IDS gene-pseudogene exchange responsible for an intragenic deletion in a Hunter patient

Detection of Structural Variants by NGS: Revealing Missing Alleles in Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a heterogeneous group of rare multisystem metabolic disorders occurring mostly in infancy and childhood, characterized by a gradual accumulation of non-degraded substrates inside the cells. Although biochemical enzymatic assays are considered the gold standard for diagnosis of symptomatic patients, genotyping is a requirement for inclusion in enzyme replacement programs and is a prerequisite for carrier tests in relatives and DNA-based prenatal diagnosis. The emerging next-generation sequencing (NGS) technologies are now offering a powerful diagnostic tool for genotyping LSDs patients by providing faster, cheaper, and higher-resolution testing options, and are allowing to unravel, in a single integrated workflow SNVs, small insertions and deletions (indels), as well as major structural variations (SVs) responsible for the pathology. Here, we summarize the current knowledge about the most recurrent and private SVs involving LSDs-related genes, review advantages and drawbacks related to the use of the NGS in the SVs detection, and discuss the challenges to bring this type of analysis in clinical diagnostics.

A 1-year and 4-month-old child with mucopolysaccharidoses type II: A clinical case report from Ethiopia

Mucopolysaccharidoses (MPSs) are a class of lysosomal storage disorders resulting in progressive disease manifestations and are caused by pathogenic variants in genes coding for enzymes needed to degrade glycosaminoglycans. While most of the seven MPSs are autosomal recessive disorders, MPS II, also known as Hunter syndrome, is inherited in an X-linked recessive manner and is the most common MPS. Here, we report a 1-year and 4-month-old boy who presented with delayed developmental milestones, back deformity, and left scrotal swelling noticed by parents at one year of age. He has coarse facial appearance with macrocephaly, widened wrists, congenital dermal melanocytosis on his back, kyphotic deformity in the thoracolumbar area and left-sided inguinal hernia all consistent with a suspected MPS II diagnosis. The MPS II diagnosis was subsequently confirmed with genetic testing of the IDS gene. To our knowledge, this is the first case of MPS II reported from Ethiopia. This case shows the importance of early clinical recognition of genetic conditions and the utility of genetic testing for confirmation. The diagnosis provided important surveillance and natural history information for the patient's providers and family.

A New Mutation in IDS Gene Causing Hunter Syndrome: A Case Report

Rationale

Mucopolysaccharidosis type II (Hunter syndrome) is an X-linked multisystem disorder, caused by deficiency of the lysosomal enzyme iduronate-2-sulfatase (I2S). The clinical manifestations of this disease are severe skeletal deformities, airway obstruction, cardiomyopathy, and neurologic deterioration.

Patient

The patient was 5 years and 6 months boy, with developmental delay, hearing loss, hepatosplenomegaly, and skeletal dysplasia. He was diagnosed with mucopolysaccharidosis type II based on clinical manifestations, biochemical and genetic analysis.

Outcomes

The patient carries a new mutation (c.879-1210_1007-218del) in hemizygosis in the IDS gene, which was defined as pathogenic according to the 2015 American College of Medical Genetics and Genomics-Association for Molecular Pathology guidelines and as responsible for the mucopolysaccharidosis type II phenotype in the patient.

Mucopolysaccharidosis Type II: One Hundred Years of Research, Diagnosis, and Treatment

Mucopolysaccharidosis type II (MPS II, Hunter syndrome) was first described by Dr. Charles Hunter in 1917. Since then, about one hundred years have passed and Hunter syndrome, although at first neglected for a few decades and afterwards mistaken for a long time for the similar disorder Hurler syndrome, has been clearly distinguished as a specific disease since 1978, when the distinct genetic causes of the two disorders were finally identified. MPS II is a rare genetic disorder, recently described as presenting an incidence rate ranging from 0.38 to 1.09 per 100,000 live male births, and it is the only X-linked-inherited mucopolysaccharidosis. The complex disease is due to a deficit of the lysosomal hydrolase iduronate 2-sulphatase, which is a crucial enzyme in the stepwise degradation of heparan and dermatan sulphate. This contributes to a heavy clinical phenotype involving most organ-systems, including the brain, in at least two-thirds of cases. In this review, we will summarize the history of the disease during this century through clinical and laboratory evaluations that allowed its definition, its correct diagnosis, a partial comprehension of its pathogenesis, and the proposition of therapeutic protocols. We will also highlight the main open issues related to the possible inclusion of MPS II in newborn screenings, the comprehension of brain pathogenesis, and treatment of the neurological compartment.

