No abstract

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.

The proposed strategy for samples from Qatar allows inclusion of remethylation disorders and vitamin B 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".

Qatar is a country in the Gulf area and member of the Gulf Cooperation Council states. The country is populated by original Qatari tribes that amount to about 200,000 people and about 600,000 expatriates mainly from Arabic and Asian countries. Inbreeding over centuries and high rates of consanguinity in the Qatari population and in some groups of expatriates, in addition to large family sizes and rapid population growth, have contributed to a high frequency of autosomal recessive disorders. In December 2003 Hamad Medical Corporation in Doha and the University Children's Hospital of Heidelberg, Germany, started an extended state-wide neonatal screening programme for metabolic and endocrine disorders, with the laboratory situated in Heidelberg, Germany. All aspects of the screening process had to be adapted to the unique situation of the laboratory being 6000 km from the birthplace of the neonates. Within 32 months, samples of 25,214 neonates were screened. In 28 cases an endocrine or metabolic diagnosis was identified (incidence 1:901, in Germany 1:1728). In particular, a variety of monogenic metabolic diseases were prevalent, with 19 patients detected giving an incidence of metabolic diseases of 1:1327 (Germany 1:2517). Each euro spent on the screening programme saved more than 25 euros in health and social costs. The programme revealed a high incidence of treatable inborn metabolic diseases in the population of Qatar. A reliable screening for classical homocystinuria showing a unique incidence of >1:3000 and for sickle cell disease has now been added.

Background: In classical homocystinuria (HCU, MIM# 236200) due to the deficiency of cystathionine bsynthase (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. Abbreviations CBSCystathionine b-synthase DBS Dried blood spots ESI-MS/ MS Electrospray ionization-tandem mass spectrometry HCU Classical homocystinuria Hcy Homocysteine