Mitochondrial gene mutations in familial non-insulin-dependent diabetes mellitus in Taiwan

High prevalence of the COII/tRNA(Lys) intergenic 9-bp deletion in mitochondrial DNA of Taiwanese patients with MELAS or MERRF syndrome

The COII/tRNA(Lys) intergenic 9-bp deletion (MIC9D) of mitochondrial DNA (mtDNA) has been established as a genetic polymorphism for Asian-Pacific populations. We investigated whether this small mtDNA deletion is co-transmitted with human diseases such as mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) and myoclonic epilepsy with ragged-red fibers (MERRF) syndromes. Forty unrelated Taiwanese families, including 12 families with MERRF and A8344G mtDNA mutation and 28 families with MELAS and A3243G mutation of mtDNA, respectively, were recruited in this study. In addition, 199 healthy subjects were recruited as control. We found that the frequency of occurrence of mtDNA with the MIC9D polymorphism in healthy subjects was 21% (41/199). However, the incidence of the MIC9D polymorphism was 67% (8/12) among the probands of all the families with MERRF syndrome (P = 0.001; OR = 8) and 39% (11/28) among the probands of the families with MELAS syndrome (P = 0.038; OR = 2). This finding indicates that the frequency of occurrence of mtDNA with the MIC9D polymorphism in patients with MERRF or MELAS syndrome is higher than that of healthy subjects. The prevalence of mitochondrial encephalomyopathies in relation to the MIC9D polymorphism of mtDNA in Taiwanese population is discussed.

Maternal transmission of diabetes

Type 2 diabetes mellitus represents a heterogeneous group of conditions characterized by impaired glucose homeostasis. The disorder runs in families but the mechanism underlying this is unknown. Many, but not all, studies have suggested that mothers are excessively implicated in the transmission of the disorder. A number of possible genetic phenomena could explain this observation, including the exclusively maternal transmission of mitochondrial DNA (mtDNA). It is now apparent that mutations in mtDNA can indeed result in maternally inherited diabetes. Although several mutations have been implicated, the strongest evidence relates to a point substitution at nucleotide position 3243 (A to G) in the mitochondrial tRNA(leu(UUR)) gene. Mitochondrial diabetes is commonly associated with nerve deafness and often presents with progressive non-autoimmune beta-cell failure. Specific treatment with Coenzyme Q10 or L-carnitine may be beneficial. Several rodent models of mitochondrial diabetes have been developed, including one in which mtDNA is specifically depleted in the pancreatic islets. Apart from severe, pathogenic mtDNA mutations, common polymorphisms in mtDNA may contribute to variations of insulin secretory capacity in normal individuals. Mitochondrial diabetes accounts for less than 1% of all diabetes and other mechanisms must underlie the maternal transmission of Type 2 diabetes. Possibilities include the role of maternally controlled environments, imprinted genes and epigenetic phenomena.

Endocrine disorders in two sisters affected by MELAS syndrome

A variety of endocrine and metabolic defects, including hypothalamopituitary hypofunction and diabetes mellitus, has been reported in association with mitochondrial disorders. We describe two sisters affected by mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome in whom DNA analysis showed an A-->G transition at the 3243rd nucleotide position on the transfer RNALeu(UUR) gene with 65% and 45% of mutant-type mitochondrial DNA present in the blood cells of the younger and the older sister, respectively. The younger sister had severe involvement of the central nervous system with mental retardation, epilepsia partialis continua, and strokelike episodes. Endocrine investigations showed an extensive neuroendocrine dysfunction with growth hormone deficiency, hypothalamopituitary hypothyroidism, prepubertal gonadotropin levels, and absence of any secondary sexual characteristics at the age of 12 6/12 years. The neurologically normal older sister was affected by diabetes mellitus and had normal hypothalamopituitary function. Our report confirms that the endocrine system can be affected differently by the same mitochondrial DNA mutation, depending on the heteroplasmia phenomenon. A complete endocrine evaluation must be performed in patients affected by mitochondrial disease and the existence of a mitochondrial disorder should be taken into account in patients with endocrine abnormalities, even if neuromuscular signs are lacking.

