Management of rib and sternal fractures

The majority of thoracic injuries encountered in the UK are secondary to blunt trauma following motor vehicle accidents. Rib fractures account for more than half of these injuries. Sternal fractures, although less common, have an increased incidence following the implementation of seat belt legislation. Both rib and sternal fractures may compromise ventilation by a variety of mechanisms. Pain leads to a reduction in lung expansion and sputum retention. Fractured ribs may cause penetrating injury resulting in a haemopneumothorax. There may also be asso ciated pulmonary contusion and flail chest, which further compromises ventilation. Sternal and rib fractures are managed by a number of specialties including accident and emergency staff, cardiothoracic and trauma surgeons, as well as anaesthetists and intensivists. The management of rib and sternal fractures principally consists of the identification and treatment of associated injuries, appropriate respiratory care and symptomatic relief. This article reviews the literature on the investigations and management of the patient with rib and sternal fractures.

Social deprivation and prognostic benefits of cardiac surgery: observational study of 44 902 patients from five hospitals over 10 years

OBJECTIVE

To assess the effects of social deprivation on survival after cardiac surgery and to examine the influence of potentially modifiable risk factors.

DESIGN

Analysis of prospectively collected data. Prognostic models used to examine the additional effect of social deprivation on the end points.

SETTING

Birmingham and north west England.

PARTICIPANTS

44 902 adults undergoing cardiac surgery, 1997-2007.

MAIN OUTCOME MEASURES

Social deprivation with census based 2001 Carstairs scores. All cause mortality in hospital and at mid-term follow-up.

RESULTS

In hospital mortality for all cardiac procedures was 3.25% and mid-term follow-up (median 1887 days; range 1180-2725 days) mortality was 12.4%. Multivariable analysis identified social deprivation as an independent predictor of mid-term mortality (hazard ratio 1.024, 95% confidence interval 1.015 to 1.033; P<0.001). Smoking (P<0.001), body mass index (BMI, P<0.001), and diabetes (P<0.001) were associated with social deprivation. Smoking at time of surgery (1.294, 1.191 to 1.407, P<0.001) and diabetes (1.305, 1.217 to 1.399, P<0.001) were independent predictors of mid-term mortality. The relation between BMI and mid-term mortality was non-linear and risks were higher in the extremes of BMI (P<0.001). Adjustment for smoking, BMI, and diabetes reduced but did not eliminate the effects of social deprivation on mid-term mortality (1.017, 1.007 to 1.026, P<0.001).

CONCLUSIONS

Smoking, extremes of BMI, and diabetes, which are potentially modifiable risk factors associated with social deprivation, are responsible for a significant reduction in survival after surgery, but even after adjustment for these variables social deprivation remains a significant independent predictor of increased risk of mortality.

Epicardial adipose tissue as a source of nuclear factor-kappaB and c-Jun N-terminal kinase mediated inflammation in patients with coronary artery disease

CONTEXT

Visceral adipose tissue (AT) is known to confer a significantly higher risk of type 2 diabetes and cardiovascular disease. Epicardial AT has been shown to be related to cardiovascular disease and myocardial function through unidentified mechanisms. Epicardial AT expresses an inflammatory profile of proteins; however, the mechanisms responsible are yet to be elucidated.

OBJECTIVES

The objectives of the study were to: 1) examine key mediators of the nuclear factor-kappaB (NFkappaB) and c-Jun N-terminal kinase (JNK) pathways in paired epicardial and gluteofemoral (thigh) AT from coronary artery disease (CAD) and control patients and 2) investigate circulating endotoxin levels in CAD and control subjects.

DESIGN

Serums and AT biopsies (epicardial and thigh) were obtained from CAD (n = 16) and non-CAD (n = 18) patients. Inflammation was assessed in tissue and serum samples through Western blot, real-time PCR, ELISAs, and activity studies.

RESULTS

Western blotting showed epicardial AT had significantly higher NFkappaB, inhibitory-kappaB kinase (IKK)-gamma, IKKbeta, and JNK-1 and -2 compared with thigh AT. Epicardial mRNA data showed strong correlations between CD-68 and toll-like receptor-2, toll-like receptor-4, and TNF-alpha. Circulating endotoxin was elevated in patients with CAD compared with matched controls [CAD: 6.80 +/- 0.28 endotoxin unit(EU)/ml vs. controls: 5.52 +/- 0.57 EU/ml; P<0.05].

