MPS I is the most common mucopolysaccharidosis worldwide, with an average incidence of about 1.7 in 100.000 live births for the severe and mild forms . The incidence of MPS I in Tunisia is also high, estimated at 0.63 in 100.000 live births , owing to the high rate of consanguinity .
To date, over 100 mutations in the IDUA gene have been described in patients with the MPS I subtypes (Human Gen Database, http://www.hgmd.org).
The molecular analysis of the six newly collected patients in Tunisian families allowed the identification of four mutations including a small insertion (1587_1588 inGC), a novel missense mutation (p.F177S) and two previously reported mutations, p.P533R and p.Y581X (Table 2). Of note, the common Caucasian IDUA mutations, p.Q70X and p.W402X, were not present .
Patient 1 from family 1 was homozygous for the previously reported p.Y581X mutation in exon 13 of the IDUA gene. The p.Y518X mutation was first described in one patient of Croatian origin. This patient was heteroallelic for the p.Y581X mutation and the Ile583delC mutation .
A patient 2 from family 2 was homozygous for the novel p.F177S mutation. This mutation causes a sever instability or loss of IDUA protein function, since the 177 residue was predicted to form the strands of the β barrel with the catalytic residues perched on the C terminal ends of the β strands . The predicted IDUA active site showed different residues, among which the residue Glu178 which is near the Phe177. This mutation presumably resulted from slipped mispairing and repairing during replication. Our study identified the p.177S mutation in patient which had the severe phenotype and undetectable IDUA activity. This concept is consistent with the fact that the phenylalanine at IDUA residue 177 is highly conserved in evolution.
A patient four from family four was homozygous for the novel 1587_1588 inGC mutation that leads to the lack of 30 aminoacid at the amino terminus of the IDUA protein (p.Leu530fs).
The insertion (GC) occurred between two direct repeats (GC) that were separated by only three bases (CCC). The patient 4 did not have detectable IDUA activity and presented with the severe Hurler phenotype.
Patients 3, 5 and 6 from families 3, 5 and 6 respectively were homozygous for the p.P533R mutation. These patients had undectable IDUA activity and the typical Hurler phenotype. The p.P533R missense mutation in exon 11 resulted in a non conservative substitution of a neutral proline for a basic arginine.
The IDUA model begins at residue 36 of the linear sequence and terminates at residue 522, as this is the longest section that can be reliably predicted on the basis of the crystal structure of of the β-D-xylosidase from Thermoanaerobacterium saccharolyticum (XyTS, EC 188.8.131.52) . As a result, residues 523-653 do not appear in the IDUA model, consistent with the finding that a proline residue in codon 533 is incapable of forming the main chain hydrogen bonds. A non conservative substitution of a neutral proline for a basic arginie could be predicted to drastically change the orientation of the secondary structure in IDUA protein leading to a severe disease phenotype.
The basis of such regional distribution of p.P533R mutation is not clear. This mutation has been identified in 92% of mutant alleles in 13 MPS I patients from Marroco , in 11% of mutant alleles of 27 MPS I patients from Sicily , in 62.5% of mutant alleles in 10 MPS I patients from Tunisia  and not identified in 3 MPS I patients from Egypt , suggesting that the p.P5323R allele possibly originated from a common founder from the Islamic occupation of Sicily .
A large number of polymorphisms and non pathogenic sequence variants have been described in the IDUA gene [7, 23, 24] (Table 2). The effect of these sequences variants on the IDUA activity has not been clearly defined especially when they are associated with specific mutations causing the Hurler/Scheie and Scheie phenotypes . The effect of the noncoding [IVS2-44 (c.300-44C > T);IVS5-45 (c.590-45G > C); IVS5-8 (c.590-8C > T);IVS7+48 (c.972+48A > G), IVS7-45 (c.973-45 G > C; IVS9+36 (c.1402+36T > C), IVS12+72 (c.1727T > G),3'UTR+44 (c.1962+44G > C)] and coding [p.A8A (c.24 A > C), p.A20A (c.60G > A); p.H33Q (c.99 T > G), p.Q433Q (c.99T > G), p.R105Q (c.314G > A); p.N181N (c.543 T > C); p.A314A (c.942G > C); p.A361 (Tc.1081G > A; p.T388T (c.116 G > C); p.V454I (c.1360G > A); p.R489R (c.1467C > T)] polymorphisms/sequence variants on IDUA expression in the Hurler patients from the Tunisian families is unknown .
Our study showed a heterogeneous pattern of mutations and polymorphisms among MPS I Tunisian patients.
The results of recent studies support the evidence of mutational heterogeneity of the IDUA, IDS and GALNS genes in patients with MPS I , MPS II  and MPS IVA  respectively. These MPS patients have different clinical presentation ranging from severe to mild. Since most patients have unique mutations, a comprehensive genotype-phenotype correlation is not feasible. At any rate, even in presence of recurrent mutations, a correlation has proved inapplicable. For example, the missense p.P533R mutation was usually reported associated with a severe phenotype , whereas this lesion identified on the contrary in our patients was found associated with mild and severe forms of the disease. This type of observation implicates others modifying or non genetic factor in the clinical presentation of disease.
A more comprehensive analysis of MPS I patients is essential. This must comprise not only a molecular screening for other known mutations, but also a complete screening for new mutations. In addition, a comprehensive clinical and epidemiological investigation could help assign the ethnic background of these patients.