Development of a newborn screening follow-up algorithm for the diagnosis of isobutyryl-CoA dehydrogenase deficiency

PURPOSE

Isobutyryl-CoA dehydrogenase deficiency is a defect in valine metabolism and was first reported in a child with cardiomyopathy, anemia, and secondary carnitine deficiency. We identified 13 isobutyryl-CoA dehydrogenase-deficient patients through newborn screening due to an elevation of C4-acylcarnitine in dried blood spots. Because C4-acylcarnitine represents both isobutyryl- and butyrylcarnitine, elevations are not specific for isobutyryl-CoA dehydrogenase deficiency but are also observed in short-chain acyl-CoA dehydrogenase deficiency. To delineate the correct diagnosis, we have developed a follow-up algorithm for abnormal C4-acylcarnitine newborn screening results based on the comparison of biomarkers for both conditions.

METHODS

Fibroblast cultures were established from infants with C4-acylcarnitine elevations, and the analysis of in vitro acylcarnitine profiles provided confirmation of either isobutyryl-CoA dehydrogenase or short-chain acyl-CoA dehydrogenase deficiency. Isobutyryl-CoA dehydrogenase deficiency was further confirmed by molecular genetic analysis of the gene encoding isobutyryl-CoA dehydrogenase (ACAD8). Plasma acylcarnitines, urine acylglycines, organic acids, and urine acylcarnitine results were compared between isobutyryl-CoA dehydrogenase- and short-chain acyl-CoA dehydrogenase-deficient patients.

RESULTS

Quantification of C4-acylcarnitine in plasma and urine as well as ethylmalonic acid in urine allows the differentiation of isobutyryl-CoA dehydrogenase-deficient from short-chain acyl-CoA dehydrogenase-deficient cases. In nine unrelated patients with isobutyryl-CoA dehydrogenase deficiency, 10 missense mutations were identified in ACAD8. To date, 10 of the 13 isobutyryl-CoA dehydrogenase-deficient patients remain asymptomatic, two were lost to follow-up, and one patient required frequent hospitalizations due to emesis and dehydration but is developing normally at 5 years of age.

CONCLUSION

Although the natural history of isobutyryl-CoA dehydrogenase deficiency must be further defined, we have developed an algorithm for rapid laboratory evaluation of neonates with an isolated elevation of C4-acylcarnitine identified through newborn screening.

Expanded newborn screening identifies maternal primary carnitine deficiency

Primary carnitine deficiency impairs fatty acid oxidation and can result in hypoglycemia, hepatic encephalopathy, cardiomyopathy and sudden death. We diagnosed primary carnitine deficiency in six unrelated women whose unaffected infants were identified with low free carnitine levels (C0) by newborn screening using tandem mass spectrometry. Given the lifetime risk of morbidity or sudden death, identification of adult patients with primary carnitine deficiency is an added benefit of expanded newborn screening programs.

Systemic correction of a fatty acid oxidation defect by intramuscular injection of a recombinant adeno-associated virus vector

Mitochondrial beta-oxidation of fatty acids is required to meet physiologic energy requirements during illness and periods of fasting or physiologic stress, and is most active in liver and striated muscle. Acyl-CoA dehydrogenases of varying chain-length specificities represent the first step in the mitochondria for each round of beta-oxidation, each of which removes two-carbon units as acetyl-CoA for entry into the tricarboxylic acid cycle. We have used recombinant adeno-associated virus (rAAV) vectors expressing short-chain acyl-CoA dehydrogenase (SCAD) to correct the accumulation of fatty acyl-CoA intermediates in deficient cell lines. The rAAV-SCAD vector was then packaged into either rAAV serotype 1 or 2 capsids and injected intramuscularly into SCAD-deficient mice. A systemic effect was observed as judged by restoration of circulating butyryl- carnitine levels to normal. Total lipid content at the injection site was also decreased as demonstrated by noninvasive magnetic resonance spectroscopy (MRS). SCAD enzyme activity in the injected muscle was found at necropsy to be above the normal control mouse level. This study is the first to demonstrate the systemic correction of a fatty acid oxidation disorder with rAAV and the utility of MRS as a noninvasive method to monitor SCAD correction after in vivo gene therapy.

