Mitochondrial disease in New Zealand: a nationwide prevalence study.

BACKGROUND: The complexities of mitochondrial disease make epidemiological studies challenging, yet this information is important in understanding the healthcare burden and addressing service and educational needs. Existing studies are limited to quaternary centres or focus on a single genotype or phenotype and estimate disease prevalence at 12.5 per 100 000. New Zealand's (NZ) size and partially integrated national healthcare system make it amenable to a nationwide prevalence study. AIM: To estimate the prevalence of molecularly confirmed and suspected mitochondrial disease on 31 December 2015 in NZ. METHODS: Cases were identified from subspecialists and laboratory databases and through interrogation of the Ministry of Health National Minimum Dataset with a focus on presentations between 2000 and 2015. Patient records were reviewed, and those with a diagnosis of 'mitochondrial disease' who were alive and residing in NZ on the prevalence date were included. These were divided into molecularly confirmed and clinically suspected cases. Official NZ estimated resident population data were used to calculate prevalence. RESULTS: Seven hundred twenty-three unique national health index numbers were identified. Five hundred five were excluded. The minimum combined prevalence for mitochondrial disease was 4.7 per 100 000 (95% confidence interval (CI): 4.1-5.4). The minimum prevalence for molecularly confirmed and suspected disease was 2.9 (95% CI 2.4-3.4) and 1.8 (95% CI 1.4-2.2) cases per 100 000 respectively. CONCLUSIONS: Within the limitations of this study, comparison to similar prevalence studies performed by specialist referral centres suggests mitochondrial disease is underdiagnosed in NZ. This highlights a need for improved education and referral pathways for mitochondrial disease in NZ.

Increased Peripheral Blood Heteroplasmy of the mt.3243A>G Mutation Is Associated with Earlier End-Stage Kidney Disease: A Case Report and Review of the Literature.

The mitochondrial DNA mutation mt.3243A>G is most commonly associated with maternally inherited diabetes and deafness (MIM 52,000), but it has protean phenotypes including renal disease due to focal segmental glomerulosclerosis. We describe monozygotic twins who both harboured this mutation and developed ESRD. Although otherwise genetically identical, the twins differed in their peripheral blood leucocyte levels of circulating mt.3243A>G heteroplasmy: 20 versus 10%, when assessed at 42 years of age. The twin with the higher heteroplasmy load developed end-stage kidney disease 15 years earlier than her sister. A review of the published literature supports a relationship between heteroplasmy level and the age at the development of the end stage of renal failure in patients with mt.3243A>G-related kidney disease.

Sudden Cardiac Death Due to Deficiency of the Mitochondrial Inorganic Pyrophosphatase PPA2.

We have used whole-exome sequencing in ten individuals from four unrelated pedigrees to identify biallelic missense mutations in the nuclear-encoded mitochondrial inorganic pyrophosphatase (PPA2) that are associated with mitochondrial disease. These individuals show a range of severity, indicating that PPA2 mutations may cause a spectrum of mitochondrial disease phenotypes. Severe symptoms include seizures, lactic acidosis, cardiac arrhythmia, and death within days of birth. In the index family, presentation was milder and manifested as cardiac fibrosis and an exquisite sensitivity to alcohol, leading to sudden arrhythmic cardiac death in the second decade of life. Comparison of normal and mutant PPA2-containing mitochondria from fibroblasts showed that the activity of inorganic pyrophosphatase was significantly reduced in affected individuals. Recombinant PPA2 enzymes modeling hypomorphic missense mutations had decreased activity that correlated with disease severity. These findings confirm the pathogenicity of PPA2 mutations and suggest that PPA2 is a cardiomyopathy-associated protein, which has a greater physiological importance in mitochondrial function than previously recognized.

Unique albumin with two silent substitutions (540Thr→Ala and 546Ala→Ser): Insights into how albumin is recycled.

OBJECTIVES: To determine the cause of an albumin abnormality detected by chance on electrospray time-of-flight mass spectrometry (TOF MS) of whole plasma, and to assess its physiological consequences. METHOD: Plasma was examined by TOF MS and tryptic mapping was used to locate mutation sites and determine the relative expression level of the variant and normal albumins. DNA sequencing was used to precisely define mutations. RESULTS: Whole protein electrospray TOF MS indicated a decrease of 14Da in the mass of albumin. Peptide mass mapping and DNA sequencing established the presence of two novel heterozygous point mutations (540Thr→Ala and 546Ala→Ser) whose combined mass changes (-30 and +16Da) indicated both mutations occurred on the same allele. Peptide ratios showed the variant albumin was present at a lower level than normal with an expression ratio of approximately 1:2 (variant:normal). Phylogenetic sequence alignments show Thr540 is highly conserved while Ala546 has wide species variation, suggesting 540Thr→Ala might compromise the protein. CONCLUSION: Both mutations occur close together in domain IIIB, a region involved in albumin scavenging and recycling. In particular, Thr540 is close to His535, a residue directly involved in pH-dependent binding and release of albumin from its recycling neonatal Fc receptor. Compromised receptor binding would explain the low albumin (34g/l) concentration and the diminished variant expression level.