Identification and Functional Characterization of IDS Gene Mutations Underlying Taiwanese Hunter Syndrome (Mucopolysaccharidosis Type II)

Hunter syndrome (mucopolysaccharidosis II; MPS II) is caused by a defect of the iduronate-2-sulfatase (IDS) gene. Few studies have reported integrated mutation data of Taiwanese MPS II phenotypes. In this study, we summarized genotype and phenotype correlations of confirmed MPS II patients and asymptomatic MPS II infants in Taiwan. Regular polymerase chain reaction and DNA sequencing were used to identify genetic abnormalities of 191 cases, including 51 unrelated patients with confirmed MPS II and 140 asymptomatic infants. IDS activity was analyzed in individual novel IDS variants using in vitro expression studies. Nineteen novel mutations were identified, in which the percentages of IDS activity of the novel missense mutations c.137A>C, c.311A>T, c.454A>C, c.797C>G, c.817C>T, c.998C>T, c.1106C>G, c.1400C>T, c.1402C>T, and c.1403G>A were significantly decreased (p < 0.001), c.254C>T and c.1025A>G were moderately decreased (p < 0.01), and c.851C>T was slightly decreased (p < 0.05) comparing with normal enzyme activity. The activities of the other six missense mutations were reduced but were insignificant. The results of genomic studies and their phenotypes were highly correlated. A greater understanding of the positive correlations may help to prevent the irreversible manifestations of Hunter syndrome, particularly in infants suspected of having asymptomatic MPS II. In addition, urinary glycosaminoglycan assay is important to diagnose Hunter syndrome since gene mutations are not definitive (could be non-pathogenic).

Wide allelic heterogeneity with predominance of large IDS gene complex rearrangements in a sample of Mexican patients with Hunter syndrome

Hunter syndrome or mucopolysaccharidosis type II (MPSII) is caused by pathogenic variants in the IDS gene. This is the first study that examines the mutational spectrum in 25 unrelated Mexican MPSII families. The responsible genotype was identified in 96% of the families (24/25) with 10 novel pathogenic variants: c.133G>C, c.1003C>T, c.1025A>C, c.463_464delinsCCGTATAGCTGG, c.754_767del, c.1132_1133del, c.1463del, c.508-1G>C, c.1006+1G>T and c.(-217_103del). Extensive IDS gene deletions were identified in four patients; using DNA microarray analysis two patients showed the loss of the entire AFF2 gene, and epilepsy developed in only one of them. Wide allelic heterogeneity was noted, with large gene alterations (e.g. IDS/IDSP1 gene inversions, partial to extensive IDS deletions, and one chimeric IDS-IDSP1 allele) that occurred at higher frequencies than previously reported (36% vs 18.9-29%). The frequency of carrier mothers (80%) is consistent with previous descriptions (>70%). Carrier assignment allowed molecular prenatal diagnoses. Notably, somatic and germline mosaicism was identified in one family, and two patients presented thrombocytopenic purpura and pancytopenia after idursulfase enzyme replacement treatment. Our findings suggest a wide allelic heterogeneity in Mexican MPSII patients; DNA microarray analysis contributes to further delineation of the resulting phenotype for IDS and neighboring loci deletions.

Identification of 11 novel mutations in 49 Korean patients with mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II) or Hunter syndrome is a rare lysosomal storage disorder caused by a deficiency of iduronate-2-sulfatase (IDS). As MPS II is X-linked, patients are usually males with heterogeneous mutations ranging from point mutations to gross deletions and recombination. In 2003, we reported a mutation analysis of 25 patients with MPS II. In this study, 31 mutations in another 49 Korean patients (45 families) with MPS II are reported: 12 missense, nine deletions, four splicing, two nonsense, two insertions, one deletion/insertion, and IDS-IDS2 recombination mutations. Among these mutations, 11 were novel ones (4 missense mutations: Ser61Pro, Pro97Arg, Pro228Ala, and Pro261Ala; 5 deletions: c.344delA, c.420delG, c.768delT, c.1112delC and c.1402delC; 1 deletion/insertion: c.1222delinsTA; and 1 insertion mutation: c.359_360insATCC). The IDS-IDS2 recombination mutations were most frequently observed; all patients with this mutation had the severe MPS II phenotype. However, most of the patients (5/7) with the G374G splicing mutation had an attenuated phenotype, except for two sibling cases with the severe phenotype. Except for a few recurrent mutations such as the G374G, R443X, L522P, and recombination mutations, each patient had a unique individual mutation. Therefore, careful interpretation of genotype-phenotype correlations is warranted.

