ABSTRACT
Mucopolysaccharidosis type IIIB (MPS IIIB), also called Sanfilippo syndrome B, is a lysosomal storage disease caused by abnormal degradation of heparan sulfate. It is characterized by progressive neurological deterioration with developmental regression and behavioral abnormalities. Additional clinical manifestations can include musculoskeletal anomalies, hearing loss, respiratory tract anomalies, and cardiovascular disease. Here, we report a second individual with MPS IIIB and chronic pancytopenia. To support our hypothesis of a pathophysiological relationship between these clinical findings, we performed hematological studies in MPS IIIB Naglu−/− mice, which revealed a microcytic anemia as well as a decreased monocyte count, without thrombocytopenia. Hematological findings are thought to be secondary to MPS IIIB even though the exact pathophysiological mechanism remains to be determined. Although it likely represents an uncommon clinical feature, we suggest that complete blood count should be considered as part of the clinical surveillance for individuals with MPS IIIB.
Keywords: lysosomal storage diseases, mucopolysaccharidosis type IIIB, pancytopenia, Sanfilippo B
We report an individual with mucopolysaccharidosis type IIIB and chronic pancytopenia. Hematological studies in a mouse model revealed microcytic anemia and decreased monocyte count. Hence, pancytopenia is thought to be secondary to mucopolysaccharidosis type IIIB, and we suggest that a complete blood count should be included in the clinical surveillance.
1. Introduction
Lysosomal storage diseases (LSD) are inborn errors of metabolism caused by lysosomal enzyme defects and often characterized by the accumulation of substrates. Over 50 different LSD have been described and their combined prevalence is estimated to be 1 in 5000 live births [1]. These conditions can affect virtually any organ or system and can present with various signs and symptoms. Pancytopenia has been described in many LSD, such as Gaucher disease (OMIM: 230800), Niemann‐Pick disease (OMIM: 257220), or Wolman disease (OMIM: 620151), mostly in the context of hypersplenism [2].
Defects in lysosomal enzymes responsible for the degradation of glycosaminoglycans (GAGs) cause mucopolysaccharidosis (MPS), a group of LSD. Mucopolysaccharidosis type IIIB (MPS IIIB), also called Sanfilippo syndrome B (OMIM: 252920), is caused by abnormal degradation of heparan sulfate and is typically characterized by progressive neurological deterioration with developmental regression and behavioral abnormalities. Multisystemic involvement is usually less marked than in other MPS, but clinical manifestations can include musculoskeletal anomalies, hearing loss, respiratory tract anomalies, and cardiovascular disease [3]. Hematological anomalies, particularly morphological ones (e.g., vacuolated lymphocytes), can be encountered in LSD and may aid in diagnostic orientation. However, pancytopenia has been reported in only a single case of MPS IIIB [4]. We describe here an additional individual with MPS IIIB and chronic pancytopenia; therefore, reinforcing this association. We subsequently performed hematological studies in a murine model and found a microcytic anemia as well as a decreased monocyte count, without thrombocytopenia.
2. Material and Method
2.1. Human Subject
This study was approved by the institutional review board of the CHU Sainte‐Justine. Written informed consent for study participation was obtained from the legal guardian. The medical chart was reviewed to collect clinical data.
2.2. Animals
In this study, we used wild‐type C57BL/6 mice (WT) and mice with the B6.129S6‐Naglutm1Efn/J mutation in the Naglu gene (MPS IIIB Naglu−/− ). MPS IIIB Naglu−/− mice were originally generated by targeted disruption of exon 6 of the Naglu gene, resulting in a null allele and complete loss of N‐alpha‐acetylglucosaminidase enzymatic activity and progressive lysosomal storage of heparan sulfate [5]. The model reproduces key features of the human Sanfilippo syndrome type B, including neuroinflammation, progressive neurodegeneration, and reduced lifespan. Homozygous MPS IIIB Naglu−/− and WT littermate mice were obtained from heterozygous crosses of B6.129S6‐Naglutm1Efn/J breeders originally purchased from The Jackson Laboratory (strain #003443). Genotyping was performed following the protocol provided by the supplier. The animals were bred and housed in the Canadian Council on Animal Care (CCAC) accredited facilities at the CHU Sainte‐Justine Research Center.
