Ethnic variability in newborn metabolic screening markers associated with false-positive outcomes

Newborn screening (NBS) programmes utilise information on a variety of clinical variables such as gestational age, sex, and birth weight to reduce false-positive screens for inborn metabolic disorders. Here we study the influence of ethnicity on metabolic marker levels in a diverse newborn population. NBS data from screen-negative singleton babies (n = 100 000) were analysed, which included blood metabolic markers measured by tandem mass spectrometry and ethnicity status reported by the parents. Metabolic marker levels were compared between major ethnic groups (Asian, Black, Hispanic, White) using effect size analysis, which controlled for group size differences and influence from clinical variables. Marker level differences found between ethnic groups were correlated to NBS data from 2532 false-positive cases for four metabolic diseases: glutaric acidemia type 1 (GA-1), methylmalonic acidemia (MMA), ornithine transcarbamylase deficiency (OTCD), and very long-chain acyl-CoA dehydrogenase deficiency (VLCADD). In the result, 79% of the metabolic markers (34 of 43) had ethnicity-related differences. Compared to the other groups, Black infants had elevated GA-1 markers (C5DC, Cohen's d = .37, P < .001), Hispanics had elevated MMA markers (C3, Cohen's d = .13, P < .001, and C3/C2, Cohen's d = .27, P < .001); and Whites had elevated VLCADD markers (C14, Cohen's d = .28, P < .001, and C14:1, Cohen's d = .22, P < .001) and decreased OTCD markers (citrulline, Cohen's d = -.26, P < .001). These findings correlated with the higher false-positive rates in Black infants for GA-1, in Hispanics for MMA, and in Whites for OTCD and for VLCADD. Web-based tools are available to analyse ethnicity-related changes in newborn metabolism and to support developing methods to identify false-positives in metabolic screening.

Metabolic Serendipities of Expanded Newborn Screening

Incidental findings on newborn screening (NBS) are results that are not the target of screening within a given NBS program, but rather are found as a result of the screening and resulting diagnostic workup for that target. These findings may not have an immediate clinical impact on the newborn, but are sometimes an additional benefit of NBS programs and may be considered secondary targets of NBS programs. This work describes four case reports that had incidental findings on the NBS, which eventually led to the diagnosis of another metabolic disease instead of the one that was initially suspected. The first case was a new defect in the cationic amino acid transporter-2 (CAT-2), which was oriented as an arginase-1 deficiency in the newborn. The second case was a maternal glutaric aciduria type 1 (GA-1) that mimicked a carnitine transporter deficiency in the newborn. The third report was a case of lysinuric protein intolerance (LPI), which appeared as high levels of citrulline on the NBS. The fourth case was a mother with homocystinuria that was diagnosed during the biochemical study of vitamin B₁₂ status. All cases provide new or interesting data that will help guide differential diagnosis in the future.

Favourable outcome in a child with symptomatic diagnosis of Glutaric aciduria type 1 despite vertical HIV infection and minor head trauma

The first case of Glutaric aciduria Type 1(GA1) in an African child was reported in 2001. GA1 has a prevalence of 1:5000 in black South Africans. Although early diagnosis is essential for a favourable outcome, newborn screening is not routine in South Africa where an estimated 320,000 children have HIV infection. Neurodevelopmental delay and encephalopathy are complications of both HIV and GA1. In such a setting it is important to recognise that HIV and GA1 can occur simultaneously. We present an HIV-infected South African male child of Xhosa descent with macrocephaly who commenced combination antiretroviral therapy (ART) at 8 weeks of age in a clinical trial which included a neurodevelopmental sub-study. He developed short-lived focal seizures at 16 months after minor head trauma. Neurological examination was normal. Neuroimaging showed temporal lobe atrophy, subtle hyperintense signal change in the globus pallidus, and focal haemosiderosis in the right Sylvian fissure region. As findings were not in keeping with HIV encephalopathy, a urine metabolic screen was undertaken which suggested GA1. Genetic testing confirmed Arg293Trp mutation. He began L-carnitine and a low protein diet as a restricted diet was not practicable. At 21 months he developed pulmonary tuberculosis, requiring 6 months treatment. He did not develop any neurologic motor symptoms. Serial neurodevelopmental and neuropsychological test scores until 9 years were similar to healthy neighbourhood controls, except for mild language delay at 3½ years. Detection of GA1, probably facilitated through participation in a clinical trial, was pivotal for a favourable outcome. The concomitant use of ART and anti-tuberculous therapy in a child with GA1 appears safe.

Low lysine diet in glutaric aciduria type I--effect on anthropometric and biochemical follow-up parameters

BACKGROUND

Metabolic treatment in glutaric aciduria type I (GA-I) including a low lysine diet with lysine-free, tryptophan-reduced amino acid supplements (AAS), carnitine supplementation and early start of emergency treatment during putatively threatening episodes of intermittent febrile illness dramatically improves the outcome and thus has been recommended by an international guideline group (Kölker et al, J Inherit Metab Dis 30:5-22, 2007). However, possible affection of linear growth, weight gain and biochemical follow-up monitoring has not been studied systematically.

