Hunter syndrome (Mucopolysaccharidosis Type II) is an X linked multisystem disorder caused due to deficiency of iduronatesulphatase. Mutation in IDS gene is responsible for this disease. No definite phenotypic-genotypic correlation has been established for point mutations in IDS gene [1], except for the pathogenic variant c.1122 C>T which is associated with a slowly progressive form of disease [2]. Mutation analysis is essential for diagnosis, carrier testing in females, and in providing prenatal diagnosis. We herein, present data on clinical features of 18 patients with Hunter syndrome from 17 families.

Clinical data from suspected Hunter syndrome patients were collected since 2007. The diagnosis was confirmed by enzyme assay of iduronate sulfatase. Informed consent was obtained and blood samples were collected from 9 families in EDTA tubes. DNA was isolated by using QIAamp DNA Mini Kit (Qiagen), which uses silica-membrane-based DNA extraction technology. Primers were designed for all 9 exons of IDS gene. In 10 patients from 9 families, PCR amplification of all 9 coding exons was done. Sanger sequencing was done using ABI 310. To look for carrier status in non-familial cases, mutation status was checked in mothers of four affected children, who had no family history. A correlation of the phenotype and genotype was attempted.

Four of the 17 families (24%) had multiple affected members. In a family with two affected siblings, the 12-year-old was more severely affected and had lower IQ than the 8- year-old sibling.

In majority of patients (78%), the age of presentation was 5 years and above. Hepatomegaly and joint contractures were present in all the patients. Fifteen children (83%) had splenomegaly at presentation and 11 (61%) had umbilical hernia. Intelligence quotient assessment revealed that 6 (33.3%) patients had normal IQ at presentation. Short stature, respiratory symptoms and macrocephaly were present in 11, 8 and 9 patients, respectively. All patients had radiological changes of dysostosis multiplex. Hearing impairment was detected in 3 out 15 patients in whom it could be assessed. Valvular heart disease was observed in two children out of 13 (15%) for whom echocardiography was done. The mean iduronate sulphatase enzyme level was 0.11 nmol/4 hr/mL.

Hemizygous mutations in IDS gene were identified in all the 10 patients with Hunter syndrome, including a pair of siblings (Table I). All of them were point mutations. Five out of nine mutations were previously reported as disease causing mutations. The four novel mutations were predicted to be pathogenic by in-silico analysis, by predicting softwares like MutationTaster, SIFT and Polyphen. Mothers of two out of four (50%) children were found to be carriers even without any family history.

TABLE I Mutation Spectrum in IDS Gene

Mucopolysaccharidosis Type II or Hunter syndrome is a common lysosomal storage disorder, which is easy to suspect clinically in any male with clinical and radiological features of mucopolysaccharidosis and a clear cornea.

Confirmation of diagnosis by enzyme assay and preferably by mutation analysis is essential not only for genetic counseling, but also for starting Enzyme Replacement Therapy (ERT) which has shown success, especially in those without neurological involvement. One third of the cases in this series had normal cognition which is comparable to Hunter Outcome survey [3]. Our only patient on ERT has shown improved physical activity and a reduction in hepatosplenomegaly, which is similar to what is reported in the literature [9].

Being an X-linked disorder, it is essential to offer carrier screening to female members in extended family on maternal side. For carrier screening and prenatal diagnosis, mutation analysis in IDS gene in the proband is essential. More than 300 pathogenic variants have been described in IDS gene, the majority of which are point mutations [6]. A great degree of phenotypic variability is observed in Hunter syndrome, but no genotype- phenotype correlation has been identified. Intra-familial variability is rare [8].

High cost of ERT and lack of its efficacy on neurological manifestation stresses the need for genetic counseling and prevention by prenatal diagnosis. Enzyme assay of chorionic villi is used for prenatal diagnosis in the absence of facilities for molecular diagnosis. However, the use of enzyme assay for prenatal diagnosis has limitations and mutation based prenatal diagnosis is more accurate. For carrier detection, enzyme assay cannot be used and mutation analysis is a must.

In the era of availability of enzyme-replacement therapy, accurate differentiation of Hunter syndrome from other mucopolysaccharidoses with overlapping clinical features is important. Mutation analysis of IDS gene helps in confirmation of the diagnosis and prevention by carrier testing and prenatal diagnosis.

Acknowledgment: Shire Plc for providing Elaprase on Global Charitable Access Program.

Contributors: DLN: Design of the work, data acquisition, analysis and interpretation, drafting the manuscript; PS: Analysis and interpretation of data, drafting the manuscript; PSG: Acquisition of data, manuscript revision; KM: Conception and design of the work, analysis and interpretation of data and revising the manuscript critically for content; SRP: Conception and design of the work, analysis and interpretation of data, revising the manuscript critically for content. SRP will be the guarantor of the study.

Funding: Indian Council of Medical Research Bio Medical Sciences (63/08/2010–BMS). Competing interests: None stated.