CONCLUSION

Epicardial AT from patients with CAD shows increased NFkappaB, IKKbeta, and JNK expression compared with both CAD thigh AT and non-CAD epicardial AT, suggesting a depot-specific as well as a disease-linked response to inflammation. These studies implicate both NFkappaB and JNK pathways in the inflammatory profile of epicardial AT and highlight the role of the macrophage in the inflammation within this tissue.

Ruptured inflammatory aortic aneurysm with aortoenteric fistula and infected with Streptococcus pneumoniae: a review of the literature

We present a case report of a 65-year-old gentleman who presented with an aortoenteric fistula along with a review of the literature. He was found, in addition, to have an inflammatory infrarenal aortic aneurysm with a posterior rupture. Cultures of the aneurysm sac confirmed a Streptococcus pneumoniae infection. The patient had previously presented with pneumonia. Antibody testing revealed an isolated pneumococcal IgG deficiency. The case demonstrates the diverse pathologies associated with aortic aneurysms and a need to be vigilant and occasionally expect the unusual.

The role of the Met98Lys optineurin variant in inherited optic nerve diseases

AIMS

To investigate the role of the common OPTN Met98Lys variant as a risk allele in open-angle glaucoma (OAG), autosomal dominant optic atrophy (ADOA) and Leber's hereditary optic neuropathy (LHON).

METHODS

The presence of the Met98Lys variant was determined in a total of 498 (128 with normal-tension glaucoma (NTG)) patients with OAG, 29 patients who had myocilin-related OAG, 101 patients from ADOA pedigrees, 157 patients from LHON pedigrees and 218 examined OAG age-matched normal controls.

RESULTS

17 of 218 (7.8%) controls had the Met98Lys variant. 28 (5.6%) patients with OAG were Met98Lys positive. More Met98Lys carriers were found in the NTG group than in the high-tension glaucoma (HTG) group (p = 0.033). However, no significant difference was observed between the NTG and control cohorts (p = 0.609). Two MYOC mutation carriers were found to have the variant. The variant was found in 1 of 10 pedigrees with ADOA and in 8 of 35 pedigrees with LHON.

CONCLUSION

Data from this study do not support a strong role for the OPTN Met98Lys variant in glaucoma, ADOA or LHON. However, a weak association was observed of the variant with NTG compared with that with HTG. Meta-analysis of all published data on the variant and glaucoma confirmed that the association, although weak, is highly statistically significant in the cohort with glaucoma versus controls.

Sequence variation in mitochondrial complex I genes: mutation or polymorphism?

BACKGROUND

Defects of the mitochondrial genome are recognised as common causes of genetic disease. Sequencing of large portions or even the entire mitochondrial genome is routine in many laboratories for the investigation of mitochondrial disease. However, establishing whether a detected sequence change is polymorphic or pathogenic is still a major difficulty because of its highly polymorphic nature. This has major implications for the patient and the family.

OBJECTIVE

To describe a scoring system for determining the likelihood that a given sequence variant in one of the seven mitochondrially encoded complex I (MTND) genes is truly pathogenic.

RESULTS

The scoring system was applied to 50 reported MTND mutations. Using this system, 21 of the mutations analysed fell into the group of neutral sequence variants, 10 were classified as possibly pathogenic, three as probably pathogenic, and 16 as almost certainly pathogenic.

CONCLUSIONS

The proposed scoring system should advance the interpretation of sequence variants and ensure that candidate pathogenic mutations are rigorously investigated.

Identification of Native American founder mtDNAs through the analysis of complete mtDNA sequences: some caveats

In this study, a detailed analysis of both previously published and new data was performed to determine whether complete, or almost complete, mtDNA sequences can resolve the long-debated issue of which Asian mtDNAs were founder sequences for the Native American mtDNA pool. Unfortunately, we now know that coding region data and their analysis are not without problems. To obtain and report reasonably correct sequences does not seem to be a trivial task, and to discriminate between Asian and Native American mtDNA ancestries may be more complex than previously believed. It is essential to take into account the effects of mutational hot spots in both the control and coding regions, so that the number of apparent Native American mtDNA founder sequences is not erroneously inflated. As we report here, a careful analysis of all available data indicates that there is very little evidence that more than five founder mtDNA sequences entered Beringia before the Last Glacial Maximum and left their traces in the current Native American mtDNA pool.

Leber's hereditary optic neuropathy triggered by antiretroviral therapy for human immunodeficiency virus

PURPOSE

To describe the clinical features of two cases of Leber's hereditary optic neuropathy (LHON) precipitated by antiretroviral treatment for human immunodeficiency virus (HIV) infection.