2.3. Blood Collection
Blood collection was performed at the terminal stage under deep anesthesia via cardiac puncture. Blood samples were directly collected into EDTA tubes and analyzed on the same day, immediately following collection. Hematological analyses were conducted at the Lady Davis Institute for Medical Research, Jewish General Hospital Sir Mortimer B. Davis (Montreal, QC, Canada), using the Vet ABC Plus hematology analyzer (SCIL/HESKA, France), which provides a three‐part differential count (lymphocytes, monocytes, and granulocytes). For each sample, 15 μL of whole blood was used according to the manufacturer's instructions. The analyzer was calibrated according to the manufacturer's specifications, and quality controls were performed prior to sample analysis.
2.4. Statistical Analysis
Data are presented as means with standard error of the mean (SEM). Statistical significance between the control (WT) and MPS IIIB Naglu−/− groups was assessed using the Student's t‐test.
3. Results
3.1. Case Report
The individual is a 16‐year‐old male. He was born at 40 weeks of gestation by caesarian section due to a large birth weight following an uncomplicated pregnancy. He had a surgery for right cryptorchidism, hydrocele, and inguinal hernia at 14 months. At 3 years old, he was diagnosed with bilateral mixed hearing loss. He then had a bilateral myringotomy with tympanoplasty, and subsequent audiology testing revealed moderate to profound bilateral sensorineural hearing loss. Adenoidectomy was performed at the same time.
He was referred to medical genetics at 7 years of age for severe global developmental delay and behavioral issues. Family history was unremarkable and there was no consanguinity. Numerous genetic and biochemical investigations were performed. Heparan sulfate was found to be elevated in urine (78.2 mg/mmol creatinine; 95th percentile for his age group was 1.5 mg/mmol creatinine), which was suggestive of MPS III. Lysosomal enzymatic activity in leukocytes was initially normal but revealed a decreased activity of N‐acetyl‐alpha‐D‐glucosaminidase when repeated. The first result was thought to be a false negative related to technical issues with the sample. A diagnosis of MPS IIIB was finally confirmed with molecular analyses showing two variants in NAGLU (OMIM: 609701): c.2135del (p.[Lys712Serfs*95]) and a 7.31 Kb deletion on 17q21.2 chromosome encompassing exons 1–5 of NAGLU.
He subsequently developed severe intellectual disability and lost his ability to speak and to walk independently. He had a first seizure at 14 years of age and was started on anticonvulsant medication. Obstructive sleep apnea was suspected based on his symptoms and on the result of an overnight pulse oximetry, but a polysomnogram could not be performed to confirm it and he was not a candidate for a continuous positive airway pressure (CPAP) due to his lack of collaboration. From the age of 16 years he was fed exclusively through a gastrostomy.
On physical examination at 16 years old, he had a head circumference of 62.5 cm (+ 3.26 SD). He had coarse facial features, hypertrichosis, and gingival hypertrophy. Abdominal examination was limited due to lack of cooperation, but mild hepatosplenomegaly was noted on previous abdominal imaging. Aortic valve disease was found on echocardiogram.
At the age of 15 years, routine blood tests revealed a pancytopenia that remained relatively stable for the next 20 months. He never needed blood product transfusion. His most recent complete blood count at 16 years old revealed a moderate pancytopenia with a leukocyte count of 3.65 × 109/L (reference range: 4.50–13.00 × 109/L), a neutrophil count of 1.1 × 109/L (reference range: 1.8–7.0 × 109/L), a hemoglobin count of 115 g/L (reference range: 130–160 g/L), and a platelet count of 81 × 109/L (reference range: 140–440 × 109/L) (Figure 1). Blood smear showed anisocytosis, poikilocytosis, atypical lymphocytes, platelet clumping, large platelets, and vacuolated neutrophils. Absolute reticulocyte count was within normal limits. He was seen in hematology, and his pancytopenia was considered to be part of his MPS IIIB disease. Given the clinical context, it was decided not to perform additional investigations such as bone marrow biopsy.
FIGURE 1.
Complete blood count trend over time. A. Hemoglobin (g/L). B. Leucocytes count (109/L). C. Platelet count (109/L). The dotted red line on each graph represents the lower limit of the reference range.