Genetic epidemiology of type 1 diabetes mellitus in Taiwan

Some environmental and genetic factors play important roles in etiopathogenesis of type 1 or insulin-dependent diabetes mellitus (IDDM). HLA genes, the IDDM1 locus located the human chromosome 6, were found to be associated with insulin-dependent diabetes mellitus. However, the incidence of IDDM varied greatly among various populations. To evaluate the pathogenetic factors contributing to the development of IDDM in Taiwan, HLA typing was performed in a group of IDDM unrelated individuals and IDDM pedigrees along with the normal controls from the northern Taiwan. DNA genotypes of class II HLA were done by polymerase-chain-reaction based oligotyping techniques. We confirmed that class II HLA genes were significantly associated with IDDM in Taiwan. To study detailed molecular structure of class II HLA molecules and disease association, we examined several amino acid residues on DQalpha and DQbeta chains and the molecular mechanisms to explain the heterozygotic effect of the DR3/DR4 and DR3/DR9 in the Chinese population. Linkage analysis in our pedigrees confirmed the association between HLA and IDDM in population association studies. Among the several class II alleles, a closer segregation of HLA-DQB1*0401 to the affected persons might suggest that HLA-DQB1*0401 itself or an allele closely linked to the DQB1 locus was the IDDM-predisposing allele in Taiwanese. For IDDM2 (INS) region, association with IDDM was not found due to that more than 90% of the population carried class I alleles. In our collection of IDDM, we found few cases (2.4%) carried mitochondrial DNA mutation. Our studies in Taiwanese confirm a multigenetic nature for IDDM.

Autoimmune IDDM in a sporadic MELAS patient with mitochondrial tRNA(Leu(UUR)) mutation

We report a 28-year-old young male with MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) presenting with two previous episodes of stroke-like manifestation, lactic acidosis and mitochondrial cardiomyopathy. He was also affected with insulin-dependent diabetes mellitus (IDDM), as diagnosed by the experience of diabetic ketoacidosis (DKA), and dependence on insulin therapy. On admission, the serum lactate level was found to be increased to 5.4 mmol/l, and plasma glucose level to 7.9 mmol/l with haemoglobin A1c 8.4%, while he was using insulin 26-30 units per day. Physical examination revealed a short stature male of height of 150 cm and weight of 49 kg. Mild mental retardation with bilateral sensorineural hearing impairment was observed. After glucagon stimulation, C-peptide levels rose from 0.46 nmol/l to 0.53 nmol/l, indicative of impaired insulin secretion. Anti-glutamate decarboxylase (anti-GAD) antibody was positive. In addition, human leucocyte associated antigen (HLA) typing showed DR3 and DR4, suggesting the strong contribution of autoimmunity to the pathogenesis of IDDM in this patient. Moreover, the result of a treadmill exercise test was positive due to inferior wall myocardial ischaemia. Cardiac catheterization and endomyocardial biopsy disclosed a normal coronary angiogram and confirmed the diagnosis of mitochondrial cardiomyopathy. Molecular genetic analysis of his family revealed a sporadic occurrence of mitochondrial DNA (mtDNA) mutation at base pair (bp) 3243. The degree of heteroplasmy of mtDNA mutation from a total of 19 passages of skin-derived fibroblasts from this patient showed a slightly downward trend. This extremely rare case of sporadic MELAS syndrome with autoimmune IDDM harbouring mtDNA mutation highlights the possible pathogenetic role of mtDNA mutations in autoimmune disease.

Screening for the mitochondrial DNA A3243G mutation in children with insulin-dependent diabetes mellitus

Since recent studies demonstrated the occurrence of the mitochondrial DNA (mtDNA) mutation A3243G in patients with adult-onset diabetes, an investigation was undertaken to determine the frequency of this mutation in a pediatric population with insulin-dependent diabetes mellitus (IDDM). DNA was extracted from peripheral blood of 270 pediatric patients with IDDM. The presence of the mtDNA A3243G mutation was screened for by minisequencing and mutation-specific ApaI endonuclease restriction after polymerase chain reaction (PCR) amplification of mtDNA. The A3243G mtDNA mutation was not found in any IDDM patients examined. This mutation is uncommon in children with IDDM from various ethnic and racial groups. Therefore, the contribution of the mutation to the pathogenesis of IDDM, if any, is minimal.