METHODS

Two cases of LHON (from an expected four new cases a year throughout Australia) were identified in men on treatment for HIV infection.

RESULTS

Two HIV-infected men were receiving combination antiretroviral therapy that included nucleoside analogues. Both patients carried the 14 484 mitochondrial DNA mutation and were distantly related (seventh cousins). Although both men presented with sequential visual loss typical of LHON and one had a known close relative affected by LHON, the correct diagnosis was delayed in both cases. The final visual outcome was profoundly reduced in both instances and cessation of antiretroviral therapy did not result in recovery of vision in one patient.

CONCLUSION

Patients with a family history of LHON who require antiretroviral treatment should be warned of the high risk of severe visual loss. The underlying mechanism of antiretroviral side effects may help characterize the other trigger factors for LHON.

The epidemiology of Leber hereditary optic neuropathy in the North East of England

We performed the first population-based clinical and molecular genetic study of Leber hereditary optic neuropathy (LHON) in a population of 2,173,800 individuals in the North East of England. We identified 16 genealogically unrelated families who harbor one of the three primary mitochondrial DNA (mtDNA) mutations that cause LHON. Two of these families were found to be linked genetically to a common maternal founder. A de novo mtDNA mutation (G3460A) was identified in one family. The minimum point prevalence of visual failure due to LHON within this population was 3.22 per 100,000 (95% CI 2.47-3.97 per 100,000), and the minimum point prevalence for mtDNA LHON mutations was 11.82 per 100,000 (95% CI 10.38-13.27 per 100,000). These results indicate that LHON is not rare but has a population prevalence similar to autosomally inherited neurological disorders. The majority of individuals harbored only mutant mtDNA (homoplasmy), but heteroplasmy was detected in approximately 12% of individuals. Overall, however, approximately 33% of families with LHON had at least one heteroplasmic individual. The high incidence of heteroplasmy in pedigrees with LHON raises the possibility that a closely related maternal relative of an index case may not harbor the mtDNA mutation, highlighting the importance of molecular genetic testing for each maternal family member seeking advice about their risks of visual failure.

mtDNA analysis of Leber hereditary optic neuropathy associated with spondyloepiphyseal dysplasia

A patient was diagnosed in 1974 with the unique combination of Leber hereditary optic neuropathy (LHON) and spondyloepiphyseal dysplasia. The entire mitochondrial DNA (mtDNA) sequence from this patient was determined in order to identify candidate pathogenic mutations. The patient's mtDNA carried the LHON mutation at nucleotide 14484, thus elucidating the etiology of his optic neuropathy. We also identified another ND6 mutation at nucleotide 14420. This latter mutation is probably a clinically benign private polymorphism, although a pathogenic role in his skeletal abnormalities or in his optic neuropathy cannot yet be ruled out.

Point mutations of the mtDNA control region in normal and neurodegenerative human brains

Recent observations in cultured human fibroblasts suggest that the accumulation of point mutations in the noncoding control region of mtDNA may be important in human aging. We studied the mtDNA control region in brain tissue from 31 normal elderly individuals, from 35 individuals who had Alzheimer disease, and from 47 individuals who had dementia with Lewy bodies. We found no evidence that these somatic mtDNA point mutations accumulate either in the brains of normal elderly individuals or in the brains of individuals with neurodegenerative disease.

The mitochondrial ND6 gene is a hot spot for mutations that cause Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a common cause of bilateral optic nerve disease. The majority of LHON patients harbour one of three point mutations of the mitochondrial DNA (mtDNA) complex I, or NADH:ubiquinone oxidoreductase (ND) genes (G11778A in ND4, G3460A in ND1, T14484C in ND6). As a consequence, screening for these mutations has become part of the routine clinical investigation of young adults who present with bilateral optic neuropathy, and the absence of these mutations is interpreted as indicating there is a low likelihood that an optic neuropathy is LHON. However, there are many individuals who develop the clinical features of LHON but who do not harbour one of these primary LHON mutations. We describe two LHON pedigrees that harbour the same novel point mutation within the mtDNA ND6 gene (A14495G). This mutation was heteroplasmic in both families, and sequencing of the mitochondrial genome confirmed that the mutation arose on two independent occasions. This is the seventh mutation in the ND6 gene that causes optic neuropathy, indicating that this gene is a hot spot for LHON mutations. Protein modelling studies indicate that all of these pathogenic mutations lie within close proximity to one another in a hydrophobic cleft or pocket. This is the first evidence for a relationship between a specific disease phenotype and a specific structural domain within a mitochondrial respiratory chain subunit. These findings suggest that the mtDNA ND6 gene should be sequenced in all patients with LHON who do not harbour one of the three common LHON mutations.

