Screening for mtDNA diabetes mutations in Pima Indians with NIDDM

Clinical features, diagnosis and management of maternally inherited diabetes and deafness (MIDD) associated with the 3243A>G mitochondrial point mutation

Maternally inherited diabetes and deafness (MIDD) affects up to 1% of patients with diabetes but is often unrecognized by physicians. It is important to make an accurate genetic diagnosis, as there are implications for clinical investigation, diagnosis, management and genetic counselling. This review summarizes the range of clinical phenotypes associated with MIDD; outlines the advances in genetic diagnosis and pathogenesis of MIDD; summarizes the published prevalence data and provides guidance on the clinical management of these patients and their families.

Mitochondrial deafness

Non-syndromic deafness can be caused by mutations in both nuclear and mitochondrial genes. More than 50 nuclear genes have been shown to be involved in non-syndromic hearing loss, but mutations in mitochondrial DNA (mtDNA) might also cause hearing impairment. As mitochondria are responsible for oxidative phosphorylation, the primary energy-producing system in all eukaryotic cells, mitochondrial dysfunction has pleiotropic effects. Many mutations in mtDNA can lead to multisystem disorders, such as Kearns-Sayre syndrome, NARP, MELAS, or MERRF syndromes, the presentation of which may include hearing loss. A more specific association of mitochondrially inherited deafness and diabetes known as MIDD syndrome can be caused by a limited number of specific mitochondrial mutations. In addition, several rare mutations in the mitochondrial MTTS1 and MTRNR1 genes have been found to be responsible for non-syndromic hearing loss. The most frequent form of non-syndromic deafness is presbyacusis, affecting more than 50% of the elderly. This age-related hearing loss is a paradigm for multifactorial inheritance, involving a multitude of inherited and acquired mutations in the nuclear and mitochondrial genomes, each with a low penetrance, in complex interplay with environmental factors, such as ototoxic medication, that accumulate with age. This study reviews the different mitochondrial mutations, leading to syndromic and especially non-syndromic deafness.

Mitochondrial deafness

The last decade has led to the identification of several mitochondrial DNA mutations associated with hearing loss. Since the only known function of the human mitochondrial chromosome is to participate in the production of chemical energy through oxidative phosphorylation, it was not unexpected that mitochondrial mutations interfering with energy production could cause systemic neuromuscular disorders, which have as one of their features hearing impairment. Surprisingly, however, inherited mitochondrial mutations also have been found to be a cause of non-syndromic hearing loss, and predispose to aminoglycoside induced hearing loss, while acquired mitochondrial mutations have been proposed as one of the causes of presbycusis. After a brief review of mitochondrial genetics, we will outline the different mitochondrial mutations associated with hearing loss, describe the audiological features, and discuss the clinical relevance of diagnosing these mutations. Clinical expression of these mitochondrial mutations is dependent on environmental exposures and nuclear-encoded modifier genes. Preventive and therapeutic strategies will depend on identification and avoidance of the environmental exposures, and the identification of the nuclear-encoded modifier genes. Experimental approaches to identify these modifier genes will be presented.

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.

Mitochondrial deafness mutations reviewed

The first molecular defect for nonsyndromic hearing loss was identified in 1993, and was a mitochondrial mutation. Since then a number of inherited mitochondrial DNA (mtDNA) mutations have been implicated in hearing loss, and acquired mtDNA mutations have been proposed as one of the causes of the hearing loss associated with aging, presbyacusis. These molecular findings have raised as many questions as they have answered, however, since the pathophysiology between the mutations and the clinical phenotype remains poorly understood. This mini-review will, after a short background review of mitochondrial genetics, (1) outline the different mtDNA mutations associated with inherited syndromic, nonsyndromic, and ototoxic hearing loss, (2) summarize the data on acquired mtDNA mutations and their possible association with presbyacusis, (3) describe the biochemical consequences of the inherited mtDNA mutations, (4) suggest the clinical implications of the identification of these mutations, and (5) discuss the penetrance and tissue specificity of the hearing associated mtDNA mutations.

Acanthosis nigricans as a risk factor for non-insulin dependent diabetes mellitus

The prevalences of obesity and of non-insulin dependent diabetes mellitus (NIDDM) have increased in the United States population over the past two decades, and thus diabetes prevention has become a major concern of public health agencies such as the National Institutes of Health. Identification of individuals at risk for diabetes is an essential first step in designing and implementing intervention programs. Insulin resistance is the hallmark of the pathophysiology of NIDDM. Subjects with hyperinsulinemia, impaired glucose tolerance, or gestational diabetes are well accepted as being at high risk for diabetes. We propose that the easily identifiable skin lesion, acanthosis nigricans, is common in the major minority groups in the United States and that its presence is a surrogate for laboratory-determined hyperinsulinemia.

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.

Controversies in counseling for mitochondrial conditions

A healthy woman sought preconceptional genetic counseling regarding a family history of a mitochondrial myopathy in her brother and retinitis pigmentosa (RP) in her two maternal aunts. Several questions were raised: (1) What is the likelihood of a familial mitochondrial condition? (2) What molecular tests or prenatal screening can we offer? (3) How would these tests help assess the likelihood of a familial mitochondrial condition? A mitochondrial mutation previously identified in the brother consisted of a heteroplasmic 2.9 kb deletion. We detected this deletion in the peripheral blood of the brother by PCR amplification of the deletion breakpoint, but not in his mother, the consultand, nor in one of the two aunts affected with RP. Although the molecular analysis was encouraging to the consultand, a familial mitochondrial disorder could not be eliminated with certainty. The pros and cons of prenatal testing for mitochondrial disorders are discussed in general, and as specifically related to this family.