3.2. Murine Model
To further characterize this observation, blood assessments were performed on seven MPS IIIB Naglu−/− mice (3 females, 4 males) and five WT mice (3 females, 2 males) aged 7–8 months. The chosen age range for the mice was selected to approximate the clinical stage of the patient. Evaluation of red blood cell parameters revealed a significant decrease in hemoglobin concentration, mean corpuscular volume, red blood cell count, and hematocrit, indicative of a microcytic anemia (Table 1). A reduction in monocyte count was observed in the white blood cell analysis. However, no thrombocytopenia was detected.
TABLE 1.
Automated full blood analysis on WT (n = 5) and MPS IIIB Naglu−/− (n = 7) mice.
| Parameters | WT | MPS IIIB Naglu−/− | p‐value |
|---|---|---|---|
| Hemoglobin (g/dL) | 14,95 ± 0,45 | 13,21 ± 0,23 | 0,0039 |
| MCV (μm3) | 47,2 ± 0,49 | 45,5 ± 0,33 | 0,0119 |
| MCH (pg) | 15,46 ± 0,25 | 15,20 ± 0,12 | 0,3247 |
| MCHC (g/dL) | 32,83 ± 0,35 | 33,39 ± 0,17 | 0,1761 |
| RBC (106/mm3) | 9,67 ± 0,24 | 8,67 ± 0,18 | 0,0064 |
| HCT (%) | 45,51 ± 1,1 | 39,69 ± 0.89 | 0,0021 |
| Platelets (103/mm3) | 874,8 ± 104,5 | 1018 ± 68,25 | 0,2571 |
| White Blood Cells (103/mm3) | 1,71 ± 0,277 | 1493 ± 0,118 | 0,4589 |
| Neutrophils (103/mm3) | 0,65 ± 0,20 | 0,617 ± 0,07 | 0,8595 |
| Lymphocytes (103/mm3) | 1 ± 0,12 | 0,783 ± 0,079 | 0,1564 |
| Monocytes (103/mm3) | 0,15 ± 0,02887 | 0,083 ± 0,01667 | 0,0630 |
Note: Values are mean ± SEM. Significant p‐value (≤ 0,05) are in bold.
Abbreviations: MCV, mean corpuscular volume; MCHC, mean corpuscular hemoglobin concentration; MCH, mean corpuscular hemoglobin; RBC, red blood cell; HCT, hematocrit.
4. Discussion
MPS IIIB is a rare neurodegenerative disorder. The highest estimated incidence is 1/100000 live births [6]. Individuals may have normal early development before developmental delay becomes evident in childhood. Regression follows developmental plateau and may start as early as three to four years old. Motor and cognitive skills are progressively lost. Although clinical severity varies among individuals, most of them will die during the second or third decade of life from neurological or respiratory complications. Behavioral issues are often a prominent part of the clinical course, with hyperactivity, aggressivity, and sleep disturbances. In addition, individuals with MPS IIIB can present with multisystemic involvement as in other MPS. Coarse facial features are frequently observed and are more marked with time. Hepatosplenomegaly, when present, does not lead to hepatic or splenic dysfunction. Musculoskeletal involvement, including mild dysostosis multiplex and carpal tunnel syndrome, is frequent although height is usually normal or near normal. Some individuals will develop epilepsy. Other clinical manifestations include hearing loss, recurrent otitis media, and respiratory tract infections, hernia, and cardiac valve disease. The pathophysiology of MPS IIIB is complex. Heparan sulfate accumulation within lysosomes leads to lysosomal dysfunction, but it also triggers secondary cellular dysregulations such as autophagy block, neuroimmune response, neuronal dysfunction, and neurodegeneration as reviewed in references [7, 8] There is currently no approved therapy for MPS IIIB, but many therapeutic approaches are being studied, including small molecules, enzyme replacement therapy, and gene therapy [3, 9, 10, 11, 12].
The individual reported here had the typical clinical evolution of individuals with MPS IIIB, with developmental delay followed by progressive loss of skills. He also presented most of the characteristic clinical features. Variants identified in NAGLU were not previously reported in the literature. The p.(Lys712Serfs*95) is a null allele and is absent from gnomAD. It is therefore predicted to be pathogenic, as it was found to be in trans with a pathogenic deletion encompassing exons 1–5 of NAGLU. Biochemical testing also supported the molecular diagnosis.
