Mucolipidosis type II and III: clinical spectrum, genetic landscape, and longitudinal outcomes in a pediatric cohort with six novel mutations

Abstract Objectives Mucolipidosis (ML) type II α/β (I-cell disease) and type III (Pseudo-Hurler polydystrophy) are rare autosomal recessive lysosomal storage disorders caused by mutations in the GNPTAB (ML III α/β) and GNPTG (ML III γ) genes, leading to impaired lysosomal enzyme trafficking. These disorders manifest as progressive multisystem diseases with skeletal, neurological, cardiovascular, and respiratory involvement. This study aims to characterise pediatric patients’ clinical, biochemical, genetic, and radiological findings with ML II α/β and IIIα/β or γ to enhance understanding of disease diagnosis and progression. Methods This retrospective cohort study included 19 pediatric patients (15 ML II α/β, 3 ML III γ, 1 ML III α/β) diagnosed at a tertiary referral center. Clinical data, including facial dysmorphism, skeletal abnormalities, organomegaly, cardiovascular involvement, and neurological findings, were recorded at baseline and follow-up. Biochemical analysis included urinary glycosaminoglycan (GAG) levels and lysosomal enzyme activities. For molecular diagnostics, next-generation sequencing (NGS) was utilized using a targeted gene panel that contained GNPTG and GNPTAB leukocytes from peripheral blood were used to harvest genomic DNA. Following the 2015 ACMG/AMP standards, variant interpretation was carried out by evaluating pathogenicity using in silico prediction methods, population frequency data, and segregation analysis. Radiological findings were evaluated via skeletal X-rays and brain MRI. Results The median age at diagnosis was 8 months (range: 1 day–252 months), with a consanguinity rate of 63.1 %. ML II α/β patients presented earlier and had more severe findings, including coarse facies, respiratory tract involvement, cardiac abnormalities, and joint contractures. Cardiac valve disease, kyphosis, and progressive hearing loss were frequent. The most common mutation in GNPTAB was c.3503_3504delTC in ML II α/β, while GNPTG variants were linked to ML III γ. Radiologic findings often included hip dysplasia; brain MRIs showed callosal and subarachnoid abnormalities in a few cases. Median survival in ML II α/β was 28 months, mainly due to respiratory failure. Enzyme assays revealed elevated β-hexosaminidase A and A+B, α-D-mannosidase, and α-L-fucosidase activities in both leukocyte and DBS samples, consistent with molecular diagnoses. Conclusions In addition to reporting six novel GNPTAB/GNPTG mutations, this study highlights the diagnostic potential of dried blood spot (DBS) samples, where elevated lysosomal enzyme activity may offer a valuable, minimally invasive screening method for ML II/III.