Neonatal screening for lysosomal storage disorders

The interest in newborn screening for lysosomal storage disorders (LSDs) has increased significantly due to newly developed enzyme replacement therapies, the need for early diagnosis, and advances in technical developments. However, testing for lysosomal storage disorders in newborn screening (NBS) raises many challenges for primary health care and their providers. The high frequency of late-onset mutations makes lysosomal storage disorders a broad health problem beyond childhood, as well as a challenge for diagnosis and therapy.

by Professor David C. Kasper

Clinical Background
Lysosomal storage disorders (LSDs) may be an attractive candidate for newborn screening (NBS). These disorders result in the accumulation of macromolecular substrates that would normally be degraded by enzymes involved in lysosomal metabolism [1]. Although individual LSDs are rare, their combined incidence has been estimated at 1 per 7,700 live births for Caucasians [2]. LSDs have a progressive course, and can present at any age affecting any number of tissues and organ systems [3]. In most cases, treatment is directed toward symptomatic care of secondary complications. The development of novel diagnostic techniques was strengthened by the availability of treatment strategies including enzyme replacement, stem cell transplantation and substrate reduction although limitations of these therapies still exist [4]. Nonetheless, early diagnosis and treatment is essential for optimal treatment thus leading to the support of implementing LSDs to the NBS panel. However, the current experience of nationwide screening for LSDs is still limited.

Laboratory diagnostics
The increased technological capacity implies that expanded NBS programmes can now identify a broader range of conditions where early detection and pre-symptomatic treatment result in clinical benefit. However, the technology for a simultaneous screening of several enzyme activities related to LSDs from more or less one single blood sample was initially complicated, time-consuming and laborious but finally new protocols and technologies are now available that allow a simplified screening procedure. For future implementation of high-throughput LSD assays in routine clinical diagnostics, sample handling and mass spectrometric analysis has to be simplified; specifically, sample pre-treatment, speed of analysis and finally detection must become more integrated [5]. In this context it is also mandatory to achieve high laboratory standards in terms of technical proficiency and reproducibility of results. Hereby, quality control materials provided by the Newborn Screening Quality Assurance Program at the Centers for Disease Control and Prevention (CDC, Atlanta, GA, USA) are available [6].

Protocols for analysing lysosomal enzyme activities evolve continuously. In addition to fluorescent methods, using, for example, 4-methylumbelliferone, efforts have been made to use tandem mass spectrometry (MS/MS) particularly for high-throughput analysis in routine newborn screening laboratories. MS/MS procedures were refined and optimised, but the complexity of sample preparation prior to mass spectrometry still remains. Drawbacks of these protocols were the need of liquid-liquid extraction (LLE), solid phase extraction (SPE), and the handling with hazardous organic compounds such as ethyl acetate. Novel aspects such as online multi-dimensional chromatography prior to flow injection analysis facilitate ease-of-use sample introduction and increased speed of analysis. Our research group previously reported the use of TurboFlow (Turbulent Flow Chromatography) for online sample clean-up to remove matrix interferences such as salts, proteins and detergents for the analysis of lysosomal enzyme activities in dried blood spot samples [7]. Subsequently, purified analytes of interest that were removed from potential matrix interferences were transferred from a TurboFlow column to an analytical column for ultra high performance liquid chromatography (UHPLC) separation prior to MS/MS analysis in order to separate enzymatic products from residual substrate. This simplified protocol has recently been evaluated in a comprehensive pilot screening of more than 8,500 newborns to demonstrate the technical feasibility and robustness [8]. Moreover, the incubation time was reduced tremendously from 12–16h to 3h [9]. However, novel buffer systems for the combined incubation of more than 6 or 9 enzymes simultaneously are on the horizon including substrates for mucopolysaccharidosis type II, IVA and VI [10]. These new buffer systems might allow the incubation of several enzymes in one reaction vial, and help to reduce costs for personnel, consumables and reagents. We conclude that multiplex MS/MS screening assays are reliable for nationwide LSD NBS, and for selective metabolite screening in high-risk population.

