Background
Among children and adolescents with diabetes mellitus (DM), type 1 DM is the most common, especially in North America and Europe [
1]. However, the annual incidence of type 1 DM varies according to the ethnic background, and worldwide incidence has increased in the past two decades [
1‐
3]. In Korea, the annual incidence of type 1 DM increased from 1.36/100,000 in 1995–2000 to 3.19/100,000 in 2012–2014 [
4]. In addition, increasing worldwide rates of child obesity have been associated with a variable increase in the prevalence of type 2 DM depending on the ethnic background and region of residence [
2].
Most DM cases are classified as type 1 or type 2 DM (i.e., of multifactorial etiology). Type 1 DM is primarily caused by the autoimmune destruction of pancreatic β-cells [
5]. A substantial proportion of type 2 DM is caused by environmental and multiple genetic defects with a smaller effect size. Compared with type 2 DM, monogenic diabetes, including maturity-onset diabetes of the young (MODY), is a rare form of DM caused by mutations in one of more than 20 genes that control either the secretion or action of insulin [
6]. Monogenic diabetes is characterized by Mendelian inheritance pattern with a large effect size of causal variants and minimal environmental contributions [
7]. The prevalence of monogenic diabetes is estimated to be 1.1–6.3% among children and adolescents with DM in Europe [
8‐
13].
It is important to make an accurate etiologic diagnosis of DM since it can affect the therapeutic decisions, the prognosis of chronic complications, and genetic counseling [
14]. However, the overlapping clinical features of various forms of DM make it difficult to perform differential etiologic diagnosis. Information on the distribution of the type of DM and the prevalence of monogenic diabetes has not been established in the Korean pediatric population. A few studies on MODY in Korea demonstrated that mutations in the major MODY genes, including some cases with
GCK mutations, were rare [
15‐
18]. Therefore, this study was performed to investigate the etiologic distribution and clinical characteristics of pediatric diabetes including monogenic diabetes at a single tertiary center over a 20-year period in Korea.
Discussion
This study described the clinical characteristics of different types of DM based on the experiences of a single tertiary center over the last 20 years. Type 1 DM accounted for most cases (74.6%) of DM in the cohort, followed by type 2 DM (17.8%). Genetic etiologies were confirmed in 14 patients (5.1%) from 13 families. Diverse genetic etiologies are associated with pediatric monogenic diabetes, and extra-pancreatic features were found to be an important clue to the diagnosis of monogenic diabetes.
The frequencies of type 1 DM, type 2 DM, and MODY were 85.6, 10.8, and 1.2%, respectively, in the SEARCH study (USA); on the other hand, these ratios were 95.5, 1.3, and 1.5%, respectively, in the SWEET study (Europe) [
30‐
32]. The diagnosis of type 2 DM (17.8%) and genetically confirmed monogenic diabetes (5.1%) was more common in the present study compared with the previous studies. The variation in the frequencies could be explained by the availability of genetic testing and the prevalence of obesity in the region. Antibody positivity has been reported in up to 15% of patients with type 2 DM, and these autoantibody-positive patients are usually younger, less overweight or obese, and have higher HbA1c levels [
33]. However, there were no significant differences in the current study.
Monogenic diabetes comprises various phenotypes including neonatal DM, MODY, and rare syndromic diabetes with extra-pancreatic features including neurological, renal, intestinal, or skeletal abnormalities [
6]. Therefore, monogenic diabetes might be initially misdiagnosed as type 1 or type 2 DM prior to the manifestation of extra-pancreatic features, as in the case of the patients with Wolfram syndrome, DEND syndrome, and IPEX syndrome in the present study.
Establishing the etiology of DM is important for therapeutic strategies, the prognosis of chronic complications [
14], and appropriate genetic counseling for monogenic diabetes [
6]. For example, the molecular diagnosis of monogenic diabetes can lead to changes in treatment, often with improved glycemic control, as some patients with monogenic diabetes carrying mutations in specific genes (e.g.,
HNF1A,
HNF4A,
KCNJ11, and
ABCC8) can be treated with oral sulfonylureas instead of insulin [
34].
MODY is defined as an autosomal dominantly inherited familial form of non-autoimmune diabetes due to a primary defect in pancreatic β-cell function with an age of onset before 25 years of age [
35]. Mutations in 14 different genes are known to be associated with MODY [
https://www.omim.org/entry/606391, accessed on January 2021]. Among them, mutations in
HNF4A,
HNF1A, and
GCK are the most common causes of MODY [
36]. The prevalence of MODY has been estimated to be 1–2% of cases of DM [
37]. In an Italian study, MODY was the second most prevalent cause (5.5%) of DM after type 1 DM; however, mutations in MODY-related genes were documented in approximately 1.9% of patients [
9]. In India, sequence variants in MODY genes were identified in 15–19% of patients with clinically diagnosed MODY in India [
38,
39]. Among them,
HNF1A or
ABCC8 mutations were the most common [
39]. The frequency of mutations in the major MODY genes (
HNF4A,
GCK, and
HNF1A) has been shown to be extremely low among Korean patients with MODY [
17,
25]. However, with the advent of next-generation sequencing, rate at which MODY is diagnosed using genetic screening has increased. In 28 patients with early-onset diabetes in Korea, four pathogenic or likely pathogenic variants were identified in three patients usingy whole exome sequencing [
40]. In targeted panel sequencing, molecular genetic diagnosis was possible in 21.1% of 109 patients who were clinically suspected to have monogenic diabetes [
41].
A diagnosis of MODY is dependent on the active referral of patients who are likely to have MODY, which suggests that some patients with MODY remain underdiagnosed [
42]. Childhood type 2 DM can be confused with MODY owing to a family history and presenting features, as well as obesity or overweight as a possible confounding factor [
11]. Clinically, MODY should be considered in patients with atypical features for type 2 DM, including diabetes onset before the age of 45 years, a normal or low BMI, lack of acanthosis nigricans, and normal serum triglyceride levels and/or normal or elevated high-density lipoprotein cholesterol concentrations [
43]. In addition, high-sensitivity C-reactive protein levels are low in
HNF1A-MODY and can be used to distinguish between
HNF1A-MODY and type 2 DM [
44]. In addition, MODY can be misclassified as type 1 DM, especially in cases involving
HNF1A mutations [
9].
However, MODY can be differentiated from type 1 DM by atypical features of type 1 DM. including the absence of pancreatic autoantibodies, low insulin requirements, evidence of endogenous insulin production with detectable serum C-peptide (> 0.6 ng/mL), and the absence of DKA [
43]. An additional, non-invasive 2-h postprandial urinary C-peptide to creatinine ratio (UCPCR) test can be used to distinguish between long-standing type 1 DM and
HNF1A-MODY and
HNF4A-MODY. A UCPCR of ≥0.2 nmol/mmol is 97% sensitive and 96% specific for differentiating
HNF1A- and
HNF4A-MODY from type 1 DM [
45].
A correct diagnosis of MODY is important for the treatment and identification of affected or at-risk family members. Despite the low frequency of MODY among pediatric DM patients, non-obese individuals with a family history of DM and those who lack the clinical characteristics of type 1 DM and type 2 DM should be evaluated for MODY using a high index of suspicion.
This study had several limitations. This study is not a national multicenter study and investigated etiologic distribution of DM in pediatric patients under 18 years of age at diagnosis who were diagnosed at a single tertiary center. Thus, the frequency of specific types of DM may not reflect the actual frequency.
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