Molecular based newborn screening in Germany: Follow-up for cystinosis

Abstract

Background: Newborn screening (NBS) programs for treatable metabolic disorders have been enormously successful, but molecular-based screening has not been broadly implemented so far.

Methods: This prospective pilot study was performed within the German NBS framework. DNA, extracted from dried blood cards was collected as part of the regular NBS program. As cystinosis has a prevalence of only 1:100,000-1:200,000, a molecular genetic assay for detection of the SMN1 gene mutation with a higher prevalence was also included in the screening process, a genetic defect that leads to spinal muscular atrophy (SMA). First tier multiplex PCR was employed for both diseases. The cystinosis screening employed assays for the three most common CTNS mutations covering 75% of German patients; in case of heterozygosity for one of these mutations, samples were screened by next generation sequencing (NGS) of the CTNS exons for 101 CTNS mutations. A detection rate of 98.5% is predicted using this approach.

Results: Between January 15, 2018 and May 31, 2019, 257,734 newborns were screened in Germany for cystinosis. One neonate was diagnosed with cystinosis, consistent with the known incidence of the disease. No false positive or false negatives were detected so far. Screening, communication of findings to parents, and confirmation of diagnosis were accomplished in a multi-disciplinary setting. This program was accomplished with the cooperation of hospitals, physicians, and parents. In the neonate diagnosed with cystinosis, oral cysteamine treatment began on day 18. After 16 months of treatment the child has no clinical signs of renal tubular Fanconi syndrome.

Conclusions: This pilot study demonstrates the efficacy of a molecular-based neonatal screening program for cystinosis using an existing national screening framework.

Keywords: Cystinosis; Efficacy; Follow-up; MS-MS, tandem mass spectroscopy; Molecular-based newborn screening; NBS, newborns screening; NGS, next generation sequencing; QA/QC, quality assurance, quality control; SMA, spinal muscular atrophy; SMN, survival motor neuron protein.

© 2019 The Authors.

Figures

Fig. 1

Clinical characteristics of individuals with cystinosis. (Top) Cystinosis patients lack a functional cystine transporter in the lysosomal membrane, so cystine remains inside the lysosome. Cysteamine converts cystine to cysteine and cysteine-cysteamine mixed disulfide, which freely exits the cystinotic lysosome via the lysine transporter. (Bottom) The biallelic mutations of the CTNS gene lead to elevated cystine levels in nearly all cells, resulting in renal damage corneal cystine crystals, myopathy, and rickets due to hypophosphatemia. (Figures used with permission of [[21], [25]]).

Fig. 2

Flowchart for pilot molecular newborn screening for cystinosis and spinal muscular atrophy (SMA). Heelstick blood spots were obtained from newborns with parental consent and processed in one of two laboratories. Positive results were conveyed to the referring clinicians and to the parents, immediately for referral to a center with expertise in cystinosis or SMA.