Retrospective determination of ceruloplasmin in newborn screening blood spots of patients with Wilson disease

Wilson disease is an autosomal recessive disorder of copper transport, caused by the reduced or absent function of the Wilson disease gene ATP7B on chromosome 13. The disease is characterized by reduced incorporation of copper into the ceruloplasmin protein and reduced excretion of copper into the bile. Wilson disease is effectively treated if detected early. Our study goals were to determine the feasibility of a population screening for Wilson disease using dried blood spots and to characterize the base-line ceruloplasmin concentration in newborn blood spots of patients with Wilson disease. Ceruloplasmin was analyzed in dried blood spots obtained from 353 Mayo Clinic pediatric volunteers aged from 3 months to 18 years and from 1045 anonymous newborn screening specimens using a sandwich enzyme-linked immunosorbent assay. The original newborn screening blood spots were retrieved from two patients with Wilson disease along with age-matched controls for ceruloplasmin determination. The mean (+/-SD) concentration of ceruloplasmin in the pediatric blood spots was 40.0+/-14.4 mg/dL (range 13.1 to >60 mg/dL) and newborn blood spots was 47.2+/-15.5mg/dL (range 6.5 to >60 mg/dL). Ceruloplasmin in the newborn blood spots from two Wilson disease patients were 2.6 and 2.8 mg/dL, respectively. The newborns affected with Wilson disease had significantly lower ceruloplasmin levels in blood spots than unaffected newborns. These findings support that presymptomatic screening for Wilson disease using dried blood spots could be possible, even in the newborn period.

In vitro characterization and in vivo expression of human very-long chain acyl-CoA dehydrogenase

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is a disorder of fatty acid beta-oxidation that can present at any age with cardiomyopathy, rhabdomyolysis, hepatic dysfunction, and/or nonketotic hypoglycemia. Through the expansion of newborn screening programs an increasing number of individuals with VLCAD deficiency are being identified prior to the onset of symptoms allowing early initiation of therapy. The development of a safe, durable, and effective VLCAD gene delivery system for use at the time of diagnosis could result in a significant improvement in the quality and duration of life for patients with VLCAD deficiency. To this end, we developed a construct containing the human VLCAD cDNA under the control of the strong CMV promoter (pCMV-hVLCAD). A novel rabbit polyclonal anti-VLCAD antibody was prepared using a 24 amino-acid peptide unique to the human VLCAD protein to study human VLCAD expression in immune competent mice. Antibody specificity was demonstrated in Western blots of human VLCAD deficient fibroblasts and in pCMV-hVLCAD transiently transfected VLCAD deficient fibroblasts. Transfected fibroblasts showed correction of the metabolic block as demonstrated by normalization of C14- and C16-acylcarnitine species in cell culture media and restoration of VLCAD activity in cells. Following tail vein injection of pCMV-hVLCAD into mice, we demonstrated expression of hVLCAD in liver. Altogether, these steps are important in the development of a durable gene therapy for VLCAD deficiency.

Quantitative determination of succinylacetone in dried blood spots for newborn screening of tyrosinemia type I

BACKGROUND

Tyrosinemia type I (TYR 1) is a severe disorder causing early death if left untreated. While tyrosine can be determined in dried blood spots (DBS), it is not a specific marker for TYR 1 and most often associated with benign transient tyrosinemia of the newborn. Succinylacetone (SUAC) is a specific marker for TYR 1 but not detectable by routine newborn screening. We developed a new assay that determines SUAC in DBS by liquid-chromatography tandem mass spectrometry (LC-MS/MS).

METHODS

Whole blood is eluted from a 3/16-in. DBS by an aqueous solution containing deuterium labeled SUAC as internal standard (IS). SUAC and IS are oximated, then extracted, butylated, and analyzed by LC-MS/MS. Quantitation is from SUAC spiked calibrator DBS over the range 0-200 microM using selected reaction monitoring of transitions m/z 212 to 156 and m/z 214 to 140 for SUAC and IS, respectively. Analysis time is 5 min. To assess the effectiveness of a two-tier screening approach for TYR 1 we applied this assay to our newborn screening program over the last 15 months.

