Heterozygous lys169Glu mutation of glucokinase gene in a Chinese family having glucokinase-maturity-onset diabetes of the young (GCK-MODY).

We report a 24-year-old female with early-onset and persistent mild fasting hyperglycemia due to glucokinase-maturity-onset diabetes of the young (GCK-MODY). A c.505A>G (p. Lys169Glu) missense mutation of the GCK gene was identified. In silico analysis indicated that the mutation affected a conserved amino acid and is disease-causing. This report describes GCK-MODY in a Chinese family and stresses that in managing this condition it is important to avoid unnecessary drug treatment and excessive anxiety about mild hyperglycemia.

Introduction

The mutations in the glucokinase (GCK) gene, which encodes glucokinase, and play the role of catalytic glucose conversion into glucose-6-phosphate,[1] leads to glucokinase-maturity-onset diabetes of the young type (GCK-MODY). This condition is characterized by mild nonprogressive hyperglycemia presenting since birth with low risk of late complications[2] and therefore is often underdiagnosed.

Genetic testing improves the accuracy of GCK-MODY diagnosis, which is important because of the principal differences in the treatment and prognosis of various types of MODY and other types of diabetes. In this report we describe our experience with managing a Chinese family in whom NM_001354800.1:c.505A>G (Lys169Glu) mutation in GCK gene was identified.

Case Report

A 24-year-old female [III-6, proband, as shown in Figure 1a], whose body mass index (BMI) was 17.2 kg/m2, was referred for molecular evaluation due to continuous moderate fasting hyperglycemia (blood glucose values: 6.4–7.3 mmol/L) diagnosed since 2 years. Her mother (II-8, 48 years old) was known to have mild fasting hyperglycemia since last 25 years. Two of her maternal aunts (II-2, 63 years old; II-10, 45 years old) were diagnosed to have mild fasting hyperglycemia since last 15 years ago. Two of her cousins (III-1, 36 years old; III-4, 29 years old) had been diagnosed as having mild fasting hyperglycemia since last 3 and 2 years, respectively. The proband's maternal grandmother was reported to have mild fasting hyperglycemia (no treatment taken) for a period of more than 10 years and had died of chronic obstructive pulmonary disease (COPD). Other members of this extended multigenerational Chinese family were also evaluated.

Figure 1

(a) Pedigree for GCK-MODY family. Squares represent males and circles represent females. Dark represents individuals with hyperglycemia. The number under each member represents the sample identifier. The arrow indicates the proband. NN: No mutation; NM: Heterozygous mutation (c.505A>G); NA: Not available for testing. (b) Sequence of a heterozygous missense mutation (c.505A>G) results in a substitution of glutamic acid (GAG) for lysine (AAG)

The study protocol was approved by the institutional ethical committee and conducted as per the guidelines of the Declaration of Helsinki. All the family members signed the informed consent.

On evaluation, some new cases (II-6 with diabetes, III-3 with diabetes, III-7 with impaired fasting glucose, III-8 with impaired glucose tolerance, and III-9 with diabetes) of hyperglycemia were discovered. All of them had no typical symptoms such as polyuria, polydipsia, and polyphagia and were not treated with any drugs. The demographic and clinical characteristics and laboratory evaluations of the family members are shown in Table 1. Glutamic acid decarboxylase antibody (GAD-Ab), islet-cell antibody (ICA), and anti-insulin antibody (IAA) of all the family members were negative.

Table 1

Demographic and clinical characteristics of the family members

Patient	Actual age/age at diagnosis (years)	Sex	Height (cm)	Weight (kg)	BMI (kg/m2)	Waistline (cm)	HbA1c (%)/(mmol/mol)	FPG (mmol/L)	2hPG (mmol/L)	Insulin (0h) (mIU/L)	Insulin (2h) (mIU/L)	C-P (0h) (ng/ml)	C-P (2h) (ng/ml)
II-2	63/48	Female	158	51.5	20.6	70	6.4/46	5.31	11.84	3.4	33.7	0.67	3.59
II-6	54/54	Female	160	62	24.2	80	6.5/48	5.71	12.78	6.1	32.5	0.72	3.51
II-7	47/-	Male	175	71	23.2	86	4.9/30	4.5	5.05	4.2	32.4	0.71	3.73
II-8	48/23	Female	155	54	22.5	72	6.5/48	5.03	9.55	4.1	25.5	0.87	4.61
III-1	36/33	Female	158	50.5	20.2	67	6.3/45	5.93	13.29	4.9	62.9	0.62	4.68
III-2	28/-	Female	166	60.5	22.0	81	5.4/36	3.89	7.3	8.3	65.1	1.01	4.27
III-3	31/31	Male	168	91	32.2	104.5	5.1/32	6.18	12.48	9.5	134.9	1.32	7.56
III-4	29/27	Female	166	49.5	18.0	66	6.3/45	7.61	13.4	10.3	61	0.9	4.56
III-5	19/-	Female	156	60.5	24.9	80	5.1/32	4.88	6.43	27.8	98.3	1.79	6.03
III-6 (proband)	24/22	Female	161	44.5	17.2	56	6.1/43	6.49	9.31	7.8	57.3	0.76	4.09
III-7	18/18	Female	164	46	17.1	59	6.2/44	6.84	7.7	9.1	38	0.96	4.56
III-8	19/19	Male	181	61.5	18.8	69	6.5/48	4.18	8.78	14.4	161.4	1.56	8.24
III-9	11/11	Female	147	37	17.1	59	6.3/45	6.4	12.85	10.6	84.1	0.96	5.24

