Coexistence of mutations R315X and g.99367-102002del both involving the exon 5 of ARSB gene caused a misdiagnosis for a MPS VI patient.

Mucopolysaccharidosis type VI (MPS VI, Maroteaux-Lamy syndrome) is a lysosomal disease due to the deficiency of arylsulfatase B (ARSB). In the absence of this enzyme, the stepwise degradation of the glycosaminoglycan (GAG) dermatan sulfate is blocked, causing a chronic progressive disorder involving multiple organs. At present, enzyme replacement therapy (ERT) represents the only therapeutic option for this disease (Harmatz et al, 2005) with urinary GAG as important biochemical parameter to assess treatment efficacy. In a recent paper where the measurement of the gene expression of TNF-α was suggested as potential disease biomarker, the genotype of a patient was reported as R315X in homozygosity (Di Natale et al, 2008). At that time the DNA of both parents was not available. Subsequently, when a molecular characterization was required from the entire family, the father resulted non-carrier for the R315X mutation. Thus the patient was considered apparently homozygous for the R315 mutation and extensive work was performed to find the second mutation. Patient cDNA revealed the presence of a fragment shorter than that obtained from the normal control: the analysis of this fragment showed that the cDNA was lacking of the entire exon 5 of the ARSB gene: this new mutation on the second allele was identified as c.899-1142del. Since the genomic DNA sequencing allowed us to exclude the presence of splicing mutations, in an attempt to find the breakpoints of a possible deletion involving the exon 5 of the gene we performed PCR analysis of polymorphisms reported in the NCBI SNP database for the ARSB gene. This allowed us to further restrict the area of the possible deletion and to identify the mutation at the genomic DNA level as g.99367-102002del; this gross deletion, involving the entire exon 5 of the gene and parts of the introns 4 and 5 leads to a frameshift starting at aa 300 resulting in a protein with 39 % aminoacids different from the normal enzyme. We conclude that, in the presence of an apparent homozygosity, it is mandatory to analyze the DNA of both parents, to avoid misdiagnoses, followed by potential pitfalls in genetic counselling.