RESULTS

The intra-assay precision was determined for three different levels of SUAC (5, 20, and 50 micromol/L) and the CV calculated to be 4.7, 2.6, and 3.1%, respectively (n=5). Inter-assay precision CVs were 12.7, 8.2, and 7.8%, respectively on the same samples. SUAC levels in DBS from 10 confirmed TYR 1 cases not treated with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) were clearly abnormal (16-150 micromol/L; mean: 61 micromol/L; controls: <5 micromol/L). Over a 15-month period, SUAC was determined in newborn screening samples with elevated tyrosine concentrations when applying different cut off values until it was settled at 150 micromol/L. No case of TYR 1 was detected in 124,780 newborns tested.

CONCLUSION

We have developed a new LC-MS/MS based method for the determination of SUAC in DBS. This assay has the potential to significantly reduce the number of false positive results in newborn screening for TYR 1 and can also be used for the laboratory follow up of patients treated for TYR 1.

Making the case for objective performance metrics in newborn screening by tandem mass spectrometry

The expansion of newborn screening programs to include multiplex testing by tandem mass spectrometry requires understanding and close monitoring of performance metrics. This is not done consistently because of lack of defined targets, and interlaboratory comparison is almost nonexistent. Between July 2004 and April 2006 (N=176,185 cases), the overall performance metrics of the Minnesota program, limited to MS/MS testing, were as follows: detection rate 1:1,816, positive predictive value 37% (54% in 2006 till date), and false positive rate 0.09%. The repeat rate and the proportion of cases with abnormal findings actually been reported are new metrics proposed here as an objective mean to express the overall noise in a program, where noise is defined as the total number of abnormal results obtained using a given set of cut-off values. On the basis of our experience, we propose the following targets as evidence of adequate analytical and postanalytical performance: detection rate 1:3,000 or higher, positive predictive value>20%, and false positive rate<0.3%.

Automated spectrophotometric analysis of mitochondrial respiratory chain complex enzyme activities in cultured skin fibroblasts

BACKGROUND

Mitochondrial respiratory chain complex (RCC) disorders may occur as commonly as 1 in 8500 individuals. Because of the great variability of phenotypic presentations, measurement of individual RCC enzyme activities is a crucial diagnostic process. Current assay methods are time-consuming and labor-intensive and thus constitute a major impediment to clinical practice. A method with a faster turnaround time would therefore be beneficial.

METHOD

We developed an automated spectrophotometric method for measuring the respiratory chain enzyme activities of complex I, complex II + III, and complex IV with the Hitachi 912, an automated spectrophotometer. Mitochondrial citrate synthase was also determined for normalization of the RCC activities.

RESULTS

A blinded method comparison with samples from an external testing center yielded a 91% concordance of interpretations. Mean intraassay imprecision (as CV; n = 20) in a single batch analysis of each RCC was 5.9%. Interassay imprecision, evaluated on 2 samples harvested and analyzed 3 times each, gave mean CVs of 10%-18%.

CONCLUSIONS

With this automated method, a panel of RCC enzyme activities can be determined in <2 h. In addition, an immunoblot assay using monoclonal antibodies against specific subunits of RCC enzyme complexes can be informative in cases of borderline enzyme activity. Our results suggest that in vitro diagnosis of RCC enzyme deficiencies in skin fibroblasts is an effective alternative to invasive muscle biopsy.

Genotypic differences of MCAD deficiency in the Asian population: novel genotype and clinical symptoms preceding newborn screening notification

PURPOSE

In contrast to its high prevalence in Caucasians, medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is reported to be an extremely rare metabolic disorder in the Asian population. The common MCAD gene (ACADM) mutation 985A>G (p.K329E), accounting for the majority of cases in Caucasians, has not been detected in this ethnic group, and the spectrum of ACADM mutations has remained unknown.

