Analysis of long-term observations of the large group of Russian patients with Hunter syndrome (mucopolysaccharidosis type II)

Analysis of fatal outcomes of patients with mucopolysaccharidosis type II according to the Russian mucopolysaccharidosis registry

BACKGROUND

Mucopolysaccharidosis type II (MPS II) is a chronic inherited disease with multiorgan involvement, a progressive course, and restricted life expectancy.

AIM

To evaluate the predictors of fatal outcomes in MPS II patients.

METHODS

In the retrospective cohort study, the clinical, laboratory data and enzyme replacement therapy (ERT) (84.2%) of about 160 patients were extracted and analyzed from the Russian MPS II registry, with death as a primary outcome. We compared patients who died (n = 20; 12.5%) with severe form (n = 13; 68.4%) and attenuated form (n = 6, 31.6%) to 140 alive patients.

RESULTS

Fatal outcomes occurred in 5%, 35%, 20%, and 40% of patients before 10, 10-14, 15-19, and ≥ 20 years. The most common causes of death were cardiovascular (29.4%), respiratory failure (17.6%), including pneumonia (17.6%), and their associations (17.6%) and MPS II progression (11.8%). Acute or chronic respiratory failure was in 53%. Died patients had higher birth weight, higher age of diagnosis, and start of ERT. Hydrocephalus, hydrocephalus bypass surgery, epilepsy, difficulty swallowing, and impaired movement after 12 years of age were significantly more common in the deceased patients. Cox regression analysis has revealed the following time-dependent covariates of the lethal outcome: 1^st-year psychomotor development delay, delayed mental and speech development, hydrocephalus, swallow disorders, impossible walking at age > 12 years, respiratory disorders, tracheostomy, neuronopathic form.

CONCLUSION

Increased birth weight, delayed diagnosis and the start of ERT, and development of neuronopathic form with impossible walking after 12 years were the main predictors of the fatal outcome.

The diagnosis and management of mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II) is a rare X-linked recessive inherited lysosomal storage disease. With pathogenic variants of the IDS gene, the activity of iduronate-2-sulfatase (IDS) is reduced or lost, causing the inability to degrade glycosaminoglycans (GAGs) in cells and influencing cell function, eventually resulting in multisystemic manifestations, such as a coarse face, dysostosis multiplex, recurrent respiratory tract infections, and hernias. Diagnosing MPS II requires a combination of clinical manifestations, imaging examinations, urinary GAGs screening, enzyme activity, and genetic testing. Currently, symptomatic treatment is the main therapeutic approach. Owing to economic and drug availability issues, only a minority of patients opt for enzyme replacement therapy or hematopoietic stem cell transplantation. The limited awareness of the disease, the lack of widespread detection technology, and uneven economic development contribute to the high rates of misdiagnosis and missed diagnosis in China.

The Spectrum of Disease-Associated Alleles in Countries with a Predominantly Slavic Population

There are more than 260 million people of Slavic descent worldwide, who reside mainly in Eastern Europe but also represent a noticeable share of the population in the USA and Canada. Slavic populations, particularly Eastern Slavs and some Western Slavs, demonstrate a surprisingly high degree of genetic homogeneity, and, consequently, remarkable contribution of recurrent alleles associated with hereditary diseases. Along with pan-European pathogenic variants with clearly elevated occurrence in Slavic people (e.g., ATP7B c.3207C>A and PAH c.1222C>T), there are at least 52 pan-Slavic germ-line mutations (e.g., NBN c.657_661del and BRCA1 c.5266dupC) as well as several disease-predisposing alleles characteristic of the particular Slavic communities (e.g., Polish SDHD c.33C>A and Russian ARSB c.1562G>A variants). From a clinical standpoint, Slavs have some features of a huge founder population, thus providing a unique opportunity for efficient genetic studies.

Molecular analysis and novel variation identification of Chinese pedigrees with mucopolysaccharidosis using targeted next-generation sequencing

BACKGROUND

Mucopolysaccharidosis (MPS) refers to a group of lysosomal storage disorders for which seven types and 11 subtypes are currently recognized. Targeted next-generation sequencing (NGS) offers an important method of disease typing, diagnosis, prenatal diagnosis, and treatment.

METHODS

Gene variations in 48 Chinese MPS patients were evaluated using NGS, and the pathogenicity of the DNA alterations was evaluated using PolyPhen2, SIFT, and Mutation Taster. The effect of amino acid substitution on protein structure was also assessed.

RESULTS

Four pedigrees with MPS I (8.3%), 28 with MPS II (58.3%), two with MPS IIIA (4.2%), two with MPS IIIB (4.2%), six with MPS IVA (12.5%), one with MPS IVB (2.1%), and five with MPS VI (10.4%) were identified. Of the 69 variations identified, 11 were novel variants (three in IDUA, five in IDS, and three in GALNS), all of which were predicted to be disease-causing except for one, and were associated with impaired protein structure and function.

CONCLUSIONS

Targeted NGS technology is effective for the gene-based testing of MPS disorders, which show high allelic heterogeneity. MPS II was the predominant form in Chinese. Our study expands the existing variation spectrum of MPS, which is important for disease management and genetic counseling.

Differences in MPS I and MPS II Disease Manifestations

Mucopolysaccharidosis (MPS) type I and II are two closely related lysosomal storage diseases associated with disrupted glycosaminoglycan catabolism. In MPS II, the first step of degradation of heparan sulfate (HS) and dermatan sulfate (DS) is blocked by a deficiency in the lysosomal enzyme iduronate 2-sulfatase (IDS), while, in MPS I, blockage of the second step is caused by a deficiency in iduronidase (IDUA). The subsequent accumulation of HS and DS causes lysosomal hypertrophy and an increase in the number of lysosomes in cells, and impacts cellular functions, like cell adhesion, endocytosis, intracellular trafficking of different molecules, intracellular ionic balance, and inflammation. Characteristic phenotypical manifestations of both MPS I and II include skeletal disease, reflected in short stature, inguinal and umbilical hernias, hydrocephalus, hearing loss, coarse facial features, protruded abdomen with hepatosplenomegaly, and neurological involvement with varying functional concerns. However, a few manifestations are disease-specific, including corneal clouding in MPS I, epidermal manifestations in MPS II, and differences in the severity and nature of behavioral concerns. These phenotypic differences appear to be related to different ratios between DS and HS, and their sulfation levels. MPS I is characterized by higher DS/HS levels and lower sulfation levels, while HS levels dominate over DS levels in MPS II and sulfation levels are higher. The high presence of DS in the cornea and its involvement in the arrangement of collagen fibrils potentially causes corneal clouding to be prevalent in MPS I, but not in MPS II. The differences in neurological involvement may be due to the increased HS levels in MPS II, because of the involvement of HS in neuronal development. Current treatment options for patients with MPS II are often restricted to enzyme replacement therapy (ERT). While ERT has beneficial effects on respiratory and cardiopulmonary function and extends the lifespan of the patients, it does not significantly affect CNS manifestations, probably because the enzyme cannot pass the blood-brain barrier at sufficient levels. Many experimental therapies, therefore, aim at delivery of IDS to the CNS in an attempt to prevent neurocognitive decline in the patients.