Mucopolysaccharidosis (MPS) type III, or Sanfilippo syndrome, is an autosomal recessive lysosomal storage disorder caused by mutations in genes encoding enzymes responsible for glycosaminoglycan (GAG) degradation. This case report describes two siblings with MPS type IIIB who exhibit a rare compound heterozygous mutation in the NAGLU gene. A 7-year-old girl and her 4-year-old brother were referred for evaluation due to learning disabilities, aggressiveness, and coarse facial features. Enzyme assay using tandem mass spectrometry on dried blood spots in both siblings revealed absent N-acetyl-α-glucosaminidase activity. Targeted sequencing confirmed the diagnosis, identifying two heterozygous mutations-an in-frame insertion and a missense mutation-in exon 3 of the NAGLU gene: c.214_237dup (p.Ala72_Gly79dup) and c.625A>C (p.Thr209Pro). This rare genetic finding in two siblings with Sanfilippo syndrome type B underscores the importance of precise mutation identification. Accurate characterization of defective gene variants may provide insights into potential targets for gene therapy in monogenic disorders.

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Background/Objectives: Sanfilippo Syndrome type B or Mucopolysaccharidosis type IIIB (MPS IIIB, OMIM 252920) is a lysosomal storage disease caused by deficiency of alpha-N-acetylglucosaminidase (NAGLU, E.C. 3.2.1.50) due to pathogenic variants in the NAGLU gene (17q21.2). The disease is characterized by progressive neurological manifestations, marked by cognitive decline, with relatively mild somatic involvement. We aim to present relevant information on a cluster of MPS IIIB identified in Ecuador, particularly regarding their clinical, biochemical, genetic, demographic, and ancestry characteristics. Methods: We present a characterization of a clinical, biochemical, genetic and demographic cluster of MPS IIIB patients in Ecuador, located in four main regions: Manabí, Guayas, Los Ríos, and Santo Domingo de los Tsáchilas. The patients included were diagnosed due to increased levels of urinary glycosaminoglycans (uGAG), plus deficient activity of NAGLU, and/or identification of biallelic pathogenic mutations in the NAGLU gene. Patients' charts were reviewed for biochemical findings, medical history, clinical manifestations and assessments. Results: We present the results of clinical, biochemical, genetic and demographic characterization of a cluster in Ecuador with 24 patients identified with Sanfilippo syndrome type IIIB, resulting in an estimated incidence of 1.5/100,000. The mean age at diagnosis was 8.8 years, with symptom onset at 4.5 years on average. All patients exhibited elevated levels of uGAG and undetectable NAGLU activity, and all of them presented the c.1487T>C (p.Leu496Pro) variant in the NAGLU gene in homozygosis, indicating a possible founder effect, with the exception of one heterozygous one (p.Leu496Pro/p.Arg482Gln). A positive correlation between age of diagnosis and the concentration of one isoform of heparan sulfate (HS-OS) was found (p < 0.05). Clinical findings included neuropsychomotor developmental delay (75%), neurological regression (65%), hepatomegaly (55%), growth deficiency (50%), coarse facies (45%) and hernia (40%). Male patients presented earlier onset of symptoms. Maternal ancestry was successfully determined for 21 of the 24 patients. The majority were of Native American ancestry (71.4%), followed by European (19%), African (4.8%), and Asian (4.8%) lineages. Haplogroup A was the most prevalent (42.9%), followed by haplogroups D (19%), C, U, and H (each 9.5%), and R and L2 (each 4.8%). Conclusions: Ancestry can indicate a possible mechanism to explain the heterogeneous symptomatic presentation. These findings highlight the need for further research on genetic and environmental influences on disease severity in this population.

Sanfilippo syndrome type B results from NAGLU mutations which cause progressive cognitive impairments and central nervous system degeneration. A 10-year-old boy presented with developmental regression, brain atrophy, intellectual disability, attention-deficit/hyperactivity disorder, and restlessness. His parents were non-consanguineous and asymptomatic. Whole-exome sequencing (WES) was performed, and variants were confirmed by Sanger sequencing. Downstream analyses integrated protein-protein interaction (PPI), gene-microRNA interaction (GMI), and drug-disease association (DDA) networks using STRING, NetworkAnalyst, and Enrichr. Two missense variants were identified including rs1358994052 (NAGLU:c.874G > A; p.Gly292Arg) and rs768918822 (NAGLU:c.1004 A > G; p.Tyr335Cys [Y335C]), classified as pathogenic and likely pathogenic, respectively, by ACMG guidelines. Both variants localize to regulatory elements. The compound heterozygote network exhibited increased PPI connectivity and the absence of hsa-miR-27a-3p in GMI analysis. DDA highlighted carcinogenesis as the top-ranked term in the compound heterozygote network, contrasting with leukemia associations in homozygous contexts. Compound heterozygous regulatory variants in NAGLU underlie diverse biochemical and neurodevelopmental phenotypes beyond enzymatic deficiency, emphasizing the value of integrative WES and systems biology approaches to refine pathogenicity assessments and guide targeted functional validation.

Sanfilippo syndrome Type-B, also known as mucopolysaccharidosis IIIB (MPS IIIB), accounts for approximately one-third of all Sanfilippo syndrome patients and is characterized by a similar natural history as Type-A. Patients suffer from developmental regression, bone malformation, organomegaly, GI distress, and profound neurological deficits. Despite human trials of enzyme replacement therapy (ERT) (SBC-103, AX250) in MPS IIIB, there is currently no FDA approved treatment and a few palliative options. The major concerns of ERT and gene therapy for the treatment of bone malformation are the inadequate biodistribution of the missing enzyme, N-acetyl-α-glucosaminidase (NAGLU), and that the skeleton is a poorly hit target tissue in ERT and gene therapy. Each of the four known human types of MPS III (A, B, C, and D) is usually regarded as having mild bone manifestations, yet it remains poorly characterized. This study aimed to determine bone mineral content (BMC), volumetric bone mineral density (vBMD), and biomechanical properties in femurs MPS IIIB C57BL/6 mice compared to phenotypic control C57BL/6 mice. Significant differences were observed in MPS IIIB mice within various cortical and cancellous bone parameters for both males and females (p < 0.05). Here, we establish some osteogenic manifestations of MPS IIIB within the mouse model by radiographic and biomechanical tests, which are also differentially affected by age and sex. This suggests that some skeletal features of the MPS IIIB mouse model may be used as biomarkers of peripheral disease correction for preclinical treatment of MPS IIIB.