METHOD

Biochemical genetic testing including plasma acylcarnitine and urine acylglycine analyses, as well as sequencing of ACADM was performed in a Korean family with a newborn who had an elevated octanoyl (C8) carnitine concentration by newborn screening (NBS). Genotyping of 50 Korean newborns with normal NBS results was performed.

RESULT

We report the identification of the first Korean patient with MCAD deficiency, caused by a novel missense mutation in ACADM, 843A>T (R281S), and a 4-bp deletion, c.449_452delCTGA. The patient became symptomatic before notification of the abnormal NBS result. Both the father and a brother who were identified as carriers for the 4-bp deletion had mildly elevated plasma C8 and C10:1 carnitine concentrations, whereas the acylcarnitine profile was normal in the mother who carries the missense mutation.

CONCLUSION

The 4-bp deletion may represent a common Asian ACADM mutation, considering that it recently has also been found in two of the three Japanese patients in whom genotyping was performed. Greater availability of MCAD mutation analysis is likely to unravel the molecular basis of MCAD deficiency in the Asian population that might differ from Caucasians.

Medium-chain acyl-CoA dehydrogenase deficiency in gene-targeted mice

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common inherited disorder of mitochondrial fatty acid β-oxidation in humans. To better understand the pathogenesis of this disease, we developed a mouse model for MCAD deficiency (MCAD^−/−) by gene targeting in embryonic stem (ES) cells. The MCAD^−/− mice developed an organic aciduria and fatty liver, and showed profound cold intolerance at 4 °C with prior fasting. The sporadic cardiac lesions seen in MCAD^−/− mice have not been reported in human MCAD patients. There was significant neonatal mortality of MCAD^−/− pups demonstrating similarities to patterns of clinical episodes and mortality in MCAD-deficient patients. The MCAD-deficient mouse reproduced important aspects of human MCAD deficiency and is a valuable model for further analysis of the roles of fatty acid oxidation and pathogenesis of human diseases involving fatty acid oxidation.

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is one of the most common inherited disorders of metabolism. This defect in fatty acid oxidation can lead to severe and sometimes fatal disease, especially in young children because they are unable to tolerate a fasting episode. Metabolic complications include very low blood glucose concentrations and generation of toxic by-products. This disorder can result in sudden infant death. Using a process known as gene targeting in mouse embryonic stem cells, the authors have developed a mouse model with the same enzyme deficiency. This mouse model of MCAD deficiency develops many of the same disease characteristics found in affected children. The MCAD-deficient mouse model shows a high rate of newborn loss, intolerance to cold, and the characteristic biochemical changes in the blood, tissues, and urine that are very similar to those found in the human disease counterpart. The MCAD-deficient mouse model will allow researchers to better understand disease mechanisms so that new preventive measures or therapies can be developed.

In vitro correction of medium chain acyl CoA dehydrogenase deficiency with a recombinant adenoviral vector

Defects of mitochondrial beta-oxidation are a growing group of disorders with variable clinical presentations ranging from mild hypotonia to sudden infant death. Current therapy involves avoidance of fasting, dietary restrictions, and cofactor supplementation. Unfortunately, times of acute illness and noncompliance can interfere with these therapies and result in a rapid clinical decline. The development of a safe, durable, and effective gene delivery system remains an attractive alternative therapy for individuals with these disorders. To this end, a recombinant first-generation adenovirus vector (Ad/cmv-hMCAD) has been prepared that constitutively expresses the human medium chain acyl CoA dehydrogenase (MCAD) protein under the control of the CMV promoter and bovine polyadenylation signal. Characterization of human fibroblasts deficient in MCAD infected with Ad/cmv-hMCAD including Western analysis, immunohistological staining visualized with confocal microscopy, electron transfer protein (ETF) reduction assay, and palmitate loading studies was performed. Infection of MCAD deficient fibroblast with Ad/cmv-hmcad resulted in the production of a 55kDa protein that co-localized in cells with a mitochondrial marker. Extracts prepared from Ad/cmv-hMCAD infected deficient fibroblasts demonstrated correction of the block seen in the MCAD catalyzed reduction of ETF in the presence of octanoyl CoA. Finally, MCAD deficient fibroblasts infected with increasing amounts of Ad/cmv-hMCAD showed a stepwise improvement of the abnormal acylcarnitine profile exhibited by the deficient cells. Together these studies demonstrate our ability to express and monitor the expression of MCAD in treated cells and support further in vivo murine studies to assess toxicity and duration of correction with this and other MCAD recombinant vectors.

