Algorithm for Pompe disease newborn screening: Results from the Taiwan screening program

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Abstract

Background

Pompe disease is caused by a deficiency in acid α-glucosidase (GAA) and results in progressive, debilitating, and often life-threatening symptoms. Newborn screening has led to the early diagnosis of Pompe disease, but the best algorithm for screening has not yet been established.

Materials and methods

GAA and neutral α-glucosidase (NAG) activities in dried blood spots (DBSs) were assayed using 4-methylumbelliferyl-β‐d-glucopyranoside as the substrate. We also measure α-galactosidase A (GLA) activity in DBSs for comparison. A total of 473,738 newborns were screened for Pompe disease, and the data were analyzed retrospectively to determine the best screening algorithm.

Results

The fluorescence assay used in the screening possessed good reproducibility, but the NAG/GAA ratio was superior in separating the true-positive from the false-positive cases. An NAG/GAA cutoff ratio  60 produces a positive predictive value (PPV) of 63.4%, and in our sample, only two cases of later-onset Pompe disease would have been missed. The GLA/GAA ratio is not as effective as the NAG/GAA ratio.

Conclusion

A suitable control enzyme can improve the performance of newborn screening. Newborn screening for Pompe disease can be performed using the NAG/GAA ratio as a cutoff even in the presence of GAA partial deficiency.

Highlights

► We propose an algorithm for Pompe screening based on data from ~470 M newborns. ► Our efforts will help others to improve quality of Pompe disease newborn screening. ► The NAG/GAA ratio cannot be replaced by other ratios, such as the GLA/GAA ratio.

Introduction

Pompe disease (also known as glycogen storage disease type II, glycogenosis II, or acid maltase deficiency) is a lysosomal storage disorder in which a deficiency in acid α-glucosidase (GAA) causes the intralysosomal accumulation of glycogen in all tissues, most notably skeletal muscles [1]. Clinically, Pompe disease presents with a wide spectrum of phenotypes, ranging from the severe and rapidly progressive infantile-onset form characterized by the presence of hypertrophic cardiomyopathy to the heterogeneous and more slowly progressive later-onset form, which typically present with no cardiac manifestations [1]. Currently, enzyme replacement therapy (ERT) with recombinant human alglucosidase alfa (Myozyme®, Genzyme, Cambridge, MA) is the only treatment for Pompe disease [2]. Treatment with alglucosidase alfa prolongs survival and ventilator-free survival and reverses cardiomegaly in infantile-onset Pompe disease (IOPD) [3], [4], [5]. When an infant is diagnosed through newborn screening and treated very early in life, normal motor function can be maintained [6]. ERT for later-onset Pompe disease (LOPD) has been associated with improved motor capability and stabilized pulmonary function, but the best outcomes were in the youngest, non-ventilated patients [7], [8]. Therefore, early diagnosis and early treatment are also important for LOPD, but the mean delay from onset of symptoms until diagnosis averages 10 years [9]. Screening high-risk patients, such as those with limb-girdle muscular dystrophy, can identify undiagnosed LOPD [10], but this diagnosis may come too late for treatment.

We recently conducted a large-scale newborn screening program for the early detection and treatment of Pompe disease [6], [11]. In addition to patients with IOPD, newborns suspected of having LOPD were also identified. We present our follow-up of 13 such newborns over a 4-year period. Four of the newborns we followed have been treated with ERT due to hypotonia or creatine kinase (CK) elevation [12]. Although newborn screening benefits these ERT-treatment patients, the initial deliberate recall rate of 0.82% [11] was unacceptable in most newborn screening programs. It was later determined that those false‐positive cases were largely due to the prevalence of one pseudodeficiency allele [13]. In this study, we retrospectively analyzed data from 473,738 newborn screenings. We demonstrate that the fluorescence enzyme activity assay used in our screening program had good reproducibility and that only the neutral alpha-glucosidase activity (NAG) to GAA ratio could serve as an effective cutoff. The cutoff can be adjusted to reduce the false‐positive rate if a small false‐negative rate can be tolerated.

Section snippets

The study population

The Newborn Screening Center at the National Taiwan University Hospital initiated a program for Pompe disease in 2005. Between its initiation and Dec. 31, 2011, the program screened 473,738 newborns by dried blood spot(s) (DBS) collected at age of 48–72 h (the 1st DBS) for routine metabolic newborn screening. Both the screening cutoff and the critical cutoff were established according to the NAG/GAA ratio. Newborns with an NAG/GAA ratio exceeding the critical cutoff at the 1st screening had an

General performance of the screening program

The critical cutoff was set as NAG/GAA ratio  100 throughout the study period. The screening cutoff was initially set as NAG/GAA  25 in Oct. 2005, but was increased to NAG/GAA  30 after Oct. 2007 to decrease the false‐positive rate. The recall rate was 0.82% initially but decreased to 0.26% after Oct. 2007. A total of 2210 newborns had inconclusive results and were recalled for a 2nd DBS (an accumulated recall rate of 0.47%) while another thirty-one newborns exceeded the critical cutoff (NAG/GAA

Discussion

Newborn screening is the only way to initiate the early diagnosis and early treatment of Pompe disease. Several methods have been tested for this purpose, including the fluorescence substrate assay used in the current study, the tandem mass substrate assay [15], the digital microfluidics assay [16], and the immunologic approach [17]. Some of the methods have claimed excellent specificity and a high PPV. However, other than the fluorescence assay, all of these methods were tested in a small

Acknowledgments

This analysis was partially supported by the Bureau of Health Promotion, Department of Health, R.O.C. (DOH100-HP-1214) and the Taiwanese National Science Council (NSC 99-2628-B-002-007-MY3).

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  • Cited by (0)

    Financial disclosure: W-L Hwu has received research grant support from Genzyme Corporation. All other authors have no conflicts of interest to declare.

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