The inheritance of mitochondrial DNA heteroplasmy: random drift, selection or both?

The mammalian mitochondrial genome (mtDNA) is a small double-stranded DNA molecule that is exclusively transmitted down the maternal line. Pathogenic mtDNA mutations are usually heteroplasmic, with a mixture of mutant and wild-type mtDNA within the same organism. A woman harbouring one of these mutations transmits a variable amount of mutant mtDNA to each offspring. This can result in a healthy child or an infant with a devastating and fatal neurological disorder. Understanding the biological basis of this uncertainty is one of the principal challenges facing scientists and clinicians in the field of mitochondrial genetics.

Mitochondrial DNA haplogroups and susceptibility to AD and dementia with Lewy bodies

The authors analyzed the relationship between nuclear genetic risk factors (apolipoprotein E genotype) and mitochondrial DNA (mtDNA) sequence variants in pathologically proved cases of AD (n = 185), dementia with Lewy bodies (DLB; n = 84), and control subjects (n = 179). Specific European mtDNA haplogroups and the A4336G mutation were not associated with an increased risk of AD. mtDNA haplogroup H was overrepresented in the DLB patients when compared with control subjects. Additional studies are needed to clarify the significance of the association.

Transmission of the human mitochondrial genome

The segregation and transmission of mitochondrial genomes in humans are complicated processes, but are particularly important for understanding the inheritance and clinical abnormalities of mitochondrial disorders. This review describes three aspects of mitochondrial genetics. First, that the segregation and transmission of mitochondrial (mt)DNA molecules are likely to be determined by their physical association within the organelles and by the dynamics of mitochondrial structure and subcellular organization. Second, that the transmission of heteroplasmic mtDNA sequence changes from one generation to the next often involves rapid shifts in allele frequency. For >20 years, the standard explanation has been that there is a developmental bottleneck in which, at some stage of oogenesis, there is a reduction in the effective number of mitochondrial units of inheritance. The third aspect is that ongoing analyses of the segregation and transmission of pathogenic mtDNA mutations indicate the operation of multiple genetic processes. Thus, the segregation and transmission of mtDNA mutations occurs predominantly, but not exclusively, under conditions of random genetic drift. However, there is also evidence for bias due to incomplete ascertainment of pedigrees and for negative selection of pathogenic mutations in rapidly dividing somatic tissues such as the white blood cell population.

Persistent heteroplasmy of a mutation in the human mtDNA control region: hypermutation as an apparent consequence of simple-repeat expansion/contraction

In the genealogical and phylogenetic analyses that are reported here, we obtained evidence for an unusual pattern of mutation/reversion in the human mitochondrial genome. The cumulative results indicate that, when there is a T-->C polymorphism at nt 16189 and a C-->T substitution at nt 16192, there is an extremely high rate of reversion (hypermutation) at the latter site. The apparent reversion rate is sufficiently high that there is persistent heteroplasmy at nt 16192 in maternal lineages and at the phylogenetic level, a situation that is similar to that observed for the rapid expansion/contraction of simple repeats within the control region. This is the first specific instance in which the mutation frequency at one site in the D-loop is markedly influenced by the local sequence "context." The 16189 T-->C polymorphism lengthens a (C:G)n simple repeat, which then undergoes expansion and contraction, probably through replication slippage. This proclivity toward expansion/contraction is more pronounced when there is a C residue, rather than a T, at nt 16192. The high T-->C reversion frequency at nt 16192 apparently is the result of polymerase misincorporation or slippage during replication, the same mechanism that also causes the expansion/contraction of this simple-repeat sequence. In addition to the first analysis of this mitochondrial hypermutation process, these results also yield mechanistic insights into the expansion/contraction of simple-repeat sequences in mtDNA.

Longitudinal analysis of the segregation of mtDNA mutations in heteroplasmic individuals

