Over ten years of newborn screening for LSDs in Tuscany (Italy): Epidemiology, novel variants, and the pseudodeficiency burden

Background: Lysosomal storage disorders (LSDs) comprise a heterogeneous group of inherited metabolic diseases that lead to severe, irreversible complications if diagnosis is delayed. Newborn screening (NBS) provides a crucial opportunity for early identification and timely initiation of disease-modifying interventions. Methods: We analyzed 234,642 newborns screened over a 10-year period in Tuscany, Italy, for Fabry disease (FD), Pompe disease (PD), and mucopolysaccharidosis type I (MPS I), using evolving methodologies, including tandem mass spectrometry and digital microfluidics-based assays. We report the epidemiological data, molecular characterization, and the clinical impact of our screening program. Approximately 18% of screened newborns were of non-Italian origin, predominantly from Chinese, North African, and sub-Saharan African communities, providing unique insights into the genetic landscape of LSDs in a multiethnic population. Results: We identified 422 initial positive screens (recall rate 0.18%), of which 170 were confirmed on second dried blood spot (DBS). Molecular analysis revealed 17 late-onset PD individuals (incidence 1:13,802), 24 FD individuals, including five classical and 19 late-onset phenotypes (incidence 1:9776 total, 1:46,928 classical). No MPS I cases were confirmed, despite 45 positive initial screens. Nineteen novel variants were detected across the three genes: 4 in GAA, 5 in GLA, and 10 in IDUA. Evidence of regional founder effects emerged for the GLA variants c.335G > A and c.640-823 T > C. Conclusions: Our 10-year experience demonstrates the effectiveness of NBS for LSDs while highlighting important challenges, particularly the high burden of pseudodeficiencies and variants of uncertain significance (VUS). In MPS I screening, the substantial prevalence of pseudodeficiency alleles underscores the need for robust second-tier testing strategies to improve positive predictive value, especially in ethnically diverse populations.