Mucopolysaccharidosis type II: an update on mutation spectrum

Mucopolysaccharidosis type II (MPS II; Hunter disease) is caused by deficiency of the enzyme iduronate-2-sulphatase (IDS) and patients present with a wide range of clinical signs and symptoms. The level of activity of IDS, however, does not allow prediction of phenotype. In our study of unrelated individuals with MPS II, alterations in the IDS gene could be identified in all 155 patients. Investigations in families in which the occurrence of MPS II was sporadic revealed mosaicism in the mothers of a small number of patients and a high frequency of de novo mutations occurring preferentially during male meiosis. Mutations identified in our patients include 27 large alterations and 128 small gene alterations (96 different alterations). These data further confirm the extreme heterogeneity of IDS gene alterations, as more than 330 have been reported to date. This genetic heterogeneity may explain the high degree of clinical heterogeneity in MPS II. Therefore, attempts have been made to establish genotype-phenotype correlations in order to provide an indication of the likely prognosis and a basis on which to evaluate treatment. To date, some progress has been made in predicting the clinical phenotype from the genotype although it remains difficult in a few individual cases. However, as the crystallographic 3D structure of IDS is yet to be determined, evaluation of the impact of mutations on IDS activity is often time consuming. Furthermore, if a given mutation is recurrent, some patients carrying the same change may present with different phenotypes, suggesting that factors other than the IDS gene (e.g. other genes, environmental factors) can modulate the clinical phenotype.

CONCLUSION

Although genotype-phenotype correlations may be difficult to establish, they will be of increasing importance for choosing the most appropriate therapy for an individual patient, as new therapeutic strategies may be targeted according to phenotype.

Clinical and molecular cytogenetic characterization of two patients with non-mutational aberrations of theFMR2 gene

We report on two patients; a female having mild mental retardation (MR) with a balanced translocation, 46,XX,t(X;15)(q28;p11.2), and a male diagnosed as having mucopolysaccharidosis type II (MPS II or Hunter syndrome) with atypical early-onset MR and a normal male karyotype. Molecular cytogenetic analyses, including fluorescence in situ hybridization and array-based comparative genomic hybridization using an in-house X-tiling array, revealed that first patient to have a breakpoint at Xq28 lying within the FMR2 gene and the second to have a small deletion at Xq28 including part of FMR2 together with the IDS gene responsible for MPS II. In Patient 1, X-chromosome inactivation predominantly occurred in the normal X in her lymphocytes, suggesting that her MR might be explained by a disruption of the FMR2 gene on der(X) t(X;15) concomitant with the predominant inactivation of the intact FMR2 gene in another allele. We compared phenotypes of Patient 2 with those of MPS II cases with deletion of the IDS gene alone reported previously, suggesting that the early-onset MR might be affected by the additional deletion of FMR2.

Characterization of iduronate-2-sulfatase gene-pseudogene recombinations in eight patients with Mucopolysaccharidosis type II revealed by a rapid PCR-based method

Various types of complex genetic rearrangements involving the iduronate-2-sulfatase (IDS) and its homologous pseudogene (IDS2, IDSP1) have so far been reported as the cause of Mucopolysaccharidosis type II (MPS2 or MPS II; Hunter syndrome). When using conventional mutational analyses, the occurrence in intronic regions of these rearrangements can be misleading. Here, we describe a rapid PCR-based method set up to detect possible gene/pseudogene recombinations among a series of Italian male patients who had negative results in the mutation analysis of the IDS gene. Our approach selected eight unrelated patients showing recombinations. The characterization of the proximal regions containing the breakpoints in the eight patients identified four different rearrangements due to both inversion and conversion events. Comparison of our data with previous publications confirmed that the recombinations between the IDS gene and the IDS2 pseudogene result from separate events, considering their occurrence at different positions within the same "hotspot" genomic region in unrelated patients. The RT-PCR analysis of the available cDNAs pointed out the different effects of similar rearrangements on the expression of the IDS gene. This method can be utilized effectively in the absence of the patients' cDNA, as well as for carrier detection among female family members. This advantageous approach reduces costs, is less time-consuming, and requires a smaller DNA quantity in comparison to the Southern blot hybridization technique often utilized for such complex rearrangements.