The mutation load of the pathogenic LHON (Leber hereditary optic neuropathy) mtDNA mutation at nucleotide 3460 has been followed over time in the WBC/platelet fraction from members of a matrilineal pedigree. Longitudinal analysis over a sampling period of five to six years indicates that, in all five heteroplasmic family members, the mutation load decreases at a mean overall rate of approximately 1% per year. There was no change in mutation load in homoplasmic wildtype or in homoplasmic mutant individuals. For the purposes of comparison, a longitudinal analysis of a silent mtDNA polymorphism at nucleotide 14560 was also carried out for members of a second matrilineal pedigree. In contrast to the results for the pathogenic mtDNA mutation, there was no change in the proportion of the silent polymorphism in the WBC/platelet fraction of four family members over a period of seven years. These results indicate that the pathogenic 3460 LHON mutation segregates under negative selection in these cell populations. One possible mechanism through which selection may operate is that, in heteroplasmic individuals, the hematopoietic stem cells are generally homoplasmic, either for the wildtype or for the mutant allele. The homoplasmic mutant stem cells, because of their mitochondrial respiratory chain defect, produce fewer mature WBCs and platelets over time than do the wildtype stem cells. Alternatively, the stem cells may be heteroplasmic and selection may act to favor proliferation of mitochondria with lower levels of the pathogenic mutation in the WBC/platelet cell populations.

Nonrandom tissue distribution of mutant mtDNA

Heteroplasmic mitochondrial DNA (mtDNA) defects are an important cause of inherited human disease. On a cellular level, the percentage of mutant mtDNA is the principal factor behind the expression of the genetic defect. Marked variation in the level of mutant mtDNA among tissues is thought to be responsible for the diverse clinical phenotypes associated with the same pathogenic mtDNA mutation. This study was designed to determine whether the percentage level of a pathogenic mtDNA molecule is determined by a purely random process. The tissue distribution of the A3243G MELAS point mutation was analyzed in five individuals who were members of a family with maternally inherited diabetes and deafness. The level of mutant mtDNA was measured in four tissues in three individuals and three tissues in two individuals. The highest level of mutant mtDNA occurred in skeletal muscle, followed by hair follicles, and then buccal mucosa, with the lowest levels in blood (leucocyte/platelet fraction). The probability of observing any strict hierarchy in family is 4.82 x 10(-5). These results indicate that the distribution of the A3243G mutation is not solely determined by random processes.

A novel mitochondrial DNA-like sequence in the human nuclear genome

We describe here a nuclear mitochondrial DNA-like sequence (numtDNA) that is nearly identical in sequence to a continuous 5842 bp segment of human mitochondrial DNA (mtDNA) that spans nucleotide positions 3914 to 9755. On the basis of evolutionary divergence among modern primates, this numtDNA molecule appears to represent mtDNA from a hominid ancestor that has been translocated to the nuclear genome during the recent evolution of humans. This numtDNA sequence harbors synonymous and nonsynonymous nucleotide substitutions relative to the authentic human mtDNA sequence, including an array of substitutions that was previously found in the cytochrome c oxidase subunit 1 and 2 genes. These substitutions were previously reported to occur in human mtDNA, but subsequently contended to be present in a nuclear pseudogene sequence. We now demonstrate their exclusive association with this 5842-bp numtDNA, which we have characterized in its entirety. This numtDNA does not appear to be expressed as a mtDNA-encoded mRNA. It is present in nuclear DNA from human blood donors, in human SH-SY5Y and A431 cell lines, and in rho(0) SH-SY5Y and rho(0) A431 cell lines that were depleted of mtDNA. The existence of human numtDNA sequences with great similarities to human mtDNA renders the amplification of pure mtDNA from cellular DNA very difficult, thereby creating the potential for confounding studies of mitochondrial diseases and population genetics.

Mitochondrial DNA analysis: polymorphisms and pathogenicity

The investigation of mtDNA disease can be relatively straightforward if a person has a recognisable phenotype and if it is possible to identify a known pathogenic mtDNA mutation. The difficulties arise when no known mtDNA defect can be found, or when the clinical abnormalities are complex and not easily matched to those of the more common mitochondrial disorders. We will describe here the difficulties that can be encountered during the identification of pathogenic mtDNA mutations and the approaches that can be used to confirm, or eliminate, a likely pathogenic role, in either single gene diseases or in multifactorial disorders.

Clinical mitochondrial genetics

The last decade has been an age of enlightenment as far as mitochondrial pathology is concerned. Well established nuclear genetic diseases, such as Friedreich's ataxia,¹² Wilson disease,³ and autosomal recessive hereditary spastic paraplegia,⁴ have been shown to have a mitochondrial basis, and we are just starting to unravel the complex nuclear genetic disorders which directly cause mitochondrial dysfunction (table 1). However, in addition to the 3 billion base pair nuclear genome, each human cell typically contains thousands of copies of a small, 16.5 kb circular molecule of double stranded DNA (fig 1). Mitochondrial DNA (mtDNA) accounts for only 1% of the total cellular nucleic acid content. It encodes for 13 polypeptides which are essential for aerobic metabolism and defects of the mitochondrial genome are an important cause of human disease.⁹²⁹³ Since the characterisation of the first pathogenic mtDNA defects in 1988,⁵¹³ over 50 point mutations and well over 100 rearrangements of the mitochondrial genome have been associated with human disease⁹⁴⁹⁵ (http://www.gen.emory.edu/mitomap.html). These disorders form the focus of this article.


