Mucopolysaccharidoses (MPSs) are a group of lysosomal storage diseases, each of which is produced by an inherited deficiency of an enzyme involved in the degradation of acid mucopolysaccharides, called glycosaminoglycans (GAGs). The mucopolysaccharidoses share many clinical features but have varying degrees of severity. Depending on the mucopolysaccharidoses subtype, affected individuals may have normal intellect or may be profoundly retarded, may experience developmental delay, or may have severe behavioral problems. Many patients have hearing loss, conductive and/or neurosensitive, communicating hydrocephalus, cloudy cornea and degeneration of the retina and glaucoma, and many other symptoms.
Mucopolysaccharidosis type II (also known as Hunter syndrome) is an X-linked multisystem disorder characterized by glycosaminoglycans (GAG) accumulation and caused by lack of the enzyme iduronate sulfatase, encoded by the IDS gene 1. Although it was once divided into two groups based on the severity of symptoms, MPS II is now considered to be a continuous spectrum of disease. MPS II is the only X-linked mucopolysaccharidosis. The estimated incidence of MPS II is between 1:100,000 and 1:170,000 of all male births worldwide 2, 3.
MPS II has multisystem involvement, with significant variability in both age of onset and rate of progression 1. CNS involvement is the most significant feature of MPS II and it manifests primarily by progressive cognitive deterioration. In individuals with the slowly progressive form of the disease, the CNS is minimally affected, and the effect of glycosaminoglycans (GAG) accumulation on other organ systems may be more progressive.
The major clinical features of MPS II include the following 1:
- Age of onset between 2 and 4 years of life
- Early developmental decline (ages 18 to 36 months)
- Coarse facial features
- Short stature and skeletal irregularities
- Obstructive airway and respiratory complications
- Joint stiffness
- Retinal degeneration without corneal clouding
IDS is the only gene in which variants are known to cause MPS II. More than 500 variants have been reported in the IDS gene so far. Sequence analysis of the entire IDS coding region detects 82% of variants in both males and females 4. Single nucleotide changes and splicing variants account for 65% of all variants, small deletions and insertions account for 17%, large (exonic or whole-gene) deletions account for 9% of all variants. Due to the presence of the pseudogene located 90kb telomeric from IDS, Southern blot analysis is used to detect complex rearrangements (accounting for 9% of all variants) resulting from recombination with the pseudogene 1, 4.
Boys with complete absence of functional enzyme as a result of gene deletion or complex gene rearrangements (~17% of affected individuals) invariably manifest the severe CNS presentation of the disease 1, 4. Several missense variants have been associated with the severe phenotype (p.Arg468Gln, p.Arg468Trp, and p.Ser333Leu). The majority of the separate point variants in the IDS gene causing MPS II are found in CpG sites as transitional events 1, 3, 4.
Variants at codon 468 (R468) of the IDS gene have been found in MPS II patients of various ethnic origins. In a patient with a severe MPS II phenotype, the most common was G-to-T transversion at nucleotide 1403, resulting in an arg468-to-leu (R468L) substitution 4, 5. In the series of Japanese patients, in 29% of patients missense variants R468 was identified and the CpG dinucleotide at this site was methylated, suggesting that this R468 codon is a variant hotspot.
At CENTOGENE we have analyzed more than 2700 individuals for the IDS gene. 19% of mucopolysaccharidosis type II-suspected individuals had pathogenic variant in IDS gene, while 9% were identified as carriers 8.
Out of all IDS identified pathogenic variants 72% were identified as substitution, 15% as deletions, 7% as gross/complex rearrangements and 3% as other type of variants, including 1% of conversations (Figure 1) 8. IDS classification of variants on protein level identified 47% missense variants, 20% frameshift variants, 13% nonsense, 12% splicing, 6% in-frame variants and 3% of variants with unknown effect (Figure 2) (CentoMD® 4.1) 8.
Figure 1. Types of IDS clinically relevant variants on DNA level (CentoMD® 4.1) 8.
Figure 2. Types of IDS clinically relevant variants on protein level (CentoMD® 4.1) 8.
Treatment and management of MPS, including MPS type II, includes special education for developmental delays, correction of hearing and vision, physical therapy, orthopedic surgery as needed, cerebrospinal fluid shunting for hydrocephalus and many others. Enzyme replacement therapy (ERT) with different enzymes are in the process of development, and many drugs are already licensed for treatment of the non-CNS manifestations of MPS I, MPS II, MPS IV and other forms of MPS.
CENTOGENE offers IDS enzyme activity testing, IDS gene sequencing and deletion/duplication analysis. The IDS gene is also part of the following NGS panels:
- AllNeuro panel
- Mucopolysaccharidosis panel
- Mental retardation, X-linked panel
The differential diagnosis of IDS-related disorders – depending on the major symptoms in the initial case – includes the following diseases:
- Lysosomal storage diseases
- Juvenile idiopathic arthritis
- Multiple sulfatase deficiency and mucolipidosis types II and III
- Spondyloepiphyseal dysplasia
- Legg-Calve-Perthes disease.
To confirm/establish the diagnosis, we offer fluorimetry-based Iduronate 2-sulfatase (IDS enzyme) activity testing, IDS gene sequencing and deletion/duplication gene testing. We also offer a broad selection of NGS panels which are designed for the molecular diagnostics of related conditions/phenotypes.
Thus, CENTOGENE offers the following testing strategy for IDS gene testing:
Step 1: Iduronate 2-sulfatase (enzyme) fluorimetry-based activity testing.
Step 2: IDS sequencing – covers the entire coding region, exon/intron boundaries and 200 bp of the gene promoter
Step 3: Deletion/duplication analysis/variant scanning of IDS
Step 4: If no variant is identified after analysis of the IDS gene, we offer an additional testing strategy: Whole genome sequencing from a single filter card that will cover the entire genic region (coding region, exon/intron boundaries, intronic and promoter) for all the genes included in the IDS gene-related Panels (e.g. Mucopolysaccharidosis panel). Copy Number Variants analysis derived from NGS data is also included.
Step 5: If no variant is identified after analysis of the Mucopolysaccharidosis panel, we further recommend continuing the bioinformatics analysis of the data using whole genome sequencing to cover those genes which are either implicated in an overlapping phenotype or could be involved in a similar pathway but are not strongly clinically implicated based on the current information in literature.
The following individuals are candidates for this IDS gene testing:
• Individuals with a family history of Mucopolysaccharidosis type II and presentation of the most common symptoms such as severe airway obstruction, skeletal deformities, cardiomyopathy, and neurologic decline.
• Individuals without a positive family history, but with symptoms resembling Mucopolysaccharidosis type II
• Individuals with a negative but suspected family history, in order to perform proper genetic counseling (prenatal analyses are recommended in families with affected individuals).
Sequencing, deletion/duplication of IDS gene and related genes should be performed in all individuals suspected of having mucopolysaccharidosis type II. In parallel, other genes reported to be related with this clinical phenotype should also be analyzed for the presence of variants, due to the overlap in many clinical features caused by those particular genes.
Confirmation of a clinical diagnosis of Mucopolysaccharidosis type II through genetic testing can allow for genetic counseling and may direct medical management. Genetic counseling can provide a patient and/or family with the natural history of MPS II disease, identify at-risk family members, provide information on reproductive risks as well as preconception/prenatal options, and allow for appropriate referral for patient support and/or resources.