Mucopolysaccharidosis type II--genotype/phenotype aspects

Establishing correlations between a patient's genotype and clinical phenotype is based on the assumption that the same clinical consequences will be observed in individuals with the same residual function of a specific metabolic step. In mucopolysaccharidosis type II (MPS II; Hunter disease), patients present with a wide clinical spectrum. Furthermore, current methods for measuring the activity of the deficient enzyme in MPS II--iduronate-2-sulphatase (IDS)--are insufficiently sensitive to differentiate between complete absence of activity and the presence of residual activity. Attempts have therefore been made to establish genotype-phenotype correlations in order to explain the large degree of heterogeneity and to serve as a better guide to prognosis on which to base genetic counselling and treatment options. Using MPS II as an example, this paper illustrates the difficulties and potential advantages of determining genotype-phenotype correlations in lysosomal storage diseases. The response of patients with MPS II to allogenic bone marrow transplantation provides some insight into the likely influence of certain genotypes on therapeutic efficacy.

CONCLUSIONS

Evaluation of residual activity of IDS in MPS II using gene analysis, expression studies and transcript analysis does not always allow prediction of a patient's phenotype. The variable response to bone marrow transplantation, however, illustrates the potential importance of determining the genotype for selecting the most appropriate therapy for individual patients.

Reciprocal and nonreciprocal recombination at the glucocerebrosidase gene region: implications for complexity in Gaucher disease

Gaucher disease results from an autosomal recessive deficiency of the lysosomal enzyme glucocerebrosidase. The glucocerebrosidase gene is located in a gene-rich region of 1q21 that contains six genes and two pseudogenes within 75 kb. The presence of contiguous, highly homologous pseudogenes for both glucocerebrosidase and metaxin at the locus increases the likelihood of DNA rearrangements in this region. These recombinations can complicate genotyping in patients with Gaucher disease and contribute to the difficulty in interpreting genotype-phenotype correlations in this disorder. In the present study, DNA samples from 240 patients with Gaucher disease were examined using several complementary approaches to identify and characterize recombinant alleles, including direct sequencing, long-template polymerase chain reaction, polymorphic microsatellite repeats, and Southern blots. Among the 480 alleles studied, 59 recombinant alleles were identified, including 34 gene conversions, 18 fusions, and 7 downstream duplications. Twenty-two percent of the patients evaluated had at least one recombinant allele. Twenty-six recombinant alleles were found among 310 alleles from patients with type 1 disease, 18 among 74 alleles from patients with type 2 disease, and 15 among 96 alleles from patients with type 3 disease. Several patients carried two recombinations or mutations on the same allele. Generally, alleles resulting from nonreciprocal recombination (gene conversion) could be distinguished from those arising by reciprocal recombination (crossover and exchange), and the length of the converted sequence was determined. Homozygosity for a recombinant allele was associated with early lethality. Ten different sites of crossover and a shared pentamer motif sequence (CACCA) that could be a hotspot for recombination were identified. These findings contribute to a better understanding of genotype-phenotype relationships in Gaucher disease and may provide insights into the mechanisms of DNA rearrangement in other disorders.

Analysis of a 43.6 kb deletion in a patient with Hunter syndrome (MPSII): identification of a fusion transcript including sequences from the gene W and the IDS gene

Mucopolysaccharidosis type II (Hunter syndrome) is an X-linked lysosomal storage disorder. A novel mutation is described in an MPS II patient in whom the disorder is caused by a 43.6 kb deletion. Southern blot analysis, PCR analysis and subsequent sequencing of the deletion junction revealed that the deletion spans exons 1-7 of the iduronate-2-sulfatase (IDS) gene, the IDS-2 locus and exons 3-5 of the recently identified gene W. Short direct repeats of 12 bp were identified at both deletion breakpoints, suggesting that the deletion is the result of an illegitimate recombination event. A sequence motif (TGAGGA) which is identical to a consensus sequence frequently associated with deletions in man was identified at both breakpoints. This further supports the notion that this motif is a hot spot for recombination. Gene expression studies by RT-PCR analysis of total RNA derived from fibroblasts of the patient revealed the presence of a novel fusion transcript. DNA sequence analysis of the cDNA demonstrated that it consists of exons derived from both the gene W and the IDS gene. A similar but longer fusion transcript containing exons 2-4 of the gene W and exons 4-9 of the IDS gene could also be detected in RNA of normal cell lines originating from different tissues. This result further demonstrates the complex gene expression profile of the IDS region, which may contribute to the observed genomic instability of this region.