Keywords: mitochondrial DNA; mitochondrial disease; heteroplasmy; genetic counselling

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

MELAS and MERRF. The relationship between maternal mutation load and the frequency of clinically affected offspring

The majority of pathogenic mitochondrial DNA (mtDNA) mutations are heteroplasmic, with both mutant and wild-type alleles present within the same individual. MtDNA is transmitted only from females to their offspring but a single female can bear offspring who harbour different levels of mutant mtDNA and have a variable phenotype. In single families, this complex genetic and phenotypic variability has confounded the identification of any relationship between the level of mutant mtDNA (mutation load) in the mother and the clinical features of her offspring. To obtain a more accurate description of the inheritance of pathogenic mtDNA mutations, we studied a large number of pedigrees that carried either the mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (A3243G MELAS) or the myoclonic epilepsy with ragged-red fibres (A8344G MERRF) mutations. We made two principal observations. First, for both mutations, higher levels of mutant mtDNA in the mothers' blood were associated with an increased frequency of affected offspring. Secondly, at any one level of maternal mutation load there was a greater frequency of affected offspring for the A3243G MELAS mutation than for the A8344G MERRF mutation. Although these results should not be used to give absolute risks to a female contemplating pregnancy, they suggest that the outcome of pregnancy is related to the level of mutant mtDNA in the mother and that the risks of having affected offspring may differ between different mtDNA mutations.

Leber hereditary optic neuropathy: respiratory chain dysfunction and degeneration of the optic nerve

Leber hereditary optic neuropathy (LHON) is an inherited form of bilateral optic atrophy in which the primary etiological event is a mutation in the mitochondrial genome. The optic neuropathy involves a loss of central vision due to degeneration of the retinal ganglion cells and optic nerve axons that subserve central vision. The primary mitochondrial mutation is necessary, but not sufficient, for manifestation of the optic neuropathy and secondary genetic and/or epigenetic risk factors are also involved, although they are poorly defined at the present time. There is broad agreement that mutations at nucleotides 3460, 11,778 and 14,484 are primary LHON mutations, but there may also be other rare primary mutations. It appears that the three primary LHON mutations are associated with respiratory chain dysfunction, but the derangement may be relatively subtle. There is also debate on whether there are mitochondrial mutations that have a secondary etiological or pathogenic role in LHON. The specific pattern of neurodegeneration in LHON may arise from a 'chokepoint' in the optic nerve in the region of the nerve head and lamina cribosa and which may be more severe in those LHON family members who become visually affected. It is hypothesized that the respiratory chain dysfunction leads to axoplasmic stasis and swelling, thereby blocking ganglion cell function and causing loss of vision. In some LHON patients, this loss of function is reversible in a substantial number of ganglion cells, but in others, a cell death pathway (probably apoptotic) is activated with subsequent extensive degeneration of the retinal ganglion cell layer and optic nerve.

Leber hereditary optic neuropathy: mitochondrial mutations and degeneration of the optic nerve

The predominant manifestation of Leber hereditary optic neuropathy (LHON) is a sudden and usually severe bilateral loss of central vision, most often in the mid-20s, that is due to a degeneration of the ganglion cell layer and optic nerve. LHON is an inherited form of blindness in which a mutation in the mitochondrial genome (mtDNA) is the primary etiological event. More than 95% of the LHON pedigrees in peoples of Northern European descent harbor one of the three mitochondrial mutations at nucleotides 3460, 11,778 and 14,484, although there are other rare primary mutations. In addition, there may be mtDNA mutations that have a secondary etiological role. The penetrance of the optic neuropathy is incomplete in LHON families, and males are affected much more often then females. The incomplete penetrance indicates that secondary etiological factors are necessary for the development of the optic neuropathy, although they are poorly understood at the present time. Several types of studies suggest that optic nerve function in LHON patients is impaired in the presymptomatic phase, probably as a result of a mitochondrial respiratory chain abnormality, although visual acuity is not compromised. In some family members, the presence of secondary etiological factors triggers a wave of optic nerve dysfunction in which vision is lost (the acute phase). Depending upon the particular primary LHON mutation that the patient carries, a variable proportion of the dysfunctional ganglion cells and optic nerve axons die during the atrophic phase, probably through an apoptotic pathway. In 11,778 LHON patients, retinal ganglion cell degeneration occurs almost without exception, and recovery of vision is extremely rare. In contrast, activation of the cell death pathway is less frequent, or less extensive, in 14,484 LHON patients and there is often a substantial recovery of vision.

