Ghassan Abdoh

Newborn Population Screening for Classic Homocystinuria by Determination of Total Homocysteine from Guthrie Cards

OBJECTIVE To allow early recognition of cystathionine beta-synthase by newborn screening. STUDY DESIGN Total homocysteine was determined in dried blood spots with a novel, robust high-performance liquid chromatography method with tandem mass spectrometry. Quantification of homocysteine was linear over a working range up to 50 micromol/L. For mutation analysis, DNA was tested for 2 mutations common in Qatar. RESULTS Both methods proved to be suitable for high throughput processing. In 2 years, 7 infants with classic homocystinuria were identified of 12,603 native Qatari infants, yielding an incidence of 1:1800. Molecular screening would have missed 1 patient homozygous for a mutation not previously identified in the Qatari population. Over a period of 3 years, a total of 14 cases of classic homocystinuria were detected by screening of homocysteine from all newborn infants born in Qatar (n = 46 406). Homocysteine was always elevated, whereas methionine was elevated in only 7 cases. CONCLUSIONS The study offers a reliable method for newborn screening for cystathionine beta-synthase deficiency, reaching a sensitivity of up to 100%, even if samples are taken within the first 3 days of life.

Molecular neonatal screening for homocystinuria in the Qatari population.

We report the results of molecular neonatal screening for homocystinuria (cystathionine beta‐synthase deficiency) in neonates of Qatari origin, developed in conjunction with a novel biochemical screening approach. DNA was extracted from dried blood spots (DBS); the prevalent Qatari CBS gene mutation p.R336C (c.1006C>T) and a second mutation were tested with specific TaqMan assays. Over a period of 2 years we screened 12,603 neonates and identified six affected neonates homozygous for p.R336C. There were 225 heterozygous carriers for p.R336C. One additional child with homocystinuria detected through biochemical screening was homozygous for a mutation not previously identified in Qatar. Homocystinuria in the Qatari population has an incidence of 1:1,800, the highest in the world and even higher than previously estimated. Allele frequency of the mutation p.R336C is approximately 1%, displaying a significant deviation from Hardy Weinberg equilibrium. In conclusion, first‐line molecular neonatal screening is technically feasible and may be developed as an option for presymptomatic identification of genetic disorders caused by specific mutations or a limited number of prevalent mutations. However, sensitivity for the diagnosis of disorders caused by various mutations is limited even in a homogeneous population such as Qatar. Hum Mutat 30:1–2, 2009.

Newborn screening for methylmalonic acidurias--optimization by statistical parameter combination.

SummaryWith the introduction of tandem mass spectrometry, newborn screening for disorders of propionate metabolism became widely available. However, there is controversy whether population screening for these disorders should be performed. The most widely used primary metabolite C3 itself has a poor specificity or lacks 100% sensitivity for milder forms and/or defects of cobalamin metabolism. Strategies to improve specificity have included the calculation of metabolite ratios (e.g. C3/C2) or second-tier strategies with analysis of methylmalonic acid or 2-methylcitric acid from the primary screening specimen. We report the results of a new statistical approach to identify parameter combinations that allow for 100% sensitivity as well as increased specificity. The promising results of this alternative approach will have to be substantiated on larger data sets.

Newborn screening for remethylation disorders and vitamin B12 deficiency-evaluation of new strategies in cohorts from Qatar and Germany.

BackgroundNewborn screening is a precondition for early diagnosis and successful treatment of remethylation disorders and classical homocystinuria (cystathionine-ß-synthase deficiency). Newborn screening for classical homocystinuria using total homocysteine measurement in dried blood spots has been very successfully performed for many years for newborns from Qatar.MethodsA new optimized newborn screening strategy for remethylation disorders and homocystinuria was developed and evaluated for newborns from Qatar using total homocysteine measurement as first-tier and methionine, methionine-phenylalanine-ratio and propionylcarnitine as second-tiers. Proposed cut-offs were also retrospectively evaluated in newborn screening samples of 12 patients with remethylation disorders and vitamin B12 deficiency from Qatar and Germany.ResultsOver a 12 months period, the proposed strategy led to a decrease in the recall rate in homocysteine screening for Qatar from 1.09% to 0.68%, while allowing for additional systematic inclusion of remethylation disorders and vitamin B12 deficiency into the screening panel for Qatar. In the evaluated period the applied strategy would have detected all patients with classical homocystinuria identified by the previous strategy and in addition 5 children with maternal nutritional vitamin B12 deficiency and one patient with an isolated remethylation disorder. Additional retrospective evaluation of newborn screening samples of 12 patients from Germany and Qatar with remethlyation disorders or vitamin B12 deficiency showed that all of these patients would have been detected by the cut-offs used in the proposed new strategy. In addition, an adapted strategy for Germany using methionine, methionine-phenylalanine-ratio and propionylcarnitine as first-tier, and homocysteine as a second-tier test was also positively evaluated retrospectively.ConclusionsThe proposed strategy for samples from Qatar allows inclusion of remethylation disorders and vitamin B12 deficiency in the screening panel, while lowering the recall rate. An adapted second-tier strategy is presented for screening in Germany and will be prospectively evaluated over the next years in a pilot project named “Newborn Screening 2020”.

Newborn Screening for Vitamin B6 Non-responsive Classical Homocystinuria: Systematical Evaluation of a Two-Tier Strategy

BACKGROUND In classical homocystinuria (HCU, MIM# 236200) due to the deficiency of cystathionine β-synthase (EC 4.2.1.22) there is a clear evidence for the success of early treatment. The aim of this study was to develop and evaluate a two-tier strategy for HCU newborn screening. METHODS We reevaluated data from our newborn screening programme for Qatar in a total number of 125,047 neonates including 30 confirmed HCU patients. Our hitherto existing screening strategy includes homocysteine (Hcy) measurements in every child, resulting in a unique dataset for evaluation of two-tier strategies. Reevaluation included methionine (Met) levels, Met to phenylalanine (Phe) ratio, and Hcy. Four HCU cases identified after database closure were also included in the evaluation. In addition, dried blood spot samples selected by Met values >P97 in the newborn screening programs in Austria, Australia, the Netherlands, and Taiwan were analyzed for Hcy. RESULTS Met to Phe ratio was found to be more effective for first sieve than Met, sorting out nearly 90% of normal samples. Only 10% of the samples would have to be processed by second-tier measurement of Hcy in dried blood spots. As no patient with HCU was found neither in the samples investigated for HCU, nor by clinical diagnosis in the other countries, the generalization of our two-tier strategy could only be tested indirectly. CONCLUSION The finally derived two-tier algorithm using Met to Phe ratio as first- and Hcy as second-tier requires 10% first-tier positives to be transferred to Hcy measurement, resulting in 100% sensitivity and specificity in HCU newborn screening.

Clinical genetics and genomic medicine in Qatar

Clinical genetics and genomic medicine in Qatar.