Fetal fatty acid oxidation disorders, their effect on maternal health and neonatal outcome: impact of expanded newborn screening on their diagnosis and management

Mitochondrial fatty acid oxidation disorders (FAOD) are recessively inherited errors of metabolism. Newborns with FAOD typically present with hypoketotic hypoglycemia, metabolic acidosis, hepatic failure, and cardiomyopathy. Late presentations include episodic myopathy, neuropathy, retinopathy, and arrhythmias. Sudden unexpected death can occur at any age and can be confused with sudden infant death syndrome. Some FAOD are associated with intrauterine growth restriction, prematurity, and pregnancy complications in the heterozygous mother, such as severe preeclampsia, acute fatty liver of pregnancy (AFLP), or hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome. Maternal pregnancy complications occur primarily in mothers carrying a fetus with long-chain l-3-hydroxyacyl CoA dehydrogenase deficiency or general trifunctional protein deficiencies. FAOD as a group represent the most common inborn errors of metabolism, and presymptomatic diagnosis of FAOD is the key to reduce morbidity and avoid mortality. The application of tandem mass spectrometry to newborn screening provides an effective means to identify most FAOD patients presymptomatically. At the beginning of 2005, 36 state newborn screening programs have mandated or adopted this technology resulting in a marked increase in the number of asymptomatic neonates with FAOD diagnosed. To ensure the long-term benefits of such screening programs, pediatricians and other health care providers must be educated about these disorders and their treatment.

Synergistic heterozygosity in mice with inherited enzyme deficiencies of mitochondrial fatty acid beta-oxidation

We have used mice with inborn errors of mitochondrial fatty acid beta-oxidation to test the concept of synergistic heterozygosity. We postulated that clinical disease can result from heterozygous mutations in more than one gene in single or related metabolic pathways. Mice with combinations of mutations in mitochondrial fatty acid beta-oxidation genes were cold challenged to test their ability to maintain normal body temperature, a sensitive indicator of overall beta-oxidation function. This included mice of the following genotypes: triple heterozygosity for mutations in very-long-chain acyl CoA dehydrogenase, long-chain acyl CoA dehydrogenase, and short-chain acyl CoA dehydrogenase genes (VLCAD+/-//LCAD+/-//SCAD+/-); double heterozygosity for mutations in VLCAD and LCAD genes (VLCAD+/-//LCAD+/-); double heterozygosity for mutations in LCAD and SCAD genes (LCAD+/-//SCAD+/-); single heterozygous mice (VLCAD+/-, LCAD+/-, SCAD+/-) and wild-type. We found that approximately 33% of mice with any of the combined mutant genotypes tested became hypothermic during a cold challenge. All wild-type and single heterozygous mice maintained normal body temperature throughout a cold challenge. Despite development of hypothermia in some double heterozygous mice, blood glucose concentrations remained normal. Biochemical screening by acylcarnitine and fatty acid analyses demonstrated results that varied by genotype. Thus, physiologic reduction of the beta-oxidation pathway, characterized as cold intolerance, occurred in mice with double or triple heterozygosity; however, the derangement was milder than in mice homozygous for any of these mutations. These results substantiate the concept of synergistic heterozygosity and illustrate the potential complexity involved in diagnosis and characterization of inborn errors of fatty acid metabolism in humans.

Mice heterozygous for a defect in mitochondrial trifunctional protein develop hepatic steatosis and insulin resistance

BACKGROUND & AIMS

Little is known about the role of mitochondrial beta-oxidation in development of nonalcoholic fatty liver disease (NAFLD). Mitochondrial trifunctional protein (MTP) catalyzes long-chain fatty acid oxidation. Recently, we generated a mouse model for MTP deficiency and reported that homozygous (MTPa-/-) mice suffer neonatal death. In this study, we investigated effects of heterozygosity for the MTP defect on hepatic oxidative stress, insulin resistance, and development of NAFLD in mice.