Mammalian mitochondrial genetics: heredity, heteroplasmy and disease

Mammalian mitochondrial DNA (mtDNA) is present at high copy number (10(3)-10(4) copies) in virtually all cells of the body. The mitochondrial genome shows strict maternal inheritance and the vast majority of copies are identical at birth (homoplasmy). Occasionally, a subpopulation of mtDNA molecules carry a pathogenic mutation. When this heteroplasmic mtDNA is present during embryogenesis, it can lead to a variety of clinical symptoms predominantly affecting muscle and nerve, but also affecting other tissues. While the importance of mitochodrial heteroplasmy in human disease is unquestioned, we remain largely ignorant of many fundamental aspects of mitochondrial genetics. How do mutations arise and can they be repaired, what influences the segregation and fixation of heteroplasmic mtDNA, do levels of heteroplasmy fluctuate during life, is it possible to modulate these levels by external intervention and, finally, can we predict the segregation and transmission of a mutant genome? The aim of this article is to summarize and discuss recent observations that have addressed several of these fundamental issues and to reiterate how much we still have to learn about mitochondrial genetics.

Molecular pathology of MELAS and MERRF. The relationship between mutation load and clinical phenotypes

Many patients with inherited mitochondrial encephalopathies have one of two pathogenic mutations of mitochondrial DNA (mtDNA): A3243G or A8344G. Individuals who harbour these mutations carry both mutant and wild-type alleles within each cell (heteroplasmy). Despite clear evidence of a direct relationship between the level of mutation and mitochondrial respiratory chain function in vitro, it has been more difficult to demonstrate a clear correlation between clinical phenotype and the level of mutant mtDNA in vivo. To address this issue, we identified 245 individuals who carry either the A3243G or A8344G mutations, and studied the relationship between the incidence of specific clinical features and the level of mutant mtDNA in blood (for A3243G, n = 73; for A8344G, n = 25) and/or skeletal muscle (for A3234G, n = 111; for A8344G, n = 55). Within this study group, the frequency of key clinical features was significantly different for individuals harbouring the A3243G and A8344G mutations. For both mutations, there was a correlation between the frequency of the more common clinical features and the level of mutant mtDNA in muscle. In contrast, we did not observe a correlation between the frequency of clinical features and the level of mutant mtDNA in blood. Therefore, measurement of the level of the A3243G and A8344G mutations in muscle will allow the identification of individuals who are at risk of developing specific complications, thus improving the prognostic advice that can be given to patients and family members who carry these mutations.

Maternally inherited diabetes and deafness: prevalence in a hospital diabetic population

Maternally inherited diabetes and deafness (MIDD) is a new sub-type of diabetes and results from an A to G substitution at position 3243 of the mitochondrial tRNA(leu(UUR)) gene. This mutation is also associated with a neurological syndrome (MELAS). Recent studies have screened carefully selected diabetic populations and have reported MIDD prevalence rates ranging from undetectable to 60%. The aim of this work was to determine the importance of this sub-type in clinical practice by screening a routine hospital diabetic population. A total of 1440 patients (IDDM and NIDDM) of North European extraction attending two hospital diabetes services were initially screened by questionnaire. This identified 445 patients with one or more features of MIDD and/or MELAS and these subjects were then genotyped. Two patients were identified with the mutation giving a prevalence rate of 0.13% for the whole study population, and 0.45% for the sample with phenotypic features of MIDD. In conclusion, therefore, the 3243 mutation is associated with the phenotypically distinct MIDD sub-type, but this is rare in the routine hospital diabetic population.

The epidemiology of nontuberculous mycobacterial lymphadenitis affecting New Zealand children 1986-95

AIMS

To study the epidemiological trends of nontuberculous mycobacterial lymphadenitis affecting New Zealand children from 1986-95.

METHODS

Cases were identified from the records of the three regional reference laboratories in New Zealand. All children of less than 16 years with a positive culture of nontuberculous mycobacteria from a lymph node tissue sample were included.

RESULTS

One hundred and sixty eight cases were identified, 43 in the first 5 years (no data available from Waikato) and 125 in the second 5 years of the study period. One hundred and fifty three (91%) of cases were in the 0-5 year age group and 101 (60%) were female. The head and neck was the most common site of infection accounting for 141 (84%) of all infection. In 161 (96%) of cases the causative organism was Mycobacterium avium intracellulare complex.