Vertebrate pseudogenes

Pseudogenes are commonly encountered during investigation of the genomes of a wide range of life forms. This review concentrates on vertebrate, and in particular mammalian, pseudogenes and describes their origin and subsequent evolution. Consideration is also given to pseudogenes that are transcribed and to the unusual group of genes that exist at the interface between functional genes and non-functional pseudogenes. As the sequences of different genomes are characterised, the recognition and interpretation of pseudogene sequences will become more important and have a greater impact in the field of molecular genetics.

Novel type of genetic rearrangement in the iduronate-2-sulfatase (IDS) gene involving deletion, duplications, and inversions

We describe a novel type of complex genetic rearrangement in the iduronate-2-sulfatase (IDS) gene of a severely affected MPSII patient. Southern blot analysis indicated an intragenic deletion of exons 5 and 6. The deletion spans 5,581 bp. Sequencing of the deletion junctions revealed a complex rearrangement involving duplications and inversions. A remaining 20 bp fragment (c) from the intron 6 sequence and two duplicated IDS gene fragments of 314 bp (a) from intron 6/exon 7 boundary and 23 bp (b) from exon 7 were found between the deletion breakpoints. Fragments a and c were placed in an inverted orientation. We suggest that the described rearrangement is a result of a nonhomologous recombination event at sites with little homology. The proposed model explaining this recombinational event involves the formation of "tetra-loop" single-stranded DNA structure during replication. The complexity of the described rearrangement and the lack of large homologous sequences at the mutational breakpoints suggest that complex molecular intermediates are formed during illegitimate recombination.

Mutation analysis in 57 unrelated patients with MPS II (Hunter's disease)

Genomic DNA from 57 unrelated MPS II (Hunter's disease) patients was analysed for mutations of the iduronate sulphatase (IDS) gene. The aim of the study was threefold: to identify the primary genetic lesion in patients, to investigate the correlation between genotype and phenotype, and most importantly, to provide reliable carrier testing for female members once the family mutation was identified. In 42 patients, point mutations were identified involving single base substitutions, deletions, or insertions. These included four new nonsense mutations (R8X, C84X, E245X, Y466X), six new missense mutations (D45N, N115Y, P228L, P266R, E434K, I485K, W502C), three new insertions (c70C71ins, c652C654ins, c709G710ins), six new deletions (c500delC, c705delC, c1023delA, c1049delA, c1141delC, c1576delG), and five new mutations involving splice sites (IVS1-2 a-->g, IVS2-10 t-->g, IVS5 + 2 t-->g L236L, IVS7 + 2 t-->c). One patient had a new seven base deletion in exon 9 (c1482-1488del). Four patients were shown to have complete deletions of the IDS gene and two deletions involved one or more exons. Previously described mutations present in these patients were Q80X, P86L, R172X, G374G, S333L, R443X, and R468Q. In eight patients, no mutation was detected throughout the entire coding region. Most mutations that result in MPS II appear to be unique. Absence of the probands' mutations in eight of nine maternal grandmothers suggests many mutations have arisen recently. Prediction of the clinical phenotype from the identified genotype was difficult in some families, and further studies using reverse transcription polymerase chain reaction are needed to confirm the predicted effects on the IDS mRNA suggested by genomic analysis.

Identification of iduronate sulfatase gene alterations in 70 unrelated Hunter patients

We studied 70 unrelated Hunter patients and found a gene alteration in every patient. The molecular heterogeneity was very important. Large gene rearrangements were identified in 14 patients. Forty-three different mutations were identified in the 56 other patients and 31 were not previously described. Deletions and insertions, splice site mutations were associated with a severe phenotype as nonsense mutations except Q531X. Only a few mutations were present in several patients making difficult genotype-phenotype correlations. Mutation identification allows accurate carrier detection improving prenatal diagnosis. The mother was not found to be a carrier in five cases among the 44 sporadic cases. Haplotype analysis demonstrated a higher frequency of mutations in male meiosis.

