic studies of late-onset diabetes, limited information is available on the genetic architecture of early-onset diabetes. For assessing the exome variants that may be specific in populations, gene panel testing is simple, is more economical, and has greater scalability compared with whole-genome or exome sequencing. Therefore, in the present study, we analysed the exome variants of 80 Chinese early-onset diabetes patients by a customized gene panel after the exclusion of type 1 diabetes (T1D) [
2,
13]. Then, 229 Chinese early-onset diabetes cases (T1D excluded) and 1679 controls were genotyped to validate the association of the
PAX4 variants detected by the gene panel with early-onset diabetes and the related clinical features.
Discussion
In the present study, we developed a customized gene panel to explore the genetic traits in Chinese patients with early-onset diabetes. We identified nine MODY and two FPLD patients with five novel mutations in 80 patients with early-onset diabetes after the exclusion of T1D patients. We further genotyped PAX4 R192H/S in 229 early-onset diabetes (T1D excluded) patients and 1679 non-diabetes controls and found that the East Asian specifically enriched PAX4 R192H/S was associated with an increased risk of early-onset diabetes with decreased C-peptide levels.
In the present study, we detected the presence of MODY1/3/4/7, of which MODY3 (6/9) was the most common type, but MODY2 was undetected, which is consistent with the findings of a previous study on MODY in Chinese patients [
29]. Previous studies have shown that MODY3 with mutations in
HNF1α and MODY2 with mutations in
GCK were the two major types of MODYs among Caucasians and Koreans [
30]. In addition, four novel pathogenic mutations of MODY3/4/7 were detected in our study. With genotyping diagnosis, six MODY3 patients and one MODY1 patient were recommended to receive hypoglycemic sulfonylurea treatment, as they were more sensitive to sulfonylurea than to other drugs [
3]. These findings indicate that the gene panel contributes to the detection of novel mutations and effectively benefits the diagnosis and precise treatment of MD.
Previous studies focused on the relatively older T2D patients (mean age > 40 or 50 years) in Korean, Chinese and Singaporean subjects demonstrated the association of
PAX4 R192H/S with T2D [
12,
31‐
33]. Another study on Thai patients found that the frequency of the
PAX4 R192H variant was higher in MODY [
34]. However, in the present study, we concentrated on young Chinese diabetic patients (mean age 23.60 ± 6.51 years) and found that
PAX4 R192H and R192S variants were associated with early-onset diabetes. Therefore, to the best of our knowledge, our research is the first study to report the missense variants R192H/S of
PAX4 associated with early-onset diabetes in the young Chinese population. Some previous studies also reported that
PAX4 R192H was associated with a younger onset age of T2D [
32‐
34]. However, we did not observe an younger onset age in
PAX4 R192H/S carriers but found a higher OR of
PAX4 R192H (OR 1.88) than those in the previous studies (OR 1.39 or 1.48) [
32,
33]. This may be due to the younger and narrower age range of the subjects enrolled in our study compared with the previous ones [
32,
33].
Paired-homeodomain transcription factor
PAX4 is a member of the paired box (
PAX) family and located at chromosome 7q32. It plays an important role in the development, differentiation, expansion and survival of islet insulin-producing β-cells [
8,
9]. During the early developmental stage of the pancreas, the
PAX4 gene suppresses the transcription activities of insulin, glucagon, somatostatin, islet amyloid polypeptide and ghrelin promoters mainly in the α-cells, thus repressing α-cell differentiation and permitting β-cell differentiation [
10,
35]. In the pancreas of
PAX4 homozygous mutant mice, insulin-producing β-cells and somatostatin-producing δ-cells are absent, but the population of glucagon-producing α-cells is high [
8]. Moreover, insulin protein levels increase, whereas glucagon protein levels remain constant in
Pax4-overexpressing islets, and
PAX4 mRNA expression is elevated in human insulinomas [
36,
37]. In 2007 and 2011, pathogenic mutations in
PAX4 were shown to lead to MODY9 in Thai and Japanese patients, respectively [
34,
38]. Previous functional experiments have shown that
PAX4 R192H can reduce the repression abilities of insulin and glucagon promoters and impair β-cell development and survival [
10,
11], consistent with lower C-peptide levels in
PAX4 R192H carriers, which may lead to a higher HbA1c level than in non-carriers as reported in our study. More importantly,
PAX4 R192S carriers showed lower FCP and 2 h C-peptide levels in early-onset diabetes patients. As far as we know, this study is the first report to describe that
PAX4 R192S carriers tend to have lower C-peptide levels in diabetes.
East Asians are more susceptible to early-onset diabetes than Europeans [
5] and exhibit reduced insulin secretion at earlier stages [
7]. In the present study,
PAX4 R192H/S was associated with early-onset diabetes in Chinese and lower C-peptide levels in early-onset diabetes patients. Also, different ethnicities have distinct frequencies of
PAX4 R192H and R192S according to the database gnomAD v2.1.1, the two variants are specifically higher in East Asians (R192H, MAF = 0.109; R912S, MAF = 0.039). However, the frequencies of the variants were extremely low in Europeans (MAF < 0.000). Therefore, we presume that the East Asian-specific enrichment of
PAX4 R192H/S may be one explanation for the differences in diabetes between the East and the West.
Our study was limited with a relatively small sample size. Hence, a large-scale study of the early-onset diabetes population from East Asia is needed to validate our findings. It may also be important to develop a humanized PAX4 R192H/S variant knock-in mouse model to illustrate the causality between the two risk variants and diabetes and the pathogenic mechanism that underlies early-onset diabetes.