METHODS

Thirty-three patients (n = 29 asymptomatic, n = 4 dystonic) with GA-I who have been identified by newborn screening in Germany from 1999 to 2009 were followed prospectively during the first six years of life. Dietary treatment protocols, anthropometrical and biochemical parameters were longitudinally evaluated.

RESULTS

Mean daily intake as percentage of guideline recommendations was excellent for lysine (asymptomatic patients: 101 %; dystonic patients: 103 %), lysine-free, tryptophan-reduced AAS (108 %; 104 %), energy (106 %; 110 %), and carnitine (92 %; 102 %). Low lysine diet did not affect weight gain (mean SDS 0.05) but mildly impaired linear growth in asymptomatic patients (mean SDS -0.38), while dystonic patients showed significantly reduced weight gain (mean SDS -1.32) and a tendency towards linear growth retardation (mean SDS -1.03). Patients treated in accordance with recent recommendations did not show relevant abnormalities of routine biochemical follow-up parameters.

INTERPRETATION

Low lysine diet promotes sufficient intake of essential nutrients and anthropometric development in asymptomatic children up to age 6 year, whereas individualized nutritional concepts are required for dystonic patients. Revised recommendations for biochemical monitoring might be required for asymptomatic patients.

Glutaric acidemia type 1: outcomes before and after expanded newborn screening

Glutaric acidemia type 1 (GA-1) is an autosomal recessive disorder of lysine, hydroxylysine, and tryptophan metabolism. Patients may present with brain atrophy, macrocephaly, and acute dystonia secondary to striatal degeneration typically triggered by an infection, fever, and/or dehydration. This disorder is identified on expanded newborn screening by increased glutarylcarnitine. We evaluated the outcome of 19 patients with GA-1. Ten patients were diagnosed by newborn screening and 9 were diagnosed clinically. DNA testing in 12 patients identified 15 different mutations in the glutaryl-CoA dehydrogenase gene. Plasma glutarylcarnitine and urinary 3-hydroxyglutaric acid were elevated in all patients. However, only 10 of 17 patients who underwent urine organic acid analysis were high excretors of glutaric acid. Levels of glutarylcarnitine in plasma correlated with the urinary excretion of glutaric and 3-hydroxyglutaric acid, but not with clinical outcome. Plasma lysine was also significantly correlated with urinary glutaric acid, but not with urinary 3-hydroxyglutaric acid. Brain magnetic resonance imaging in all patients showed wide Sylvian fissures before treatment, which normalized by 4 years of age in treated patients. The occurrence of three adverse outcomes (oral motor function, ambulatory capability, and dystonic movements) was on average reduced by 75% (relative risk 0.25 to 0.28) in patients identified by newborn screening compared to patients diagnosed before newborn screening (Fisher's exact test; p=0.0055 for oral motor function and ambulatory capability; p=0.023 for dystonic movements). Newborn screening is effective in the prevention of complications in patients with GA-1 when coupled with treatment strategies.

An improved LC-MS/MS method for the detection of classic and low excretor glutaric acidemia type 1

Glutaric acidemia type I (GA1) is associated with elevated glutarylcarnitine (C5DC), typically measured as its butylester by acylcarnitine profile analysis using tandem mass spectrometry (MS/MS) and the precursor-product ion pair of m/z 388-85. This method neither distinguishes between C5DC and its isomer 3-hydroxydecanoylcarnitine (C10-OH) nor reliably detects the low-excretor variant of GA1, leading to both false-positive and false-negative results when testing for GA1. To overcome these limitations, we developed an LC-MS/MS method that discriminates C5DC from C10-OH by the use of precursor-product ion pairs specific for butylated C5DC (m/z 388-115) and underivatized C10-OH (m/z 332-85). The C5DC method was validated over the linearity range of 0.025-20 μM with a lower limit of quantification (LOQ) of 0.025 μM. Excellent precision and accuracy were also observed. We tested plasma samples from 10 patients with confirmed GA1 (including 3 with the low-excretor variant), 21 patients with mild elevations of C5DC or C10-OH by routine acylcarnitine analysis for which GA1 ultimately was excluded, and 29 normal controls. By using the m/z 388-115 ion pair, all cases of GA1, including the low-excretor variant, were reliably distinguished from normal controls. By using the m/z 388-85 pair, patients with ambiguous elevations of C5DC or C10-OH demonstrated clearly elevated levels of C10-OH (m/z 332-85) but normal C5DC (m/z 388-115), confirming that the apparent elevation of C5DC is due to interference by C10-OH. Our method results in excellent detection of GA1, including the low-excretor variant, and also provides a means to discriminate C5DC and C10-OH in follow-up testing and routine acylcarnitine studies.