Emerin deletions occurring on both Xq28 inversion backgrounds

Emery-Dreifuss muscular dystrophy (EMD) is an X-linked disorder characterized by contractures, progressive weakness and cardiomyopathy. EMD is caused by mutations in the 2 kb emerin gene that is located within human Xq28. Emerin is immediately distal to the 26 kb filamin gene, and flanking the filamin-emerin region are two large inverted repeats. This entire region previously has been found to be inverted in approximately 20% of X chromosomes, presumably mediated by the inverted repeats. Only one complete emerin deletion has been reported previously. It was found to be due to a complex rearrangement involving the inverted repeats which partially duplicated filamin. We report here two additional EMD patients who have large deletions of 20 and 34 kb, respectively. Unlike the previously reported deletion, these deletions appear to be simple deletions, with each breakpoint junction showing only 2 bp of overlap, suggesting an end-joining mechanism. However, the two deletions were found on each of the two inverted backgrounds. The 20 kb deletion includes the entire emerin gene and extends well into most of the distal inverted repeat. In contrast, the 34 kb deletion occurs on the inverted X chromosome and extends centromeric, well beyond the proximal inverted repeat. In addition, at least three nearby putative genes detected by previous sequence analysis are deleted among these patients but without obvious deviation from a typical EMD phenotype. Similarly to the previously reported deletion, filamin remains intact in these two deletions. All three deletions involve distinct breakpoints within the 4.7 kb filamin-emerin intergenic region, suggesting that loss of filamin is a lethal event. Thus, the close proximity of filamin to emerin may place constraints upon potential emerin deletions and probably accounts for the rarity of complete emerin deletions in EMD patients.

Two distinct deletions in the IDS gene and the gene W: a novel type of mutation associated with the Hunter syndrome

A novel mutation has been identified in a patient with the Hunter syndrome (mucopolysaccharidosis type II), in whom the disorder is associated with two distinct deletions separated by 30 kb. The deletions were characterized by Southern blot and PCR analyses, and the nucleotide sequences at both junctions were determined. The first deletion, corresponding to a loss of 3152 bp of DNA, included exons 5 and 6 of the iduronate-2-sulfatase (IDS) gene. The second deletion was 3603 bp long and included exons 3 and 4 of gene W, which is located in the DXS466 locus telomeric of the IDS gene. Both deletions are the result of nonhomologous (illegitimate) recombination events between short direct repeats at the deletion breakpoints. An interesting finding was the presence of the heptamer sequence 5'-TACTCTA-3' present at both deletion junctions, suggesting that this motif might be a hot spot for recombination. We propose that the double deletion is the result of homology-associated nonhomologous recombinations caused by the presence of large duplicated regions in Xq27.3-q28.

Molecular and phenotypic variation in patients with severe Hunter syndrome

Severe Hunter syndrome is a fatal X-linked lysosomal storage disorder caused by iduronate-2-sulphatase (IDS) deficiency. Patients with complete deletion of the IDS locus often have atypical phenotypes including ptosis, obstructive sleep apnoea, and the occurrence of seizures. We have used genomic DNA sequencing to identify several new genes in the IDS region. DNA deletion patients with atypical symptoms have been analysed to determine whether these atypical symptoms could be due to involvement of these other loci. The occurrence of seizures in two individuals correlated with a deletion extending proximal of IDS, up to and including part of the FMR2 locus. Other (non-seizure) symptoms were associated with distal deletions. In addition, a group of patients with no variant symptoms, and a characteristic rearrangement involving a recombination between the IDS gene and an adjacent IDS pseudogene (IDS psi), showed normal expression of loci distal to IDS. Together, these results identify FMR2 as a candidate gene for seizures, when mutated along with IDS.

Germline and somatic mosaicism in a female carrier of Hunter disease

Carrier detection in a mucopolysaccharidosis type II family (Hunter disease) allowed the identification of germline and somatic mosaicism in the patient's mother: the R443X mutation was found in a varying proportion in tested tissue (7% in leucocytes, lymphocytes, and lymphoblastoid cells, and 22% in fibroblasts). The proband's sister carries the at risk allele (determined by haplotype analysis), but not the mutation. In sporadic cases of X linked diseases, germline mosaicism of the proband's mother is difficult to exclude and should be considered in genetic counselling.