METHODS

We evaluated liver histopathology, serum alanine aminotransferase (ALT), glucose, fatty acids, and insulin levels in MTPa+/- and MTPa+/+ littermates. Insulin resistance was evaluated using glucose tolerance test (GTT) and insulin tolerance test (ITT). Liver tissues were used to measure triglyceride and fatty acid content, activity of superoxide dismutases (SOD) and glutathione peroxidase (GPx), glutathione (GSH), and cytochrome P-450 2E1 expression.

RESULTS

Aging but not young MTPa+/- mice developed hepatic steatosis with elevated ALT, basal hyperinsulinemia, and increased insulin area under curve (AUC) on GTT compared with MTPa+/+ littermates. In response to insulin challenge, aging MTPa+/- mice had slower rate of glucose disappearance and increased glucose AUC. Significant hepatic steatosis and insulin resistance developed concomitantly in the MTPa+/- mice at 9-10 months of age. Aging MTPa+/- mice had higher antioxidant activity of total SOD and GPx, lower GSH, and increased expression of cytochrome P-450 2E1, consistent with increased hepatic oxidative stress.

CONCLUSIONS

Heterozygosity for beta-oxidation defects predisposes to NAFLD and insulin resistance in aging mice. Impairment of mitochondrial beta-oxidation may play an important role in pathogenesis of NAFLD.

Management and outcome of neonatal-onset ornithine transcarbamylase deficiency following liver transplantation at 60 days of life

Neonatal hyperammonemia secondary to X-linked ornithine transcarbamylase (OTC) deficiency carries a high risk of morbidity and mortality. Results of medical therapy are less than satisfactory. Experience with liver transplantation in very young affected infants is limited. We report a male newborn with severe OTC deficiency who underwent successful orthotopic, cadaveric liver transplantation at the age of 60 days. Although technically challenging in the neonatal period, liver transplantation should be considered early as the most promising treatment approach currently available.

The urinary excretion of glutarylcarnitine is an informative tool in the biochemical diagnosis of glutaric acidemia type I

Glutaric acidemia type I (GA-1) is a progressive neurodegenerative inborn error of metabolism that typically manifests acutely in infants during an intercurrent illness. The diagnosis is established biochemically by the detection of glutaric acid and 3-hydroxy glutaric acid in urine and glutarylcarnitine in plasma. However, some patients excrete only small amounts of glutaric acid and may be overlooked, especially if the plasma concentration of glutarylcarnitine is not elevated. To test the hypothesis that measuring the excretion of glutarylcarnitine may improve the recognition of GA-1 patients without significant glutaric aciduria, urine glutarylcarnitine was analyzed in 14 cases. Five of them lacked significant glutaric aciduria, 9 (of 10 available) had a normal plasma glutarylcarnitine concentration. As controls, we also evaluated 54 subjects with glutaric aciduria secondary to other causes (16-7509 mmol/mol creatinine; reference range: <15; no significant amounts of 3-hydroxy glutaric acid detectable). The excretion of glutarylcarnitine was significantly elevated in all GA-1 patients (14-522 mmol/mol creatinine; reference range: <5.2) and in none of the controls with glutaric aciduria. These findings suggest that the urinary excretion of glutarylcarnitine is a specific biochemical marker of GA-1 which could be particularly useful in the work up of patients with suggestive clinical manifestations but without glutaric aciduria and with normal plasma acylcarnitine profiles.