Bisalbuminaemia due to novel mutation at a critical residue involved in recycling; Albumin Lyon (510His→Arg).

OBJECTIVES: To define the underlying cause of bisalbuminaemia in an individual presenting with spontaneous venous thrombosis. METHOD: Plasma was examined by electrospray time-of-flight mass spectrometry (TOF MS) to assess albumin mutations and to quantify variant expression level. Tryptic peptide mapping and DNA sequencing were used to precisely define the mutation. RESULTS: Whole protein MS indicated a 19Da increase in the mass of 50% of the albumin molecules suggesting a His→Arg substitution. A novel heterozygous 510His→Arg mutation was identified by peptide mass mapping and confirmed by DNA sequencing of exon 12 of the albumin gene. CONCLUSION: The nature and location of the mutation suggest it would have no direct influence on haemostasis through altered warfarin binding or increased fibrinogen attachment and it appears to be incidental to the thrombotic phenotype. However the highly conserved His510 residue is recognised as being of critical importance in albumin recycling through interaction with its savaging neonatal Fc receptor. The normal albumin level of 41.1g/l and the coequal expression of albumin Lyon demonstrate that the conservative 510His→Arg substitution does not interfere with the pH dependant capture and release of albumin by the receptor.

Familial dysalbuminaemic hyperthyroxinaemia: a rapid and novel mass spectrometry approach to diagnosis.

BACKGROUND: Familial dysalbuminaemic hyperthyroxinaemia is an important cause of discordant thyroid function test results (due to an inherited albumin variant); however, the diagnosis can be challenging. A 51-year-old man had persistently elevated free thyroxine (T4), with discordant normal thyroid-stimulating hormone and normal free triiodothyronine. He was clinically euthyroid and had a daughter with similar thyroid function test results. We aimed to apply a whole protein mass spectrometry method to investigate this case of suspected familial dysalbuminaemic hyperthyroxinaemia. METHODS: Intact serum albumin was assessed directly using electrospray time-of-flight mass spectrometry. Results were confirmed using tryptic peptide m/z mapping and targeted DNA sequencing (exons 3 and 7 of the albumin gene). We also used this sequencing to screen 14 archived DNA samples that were negative for thyroid hormone receptor mutations (in suspected thyroid hormone resistance). RESULTS: Mass spectrometry analysis demonstrated heterozygosity for an albumin variant with a 19 Da decrease in mass, indicative of an Arg→His substitution. The familial dysalbuminaemic hyperthyroxinaemia variant was confirmed with peptide mapping (showing the precise location of the substitution, 218Arg→His) and DNA sequencing (showing guanine to adenine transition at codon 218 of exon 7). The same familial dysalbuminaemic hyperthyroxinaemia variant was identified in one additional screened sample. CONCLUSIONS: Time-of-flight mass spectrometry is a novel procedure for diagnosing familial dysalbuminaemic hyperthyroxinaemia. The test is rapid (<10 min), can be performed on <2 µL of serum and requires minimal sample preparation.

Novel silent albumin variant (191Ala→Thr) detected by TOF MS of whole plasma.

BACKGROUND: Serum protein electrophoresis occasionally reveals multiple albumin bands referred to as bis- or alloalbuminaemia, and whilst the condition can be inherited it may also be acquired. METHODS: We present a new high resolution approach to the investigation of qualitative changes in albumin structure. The on-line reverse phase time-of-flight mass spectrometry procedure (TOF MS) elaborated here requires <0.2 µl of plasma and takes ~10 min to perform. Two plasma samples with classical bisalbuminaemia were used to verify the efficacy of the procedure for detecting genetic variants. When a novel mutation was detected, mass mapping of native unreduced albumin was used to pinpoint its location and inform targeted DNA sequencing of the albumin gene. RESULTS: Normal serum albumin showed its expected major isoform at 66,439 Da and the electrophoretic variants showed co-equal expression of additional components at -14 and +744 Da respectively. Surprisingly, one of the supposed controls showed paired albumin peaks at 66,439 and 66,469 Da and this 30 Da increase in mass was localised to between Arg(114) and Arg(197) and confirmed as being due to a novel 191Ala→Thr (+30 Da) substitution through DNA sequencing. CONCLUSIONS: The electrospray TOF MS approach developed here provides a rapid, sensitive and extremely precise method of revealing minute changes in albumin primary structure.

Growth and development in a child with resistance to thyroid hormone and ectopic thyroid gland.