Safety, efficacy and physiological actions of a lysine-free, arginine-rich formula to treat glutaryl-CoA dehydrogenase deficiency: focus on cerebral amino acid influx

Striatal degeneration from glutaryl-CoA dehydrogenase deficiency (glutaric aciduria type 1, GA1) is associated with cerebral formation and entrapment of glutaryl-CoA and its derivatives that depend on cerebral lysine influx. In 2006 we designed a lysine-free study formula enriched with arginine to selectively block lysine transport across cerebral endothelia and thereby limit glutaryl-CoA production by brain. Between 2006 and present, we treated twelve consecutive children with study formula (LYSx group) while holding all other treatment practices constant. Clinical and biochemical outcomes were compared to 25 GA1 patients (PROx group) treated between 1995 and 2005 with natural protein restriction (dietary lysine/arginine ratio of 1.7±0.3 mg:mg). We used published kinetic parameters of the y+and LAT1 blood-brain barrier transporters to model the influx of amino acids into the brain. Arginine fortification to achieve a mean dietary lysine/arginine ratio of 0.7±0.2 mg:mg was neuroprotective. All 12 LYSx patients are physically and neurologically healthy after 28 aggregate patient-years of follow up (current ages 28±21 months) and there were no adverse events related to formula use. This represents a 36% reduction of neurological risk (95% confidence interval 14-52%, p=0.018) that we can directly attribute to altered amino acid intake. During the first year of life, 20% lower lysine intake and two-fold higher arginine intake by LYSx patients were associated with 50% lower plasma lysine, 3-fold lower plasma lysine/arginine concentration ratio, 42% lower mean calculated cerebral lysine influx, 54% higher calculated cerebral arginine influx, 15-26% higher calculated cerebral influx of several anaplerotic precursors (isoleucine, threonine, methionine, and leucine), 50% less 3-hydroxyglutarate excretion, and a 3-fold lower hospitalization rate (0.8 versus 2.3 hospitalizations per patient per year). The relationship between arginine fortification and plasma lysine indicates that transport competition exists at both cerebrovascular and gastrointestinal barriers, suggesting their co-administration is key to efficacy. Monitoring the ratio between lysine and arginine in diet and plasma may prove a useful strategy for treating children with GA1.

False-positive newborn screening mimicking glutaric aciduria type I in infants with renal insufficiency

Glutaric aciduria type I (GA I), an autosomal-recessive deficiency of glutaryl-CoA-dehydrogenase, leads to encephalopathic crises resulting in irreversible neurological damage. As early diagnosis and implementation of appropriate treatment has significant benefit for these patients, GA I has been implemented in the extended newborn screening program in several countries. Screening parameter is glutarylcarnitine (C5DC) with its ratios. From 1 January 2005 until 31 December 2008, 173,846 newborns were examined by neonatal screening in our screening center. C5DC and/or at least three C5DC/acylcarnitine ratios were increased in 53 newborns (0.03%) and persisted in 11 infants after recall. GA I was not confirmed in any of these infants, but all 11 infants were suffering from renal insufficiency due to congenital (5/11) or acquired (6/11) renal disease. C5DC was shown to be significantly associated with renal affection and was significantly higher in infants with congenital renal insufficiency than in those with acquired renal insufficiency (p = 0.011). Creatinine correlated significantly with C5DC (p = 0.001) and all C5DC/acylcarnitine ratios, mainly with C5DC/(C8 + C10), C5DC/C0, C5DC/C2, C5DC/C4, and C5DC/C8 (for all: p = 0.001). Glutarylcarnitinemia associated with renal insufficiency has not yet been studied systematically. Renal damage in neonates might lead to disturbances in renal transporter systems of glutaric acid and its metabolites and a decreased excretion of C5DC, thus resulting in an increase of plasma C5DC. Therefore, newborns presenting with a positive screening indicating GA I may be considered not only to suffer from GA I but from renal insufficiency as well.

Macrocephaly and bitemporal arachnoid cysts not associated with glutaric aciduria type I in a child

A 45-month-old child who had bitemporal arachnoid cysts and macrocephaly unrelated to glutaric aciduria type 1 (GA 1) was reported. The patient was admitted to the emergency unit after head trauma at 11 months of age. CT and MRI showed bitemporal arachnoid cysts (BACs). Acylcarnitine profile was normal in serum using tandem mass spectrometry. Urine and blood screening tests were within normal range for metabolic disorders. There were no unusual organic acids in urine and blood samples. No additional clinical findings of metabolic disorders such as GA 1 developed during follow-up. The majority of children affected with GA 1 have macrocephaly and BACs on CT or MRI. These signs should alert neurosurgeons to the possibility of GA 1. Neurosurgeons evaluating patients with head trauma or suspected non-accidental head injury should include GA 1 in the differential diagnosis of BACs associated with macrocephaly, and an evaluation should be performed.

Glutaryl-CoA dehydrogenase deficiency and newborn screening: retrospective analysis of a low excretor provides further evidence that some cases may be missed

Glutaryl-CoA dehydrogenase deficiency (GA-I) is associated with the onset of irreversible, disabling dystonia between 3 and 18 months of age. Presymptomatic identification and treatment can prevent the devastating disability associated with this disorder. We report the retrospective analysis of the newborn blood spot of an affected child with a low excretor phenotype. The level of glutarylcarnitine was below the newborn screening program cut-off. This suggests that some cases of GA-I may be missed by newborn screening by tandem mass spectrometry.