Pancytopenia has been frequently reported in some LSD, such as Gaucher disease, Niemann‐Pick disease, and Wolman disease. Underlying pathophysiological mechanisms include hypersplenism in the context of splenomegaly and bone marrow infiltration with abnormal cells (e.g., Gaucher cells in Gaucher disease) [2, 13]. However, pancytopenia is not a typical feature of MPS. Routine complete blood count is not part of the recommended evaluations of individuals with MPS IIIB [9] and has been described only once in MPS IIIB. Merdin et al. reported an 18‐year‐old female with MPS IIIB and mild pancytopenia. She had hepatosplenomegaly on physical exam. A bone marrow biopsy was performed, revealing hypocellular bone marrow with a marked reduction in the granulocytic lineage, although maturation was intact. In the megakaryocytic lineage, signs of dysmegakaryopoiesis were observed. The pancytopenia remained stable for years [4]. This case report suggests that the origin of the pancytopenia is central, with a global defect in hematopoiesis. In the individual that we presented here, no bone marrow biopsy was performed given the severity of his neurological disease. It therefore remains unknown if bone marrow abnormality, such as GAG infiltration with reduced production, could have contributed to the phenotype.
In most LSD, splenomegaly contributes to pancytopenia (e.g., in Gaucher disease). However, in this case, splenomegaly is unlikely to be the primary cause, as it does not appear to be severe enough to fully explain the pancytopenia. In the MPS I mouse model, heparan sulfate accumulation has been identified in the bone marrow, affecting the migration of hematopoietic stem and progenitor cells [14]. Moreover, our unpublished data suggest that the level of heparan sulfate in the bone marrow of MPS IIIC mice is also increased, implying a possible causal role for this accumulation.
To support the hypothesis that pancytopenia is secondary to the disease, we investigated the well‐established murine model available at our center. This revealed a mild microcytic anemia without evidence of pancytopenia. Although reticulocyte counts were not performed, these observations suggest a central etiology, likely inflammation. In other naturally occurring animal models of MPS IIIB, such as the emu [15, 16], Schipperke dogs [17], and bulls [18], pancytopenia has not been reported. However, a mild regenerative anemia, accompanied by leukocytosis, was observed in one emu, which was attributed to an inflammatory response [16]. Notably, bone marrow evaluations were not explicitly documented in these models.
In conclusion, we presented here an individual with MPS IIIB and pancytopenia. This association was previously reported only once in the literature. Pancytopenia is thought to be secondary to MPS IIIB even though the exact pathophysiological mechanism remains to be determined. Although it likely represents an uncommon clinical feature, we suggest that a hematological workup, with a complete blood count, should be considered as part of the clinical surveillance for individuals with MPS IIIB.
Author Contributions
E.B.L. conceived and designed the analysis, collected the data, performed the analysis, and wrote the manuscript. P.D. conceived and designed the analysis, collected the data, performed the analysis, and wrote the manuscript. A.V.P. conceived and designed the analysis, and reviewed the manuscript. P.M.C. conceived and designed the analysis, and reviewed the manuscript.
Ethics Statement
This project was approved by the institutional review board of CHU Sainte‐Justine.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgements
We would like to thank the family for participating in this study and all clinicians involved in the care of this child.
Beauregard‐Lacroix É., Dubot P., Pshezhetsky A. V., and Campeau P. M., “Mucopolysaccharidosis Type IIIB With Pancytopenia: A Case Report and Hematological Correlations in Mice,” Clinical Genetics 108, no. 5 (2025): 604–608, 10.1111/cge.14773.
Funding: This work has been partially supported by the operating grant PJT‐180546 from the Canadian Institutes of Health Research to A.V.P., Elisa Linton Research Chair in Lysosomal Diseases and gifts from Cure Sanfilippo Foundation and Sanfilippo Children's Foundation (Australia) to A.V.P. P.D. was supported by post‐doctoral scholarships from the CHU Ste‐Justine Foundation and the Vaincre les Maladies Lysosomales Foundation. P.M.C. is supported by a Senior Clinical Research Scholar award from the Fonds de Recherche du Quebec ‐ Santé (award 313617, https://doi.org/10.69777/313617).
Éliane Beauregard‐Lacroix and Patricia Dubot are contributted equally.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.