CONCLUSION

Nontuberculous mycobacterial infections cause a subacute lymphadenitis in preschool children, usually affecting the lymph nodes of the head and neck. The annual number of microbiologically confirmed cases in New Zealand had increased substantially over recent years, most notably since 1992. The reason for the increase is unknown but possible explanations include increased awareness of mycobacterial disease, external factors causing either changes in the distribution or virulence of mycobacteria in the environment and alterations in the human immune response.

Mutations in mitochondrial cytochrome c oxidase genes segregate with late-onset Alzheimer disease

Mounting evidence suggests that defects in energy metabolism contribute to the pathogenesis of Alzheimer disease (AD). Cytochrome c oxidase (CO) is kinetically abnormal, and its activity is decreased in brain and peripheral tissue in late-onset AD. CO is encoded by both the mitochondrial and the nuclear genomes. Its catalytic centers, however, are encoded exclusively by two mitochondrial genes, CO1 and CO2 (encoding CO subunits I and II, respectively). We searched these genes, as well as other mitochondrial genes, for mutations that might alter CO activity and cosegregate with AD. In the present study, specific missense mutations in the mitochondrial CO1 and CO2 genes but not the CO3 gene were found to segregate at a higher frequency with AD compared with other neurodegenerative or metabolic diseases. These mutations appear together in the same mitochondrial DNA molecule and define a unique mutant mitochondrial genome. Asymptomatic offspring of AD mothers had higher levels of these mutations than offspring of AD fathers, suggesting that these mutations can be maternally inherited. Cell lines expressing these mutant mitochondrial DNA molecules exhibited a specific decrease in CO activity and increased production of reactive oxygen species. We suggest that specific point mutations in the CO1 and CO2 genes cause the CO defect in AD. A CO defect may represent a primary etiologic event, directly participating in a cascade of events that results in AD.

Leber hereditary optic neuropathy: how do mitochondrial DNA mutations cause degeneration of the optic nerve?

Leber hereditary optic neuropathy (LHON) is an inherited form of bilateral optic atrophy in which the primary etiological event is a mutation in the mitochondrial genome. The optic neuropathy involves a loss of central vision due to degeneration of the retinal ganglion cells and optic nerve axons that subserve central vision. The primary mitochondrial mutation is necessary--but not sufficient--for development of the optic neuropathy, and secondary genetic and/or epigenetic risk factors must also be present although they are poorly defined at the present time. There is broad agreement that mutations at nucleotides 3460, 11778, and 14484 are primary LHON mutations, but there may also be other rare primary mutations. It appears that the three primary LHON mutations are associated with respiratory chain dysfunction, but the derangements may be relatively subtle. There is also debate on whether there are mitochondrial mutations that have a secondary etiological or pathogenic role in LHON. The specific pattern of the optic neuropathy may arise from a "chokepoint" in the optic nerve in the region of the nerve head and lamina cribosa, and which may be more severe in those LHON family members who become visually affected. It is hypothesized that the respiratory chain dysfunction leads to axoplasmic stasis and swelling, thereby blocking ganglion cell function and causing loss of vision. In some LHON patients, this loss of function is reversible in a substantial number of ganglion cells, but in others, a cell death pathway (probably apoptotic) is activated with subsequent extensive degeneration of the retinal ganglion cell layer and optic nerve.

How rapidly does the human mitochondrial genome evolve?

The results of an empirical nucleotide-sequencing approach indicate that the evolution of the human mitochondrial noncoding D-loop is both more rapid and more complex than is revealed by standard phylogenetic approaches. The nucleotide sequence of the D-loop region of the mitochondrial genome was determined for 45 members of a large matrilineal Leber hereditary optic neuropathy pedigree. Two germ-line mutations have arisen in members of one branch of the family, thereby leading to triplasmic descendants with three mitochondrial genotypes. Segregation toward the homoplasmic state can occur within a single generation in some of these descendants, a result that suggests rapid fixation of mitochondrial mutations as a result of developmental bottlenecking. However, slow segregation was observed in other offspring, and therefore no single or simple pattern of segregation can be generalized from the available data. Evidence for rare mtDNA recombination within the D-loop was obtained for one family member. In addition to these germ-line mutations, a somatic mutation was found in the D-loop of one family member. When this genealogical approach was applied to the nucleotide sequences of mitochondrial coding regions, the results again indicated a very rapid rate of evolution.