In our experience, comparing biochemical with genetic data of affected patients, we did not observe any correlation between mutation and lack of enzyme activity measured biochemically by MS/MS, nor could type of mutation be estimated by the level of decreased enzyme activity. However, it is mandatory to confirm biochemically suspected cases by genetic mutation analysis.

The nationwide LSD screening experience
The nationwide screening for LSDs is the beginning of a new category of disorders that will confront us with challenging topics regarding NBS. Currently, routine newborn screening for LSDs has been introduced for Pompe disease in Taiwan and for Krabbe disease in the State of New York, respectively. The Austrian Newborn Screening Center and others, for example in Washington State and Italy, have successfully started pilot studies using multiplexed MS/MS screening assays.

We report the results of a comprehensive pilot screening of ~35,000 newborns for four LSDs using a multiplex MS/MS based assay including genetic mutation analysis [11]. Our results revealed a surprisingly high number of enzyme deficiencies among a predominantly Caucasian population in a Central European country. The results finally confirmed 15 newborns with at least one mutation including diminished lysosomal enzyme activity, demonstrating the high overall incidence of 1 : 2315 among the Austrian population. Frequency, positive predictive value and technical practicability make nationwide NBS for LSDs technical feasible. In our screening, the positive cases contribute predominantly to Fabry disease with an incidence of late-onset Fabry disease of 1 : 4100 among the Austrian population. Fabry disease is found among all ethnic, racial, and demographic groups and is not restricted to a specific ethnic background. Our results are concurrent with those from Spada et al. who reported a high incidence of 1 : 3100 for late-onset and 1 : 37 000 for the classic phenotype [12]. Furthermore, several studies have shown that patients with renal insufficiency, cerebral infarctions, or left ventricular hypertrophy of unknown aetiology might suffer from Fabry disease [13]. We conclude that a putative NBS may be beneficial to identify severe clinical cases and but has the drawbacks of detecting mild forms, late onsets and asymptomatic cases.

Future perspectives
The high incidence of the late-onset phenotypes in Fabry, Gaucher and Pompe disease raises the question when genetic screening for this disease should be undertaken, in the neonatal period or at early maturity. Clearly, early detection, genetic counselling, and therapeutic intervention are beneficial for the classic phenotype but the time of screening for the late-onset variants of Fabry and other treatable diseases may raise concerns. A recent study revealed that long-term treatment led to substantial and sustained clinical benefits; however advanced cardiac and renal disease cannot be reversed later on making early diagnosis crucial. NBS is less controversial for infantile Pompe. In Taiwan, first prospective Pompe screening including the initiation of treatment before onset of obvious symptoms and significant irreversible muscle damage clearly demonstrated the benefit for infants. The central nervous system cannot be treated by enzyme replacement therapies for neuronopathic LSDs like for Gaucher II and Niemann-Pick A, and thus highlights the importance of consented genotyping and phenotype prediction after biochemical first-line screening. Apart the potential clinical benefit for patients, NBS for LSDs can provide reproductive risk information for parents and future adults. This situation is common for screening of metabolic disorders as they are inherited predominately in a recessive manner.
In conclusion, our study shows that Pompe, Gaucher and Fabry are frequent disorders with great public health implications. Even though the American College of Medical Genetics (ACMG) ranked LSDs with low priority in 2006, two LSDs including Pompe and Krabbe were finally nominated for consideration by the federal advisory committee. Currently, three states initiated NBS for LSDs, three other states have passed legislation [14]. LSDs belong to a new category of disorders for which population-based screening assays exist, and new high-throughput screening assays and novel treatment strategies are on the horizon for many others. Challenges of the future may include the implementation of the LSDs in routine NBS, dealing with the identification of late-onset phenotypes, and optimal therapy schemes potentially including cost-intensive enzyme replacement therapies.

References
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The author
David Kasper, PhD
Department of Pediatrics and Adolescent Medicine, Medical University of Vienna, Währinger Gürtel 18–20, A-1090 Vienna, Austria
e-mail: david.kasper@meduniwien.ac.at