The reference ranges for HbA1c is 4%-6% (20-42 mmol/mol), for FPG 3.9-6.1 mmol/L, for 2hPG <7.8 mmol/L, for insulin (0h) 3.0-25 mIU/L, for C-P (0) 0.81-3.85ng/ml. FPG: Fasting plasma glucose, 2hPG: 2h plasma glucose of OGTT, C-P: C peptide, BMI: Body mass index, HbA1c: Hemoglobin A1c

Direct sequencing of the entire coding region and intron-exon boundaries of the GCK gene (RefSeq NM_000162.3) was performed as described before[3] in this family and 200 healthy controls. A heterozygous missense mutation, i.e., NM_001354800.1:c.505A>G (p. Lys169Glu) as the Human Genome Variation Society name of this variant [Figure 1b], was identified. However, this mutation was not found in the 200 unrelated healthy individuals used as controls. In silico analysis using five different computational tools (MutationTaster, PolyPhen-2, SIFT, PROVEAN, and Mutation Assessor) indicated that the mutation affected a conserved amino acid and is disease-causing. According to the standardization developed by ACMG/AMP, the variant is categorized as pathogenic based on a 5-tier system.[4]

Discussion

This was a family of early-onset diabetes, in which most family members were occasionally found to have elevated fasting blood glucose levels before the age of 25 years. Furthermore, three successive generations of family members were involved. The involved patients, especially those with a hyperglycemia history of more than 5 years, without any drug treatment, did not have any typical symptoms such as polyuria, polydipsia, and polyphagia and no progression of the condition. These clinical manifestations strongly indicated the possibility of GCK-MODY.

GCK-MODY is a monogenic subtype of diabetes, which is generated by heterozygous inactivating mutations in the gene encodes of GCK. The location of the GCK gene is on chromosome 7p15.3–p15.1. It comprises 12 exons (1a, 1b, 1c, and 2–10), spaned ~45,168 bp. The GCK gene encodes a 465-amino acid protein and has three tissue-specific isoforms.[5] GCK, as a glucose sensor in the pancreas and liver, plays an important regulatory enzymatic role in regulating insulin secretion.[6] GCK mutations result in a mild hyperglycemic phenotype because the threshold for glucose-induced insulin release is elevated.

Up to now, it is reported that more than 620 GCK gene mutations have occurred in over 1400 patients with GCK-MODY.[57] In view of a great degree of allelic heterogeneity of GCK-MODY, it was necessary to conduct a direct sequencing analysis of the GCK gene in this family. The sequencing data revealed a mutation of p. Lys169Glu on exon 5 of GCK gene.

GCK consists of a large and a small domain separated by a deep cleft where glucose binds.[8] Moreover, the solved crystal structure of GCK revealed that the residue K169 of the small domain plays a pivotal role as forming part of the glucose-binding site. This c.505A>G point mutation is a missense mutation at amino acid position 169 replacing lysine with glutamic acid (p. Lys169Glu) in a highly conserved glucose and adenosine triphosphate (ATP)-binding site of the enzyme, which suggested that the dimensional conformation of GCK Lys169Glu might be changed[9] despite the lack of functional assessment of GCK activity. To address the pathogenic relevance of Lys169Glu mutation, two different in silico analysis programs, MutationTaster and Polyphen2 Web interface, had been applied. Both the analyses predicted that the Lys169Glu mutation affected a conserved amino acid and is disease-causing. Cosegregation with hyperglycemia in the affected family [Figure 1a] strongly indicated that the mutation was causative of hyperglycemia.

In this study, no GCK mutation was observed in one of the proband's cousins (III-3). Compared with his family members, he had higher waist circumference and BMI whose Glutamic Acid Decarboxylase Antibodies (GAD-Ab), Islet Cell Cytoplasmic Autoantibodies (ICA), and Islet Autoantibodies (IAA) laboratory results were negative. These clinical features are not like of the GCK-MODY phenotype. Therefore, we speculated that he suffered from type 2 diabetes, as Asian people are known to develop insulin resistance at a relatively lower BMI and have a higher incidence of type 2 diabetes.[10]

No functional analysis of this GCK mutation was done, and thus, the absence of this type of GCK activity probably may not apply in GCK-MODY cases with other types of GCK mutations. We did not find mutations in other genes such as HNF4A, HNF1A, IPF1, HNF1B, NEUROD1, and PAX4.

Our study revealed a relatively good prognosis in patients with Lys169Glu mutation in GCK gene, which helps to avoid unnecessary medical therapy and overanxiety for mild hyperglycemia.

Declaration of patient consent

The authors certify that appropriate patient consent was obtained.

Financial support and sponsorship

This work was supported by Zhejiang Medical Science and Technology Projects (2018KY056; 2017KY324; 2017KY049) and the Natural Science Foundation of Zhejiang Province (LY13H070001).

Conflicts of interest

There are no conflicts of interest.