Influence of dietary fatty acid chain-length on metabolic tolerance in mouse models of inherited defects in mitochondrial fatty acid beta-oxidation

Fasting-induced metabolic disease of all inherited deficiencies of the acyl-CoA dehydrogenases is characterized by hypoglycemia, hypoketonemia, and organic aciduria. Mice with these enzyme deficiencies are cold intolerant. To evaluate the potential role that dietary fatty acid chain-length has on a patient's ability to compensate during a metabolic challenge, we fed long-chain acyl CoA dehydrogenase (LCAD) deficient and short-chain acyl CoA dehydrogenase (SCAD) deficient mice a diet rich in medium-chain triglycerides (MCT) or long-chain triglycerides (LCT). To elucidate the importance of maintaining adequate serum glucose concentrations on compensation mechanisms during metabolic challenge, we treated LCAD-/- mice with a solution of 12.5% glucose or saline prior to fasting and a cold-challenge. We found that feeding SCAD deficient mice the LCT diet from weaning increased survival from 40 to 94% during metabolic challenge of cold tolerance. In contrast, there was no benefit to feeding the MCT diet at weaning to LCAD-/- mice; however, there was significant benefit when LCAD-/- mice were fed the MCT diet from the beginning of gestation. Survival during cold-challenge increased from 50 to 93%. In the LCAD-/- mice treated with glucose, despite maintaining serum glucose concentrations at normal or higher concentrations, the LCAD-/- mice were still unable to compensate during metabolic challenge. These results indicate the important influences dietary fatty acids may have by providing enhanced metabolic tolerance in patients with inborn errors of fatty acid oxidation. Furthermore, these studies demonstrate that there may be crucial variables involved in the treatment of these patients, including the patient's specific enzyme deficiency, the quantity and chain-length of dietary fat, which may provide positive effects, as well as the time in development when it was administered.

A common mutation is associated with a mild, potentially asymptomatic phenotype in patients with isovaleric acidemia diagnosed by newborn screening

Isovaleric acidemia (IVA) is an inborn error of leucine metabolism that can cause significant morbidity and mortality. Since the implementation, in many states and countries, of newborn screening (NBS) by tandem mass spectrometry, IVA can now be diagnosed presymptomatically. Molecular genetic analysis of the IVD gene for 19 subjects whose condition was detected through NBS led to the identification of one recurring mutation, 932C-->T (A282V), in 47% of mutant alleles. Surprisingly, family studies identified six healthy older siblings with identical genotype and biochemical evidence of IVA. Our findings indicate the frequent occurrence of a novel mild and potentially asymptomatic phenotype of IVA. This has significant consequences for patient management and counseling.

Carnitine content and expression of mitochondrial beta-oxidation enzymes in placentas of wild-type (OCTN2(+/+)) and OCTN2 Null (OCTN2(-/-)) Mice

Placenta requires energy to support its rapid growth, maturation, and transport function. Fatty acids are used as energy substrates in placenta, but little is known about the role played by carnitine in this process. We have investigated the role of carnitine in the expression of the enzymes involved in fatty acid beta-oxidation in placenta of OCTN2(-/-) mice with defective carnitine transporter (OCTN2). Heterozygous (OCTN2(+/-)) female mice were mated with heterozygous (OCTN2(+/-)) male mice. Pregnant mice were killed and fetuses and placentas were collected. Carnitine was measured using HPLC and tandem mass spectrometry. Immunohistochemistry was used to detect enzyme expression. Enzyme activities were measured spectrophotometrically. The fetal and placental weights were similar among the three genotypes (OCTN2(+/+), OCTN2(+/-), and OCTN2(-/-)). The levels of carnitine were markedly reduced (<20%) in homozygous OCTN2(-/-) null fetuses and placentas compared with wild-type OCTN2(+/+) controls. However, carnitine concentration in placenta was 2- to 7-fold higher than in the fetus in all three genotypes. Immunohistochemistry revealed that beta-oxidation enzymes are expressed in trophoblast cells. Catalytic activities of these enzymes were present at comparable levels in wild-type (OCTN2(+/+)) and homozygous (OCTN2(-/-)) mouse placentas, with the exception of SCHAD, for which activity was significantly higher in OCTN2(-/-) placentas than in OCTN2(+/+) placentas. These data show that placental OCTN2 is obligatory for accumulation of carnitine in placenta and fetus, that fatty acid beta-oxidation enzymes are expressed in placenta, and that reduced carnitine levels up-regulate the expression of SCHAD in placenta.

Recent developments and new applications of tandem mass spectrometry in newborn screening

PURPOSE OF REVIEW

To summarize recent developments in the field of newborn screening related to the use of tandem mass spectrometry as an analytic platform.