Resistance to thyroid hormone is an uncommon problem, which has rarely been associated with thyroid dysgenesis. We report a case with both thyroid gland ectopy and resistance to thyroid hormone and, thus, a reduced capacity to produce and respond to thyroid hormone. The patient presented at 2 years of age with developmental delay, dysmorphic features, and elevation in both thyroxine and thyrotropin. We document her response to therapy with thyroxine, with particular regard to her growth and development. Persistent elevation of thyrotropin is commonly recognized during treatment of congenital hypothyroidism. Resistance to thyroid hormone may be an important additional diagnosis to consider in cases where thyrotropin remains persistently elevated.

Association between endothelin type A receptor haplotypes and mortality in coronary heart disease.

AIMS: The endothelin type A receptor, encoded by EDNRA, mediates the effects of endothelin-1 to promote vasoconstriction, vascular cell growth, adhesion, fibrosis and thrombosis. We investigated the association between EDNRA haplotype and cardiovascular outcomes in patients with coronary artery disease. METHODS: Coronary disease patients (n = 1007) were genotyped for the His323His (rs5333) variant and one tag SNP from each of the major EDNRA haplotype blocks (rs6537484, rs1568136, rs5335 and rs10003447). EDNRA haplotype associations with clinical history, natriuretic peptides cardiac function and cardiovascular outcomes were tested over a median 3.8 years. RESULTS: Univariate analysis identified a 'low-risk' EDNRA haplotype associated with later age of Type 2 diabetes onset (p = 0.004) smaller BMI (p = 0.021), and reduced mortality (log rank p = 0.001). Cox proportional hazards analysis including established cardiovascular risk factors revealed an independent association between haplotype and mortality (p < 0.0001). CONCLUSION: These data highlight the potential importance of the endothelin system, and in particular EDNRA in coronary disease.

Intermittent severe, symptomatic hyponatraemia due to the nephrogenic syndrome of inappropriate antidiuresis.

A 20-year-old fit male soldier presented on two separate occasions 16 months apart with severe, symptomatic hyponatraemia and a clinical and biochemical picture consistent with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In the intervening period, repeated plasma sodium values were in the reference range. Intensive investigation failed to reveal a cause for SIADH that was initially considered idiopathic. The description of a family comprising several adults with intermittent or water load induced-hyponatraemia associated with an activating mutation in the arginine vasopressin (AVP) receptor type 2 (AVPR2) raised the question of whether our patient could have a similar 'nephrogenic syndrome of inappropriate antidiuresis'. Mutational screening of AVPR2 in our patient revealed a single missense mutation (R137C) in the second intracellular loop, which has been associated with constitutive activation of the AVPR2. In conclusion, adults with intermittent, severe hyponatraemia may have a constitutively activating mutation in the AVPR2 with resultant nephrogenic syndrome of inappropriate antidiuresis. Patients with idiopathic SIADH, particularly those with unmeasurable circulating AVP concentrations, should be considered for mutational screening of AVPR2.

Clinical indications for the investigation of porphyria: case examples and evolving laboratory approaches to its diagnosis in New Zealand.

Patients with porphyria present in a diverse and unusual variety of ways and most clinicians will see only a few cases, if any, during their professional lives. Porphyria may present (1) with acute symptoms, which may be abdominal pain, neurological or psychiatric; (2) with skin rash or photosensitivity; or (3) with a putative family history. Screening for latent porphyria has been greatly facilitated by fluorescence emission scanning of plasma and by mutational analysis. Our reference laboratory has recently diagnosed several cases of the less common types of porphyria, which we postulate is due to the availability of these methods and to the changing population of New Zealand. Accurate screening and diagnosis of porphyria is important, as an acute porphyric attack is life-threatening and preventable. Retrospective diagnosis may be difficult.

Recurrent myoglobinuria due to carnitine palmitoyltransferase II deficiency: clinical, biochemical, and genetic features of adult-onset cases.

AIMS: To create awareness of adult-onset carnitine palmitoyltransferase II (CPT II) deficiency by describing the clinical, biochemical, and genetic features of three New Zealand patients with this disorder. METHODS: Review of case notes, creatine kinase (CK) values, CPT II assay results, genetic mutation analyses, and the literature. RESULTS: Three patients with CPT II deficiency were encountered by the authors over a 7-year period. Onset of symptoms was between ages 10 and 17 years. Each patient reported exertional myalgia, and had had at least two episodes of myoglobinuria following exertion or infection. Interictal neurological examinations, CK values, and routine muscle histology were normal. CPT II activities ranged from 3.8 to 9.7 pmol/min/mg protein (normal+/-SD=162.9+/-51.0 pmol/min/mg protein). Genetic analysis showed that one patient was homozygous and two were heterozygous for S113L, the common mutation in CPT II deficiency. CONCLUSIONS: CPT II deficiency should be suspected in patients with persistent exertional myalgia who have one or more episodes of myoglobinuria. The diagnosis is confirmed using a combination of enzyme assay and genetic testing.