RECENT FINDINGS

Novel inborn errors of metabolism with informative amino acid and/or acylcarnitine profiles have been characterized, increasing the complexity of the differential diagnosis of abnormal results. In addition, methods have been developed for the analysis in dried blood spots of steroids and lysosomal enzymes. Previously unrecognized genotype/phenotype correlations have been found among cohorts of patients whose conditions were diagnosed by screening rather than clinically. Several government entities and professional organizations have issued position statements on newborn screening, and worldwide outcome studies continue to underscore the clinical and financial benefits of expanded newborn screening.

SUMMARY

Although it is done inconsistently, newborn screening in the United States is undergoing a rapid expansion driven by the introduction of tandem mass spectrometry in at least 34 state programs. This technology is also used to detect disease markers beyond acylcarnitines and amino acids, as both primary and second-tier tests. In addition to analytic improvements, there is a trend toward the development of joint programs not limited to contiguous geographic areas, often based upon public-private partnerships. This review will summarize several new developments in the field that have occurred since early 2003 and will mention others likely to occur in the near future.

Steroid profiling by tandem mass spectrometry improves the positive predictive value of newborn screening for congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is primarily caused by 21-hydroxylase deficiency and leads to an accumulation of 17-hydroxyprogesterone and reduced cortisol levels. Newborn screening for CAH is traditionally based on measuring 17-hydroxyprogesterone by different immunoassays. Despite attempts to adjust cutoff levels for birth weight, gestational age, and stress factors, the positive predictive value for CAH screening remains less than 1%. To improve this situation, we developed a method using liquid chromatography-tandem mass spectrometry to measure 17-hydroxyprogesterone, androstenedione, and cortisol simultaneously in blood spots. A total of 1222 leftover blood spots from six different screening programs using different immunoassays (fluorescent immunoassay and ELISA) were reanalyzed in a blinded fashion by liquid chromatography-tandem mass spectrometry. Thirty-one samples were from babies with CAH, 190 had yielded false-positive results by immunoassay, and the remaining 1001 samples were from babies with normal screening results. Steroid profiling allowed for an elimination of 169 (89%) of the false-positive results and for an improvement of the positive predictive value from the reported 0.5 to 4.7%. Although this method is not suitable for mass screening due to the length of the analysis (12 min), it can be used as a second-tier test of blood spots with positive results for CAH by the conventional methods. This would prevent unnecessary blood draws, medical evaluations, and stress to families.

Rapid, comprehensive screening of the human medium chain acyl-CoA dehydrogenase gene

Newborn screening by tandem mass spectrometry (MS/MS) identifies patients with medium chain acyl-CoA dehydrogenase (MCAD) deficiency the most frequently observed disorder of fatty acid oxidation. Molecular genetic analysis is becoming a common tool to confirm those identified as affected by prospective screening and for carrier detection in family studies. The A985G (K304E) mutation accounts for approximately 80% of mutant alleles in MCAD deficient patients, presenting symptomatically, while greater variability of mutant alleles is observed among cases identified through prospective screening. Aside from A985G, the mutation spectrum in MCAD deficient patients is heterogeneous such that comprehensive gene analysis is required. Traditionally the MCAD gene is assayed by sequencing the entire coding region. Although effective and definitive, this approach is expensive, turn around time is slow, and is poorly amenable to a clinical service molecular genetics laboratory. Dye-binding/high-resolution thermal denaturation is a rapid and homogeneous method by which to scan a PCR product for evidence of sequence aberration. PCR is performed in capillaries in the presence of the dsDNA-binding dye LCGreen I and subsequently the DNA/dye complexes are analyzed by high-resolution thermal denaturation. DNA sequencing was limited to fragments displaying abnormal melting profiles. Of 18 specimens analyzed, 11 have a genotype consistent with MCAD deficiency and seven have a genotype consistent with carrier status. Clinical and biochemical data corroborate that the genotype results identified the affected patients and differentiates them from carriers. The entire process is homogeneous requiring no post-PCR manipulation and is completed in under 3 h.