Clinical and genetic characteristics of Tunisian children with infantile nephropathic cystinosis.

BACKGROUND: Nephropathic cystinosis is an autosomal recessive disease caused by a mutation in the CTNS gene which encodes cystinosin, a lysosomal cystine transporter. The spectrum of mutations in the CTNS gene is not well defined in the North African population. Here, we investigated twelve patients with nephropathic cystinosis belonging to eight Tunisian families in order to analyze the clinical and genetic characteristics of Tunisian children with infantile nephropathic cystinosis. METHODS: Clinical data were collected retrospectively. Molecular analysis of the CTNS gene was performed by Sanger sequencing. RESULTS: We describe a new splicing mutation c.971-1G > C in the homozygous state in 6/12 patients which seems to be a founder mutation. The reported deletion of 23nt c.771_793 Del (p.Gly258Serfs*30) was detected in a homozygous state in one patient and in a heterozygous compound state with the c.971-1G > C mutation in 3/12 patients. Two of 12 patients have a deletion of exons 4 and 5 of the CTNS gene. None of our patients had the most common 57-kb deletion. CONCLUSIONS: The mutational spectrum in the Tunisian population is different from previously described populations. Thus, a molecular diagnostic strategy must be implemented in Tunisia, by targeting as a priority the common mutations described in this country. Such a strategy will allow a cost-effective diagnosis confirmation as well as early administration of treatment with oral cysteamine. A higher resolution version of the Graphical abstract is available as Supplementary information.

Molecular autopsy and clinical family screening in a case of sudden cardiac death reveals ACTN2 mutation related to hypertrophic/dilated cardiomyopathy and a novel LZTR1 variant associated with Noonan syndrome.

BACKGROUND: Genetic cardiac diseases are the main trigger of sudden cardiac death (SCD) in young adults. Hypertrophic cardiomyopathy (HCM) is the most prevalent cardiomyopathy and accounts for 0.5 to 1% of SCD cases per year. METHODS: Herein, we report a family with a marked history of SCD focusing on one SCD young adult case and one pediatric case with HCM. RESULTS: For the deceased young adult, postmortem whole-exome sequencing (WES) revealed a missense variant in the ACTN2 gene: c.355G > A; p.(Ala119Thr) confirming the mixed hypertrophic/dilated cardiomyopathy phenotype detected in the autopsy. For the pediatric case, WES allowed us the identification of a novel frameshift variant in the LZTR1 gene: c.1745delT; p.(Val582Glyfs*10) which confirms a clinical suspicion of HCM related to Noonan syndrome. CONCLUSION: The present study adds further evidence on the pathogenicity of ACTN2: p. Ala119Thr variant in SCD and expands the mutational spectrum of the LZTR1 gene related to Noonan syndrome.

Novel RAB3GAP1 Mutation in the First Tunisian Family With Warburg Micro Syndrome.

BACKGROUND AND PURPOSE: Warburg Micro syndrome (WARBM) is a rare autosomal recessive genetic disease characterized by ocular, neurologic, and endocrine anomalies. WARBM is a phenotypically and genetically heterogeneous syndrome caused by mutations in RAB3GAP1, RAB3GAP2, RAB18, and TBC1D20. Here we present the clinical and genetic characterization of a consanguineous Tunisian family with a WARBM phenotype presenting two pathogenic variations, one of which is on RAB3GAP1. METHODS: We applied whole-exome sequencing (WES) to two affected young males presenting a WARBM-compatible phenotype. RESULTS: We reveal a new variation in RAB3GAP1 (NM_012233.3: c.297del, p.Gln99fs) and another variation in ABCD1 (NM_000033: c.896A>G, p.His299Arg). Each of these mutations, which in silico predictions concluded as being pathogenic variations, affects a critical protein region. Both affected males presented a WARBM-compatible phenotype, with severe intellectual disability, severe developmental delay, postnatal growth delay, postnatal microcephaly, congenital bilateral cataracts, general hypotonia, and a thin corpus callosum without a splenium. However, intrafamilial clinical heterogeneity was present, since only the oldest child had large ears, microphthalmia, foot deformities, and a genital anomaly, and only the youngest child had microcornea. Despite the mutation identified in ABCD1, our patients did not have any X-linked symptoms of adrenoleukodystrophy disorder that are usually caused by ABCD1 mutations, which prompted our interest in clinical monitoring. CONCLUSIONS: WES analysis of a consanguineous Tunisian family with WARBM revealed a novel variation in RAB3GAP1 (NM_012233.3: c.297del, p.Gln99fs) that is most likely pathogenic and allowed us to confirm the diagnosis of WARBM.

3M syndrome: A Tunisian seven-cases series.

3M syndrome (3MS) is a rare autosomal recessive primordial growth disorder characterized by a severe pre- and post-natal growth deficiency, minor dysmorphisms and skeletal abnormalities, contrasting with normal intellect and endocrine function. Three different genes have been so far involved in the disease, with mutations in CUL7, OBSL1 and CCDC8. The CUL7 gene mutations are accountable for 77,5% of the genetically confirmed patients, with a founder mutation identified in exon 24 for the Maghreb families. The follow up is mainly orthopedic with possible GH-based treatment. The objective of this report was to carry out a clinical analysis of a series of Tunisian patients with features evoking 3MS and to perform a molecular analysis of the CLU7 exon 24. We carried out a descriptive retrospective study including Tunisian patients who consulted at the congenital disorders and hereditary diseases department of Charles Nicolle's hospital, Tunis, Tunisia, for intra-uterine onset growth retardation with normal intellect. We selected the patients having characteristic 3MS facial dysmorphia. The molecular analysis of the CUL7 exon 24 was performed using PCR and Sanger sequencing searching the founder mutation c.4451_4452delTG. Seven patients were included in this study. Consanguinity was noted for four families. The mean age at the first consult was 2.5 years. All the patients had an intra-uterine onset growth retardation with a preserved head circumference. All patients presented facial dysmorphia of 3MS, with a prominent forehead (7/7), a triangular face (6/7), an underdeveloped midface (7/7), a fleshy tipped nose (5/7), anteverted nares (6/7), a long philtrum (7/7) and full lips (4/7). All the patients presented skeletal abnormalities with various severities such as lumbar lordosis, hyperextensible joints, short thorax, square shoulders, hip dislocation, and prominent heels. Less frequent features were noted such as spina bifida occulta in one case, and single transverse palmar crease in 4 cases. One GH treatment response was reported. The molecular genetic analysis of the CUL7 gene (exon 24) revealed the founder mutation for all the patients which reinforces the hypothesis of founder effect for 3MS in the Tunisian population.

Spectrum of Genetic Diseases in Tunisia: Current Situation and Main Milestones Achieved.

Genetic diseases in Tunisia are a real public health problem given their chronicity and the lack of knowledge concerning their prevalence and etiology, and the high rates of consanguinity. Hence, we performed systematic reviews of the literature in order to provide a more recent spectrum of these disorders and to expose the challenges that still exist to tackle these kinds of diseases. A manual textual data mining was conducted using MeSH and PubMed databases. Collected data were classified according to the CIM-10 classification and the transmission mode. The spectrum of these diseases is estimated to be 589 entities. This suggests remarkable progress through the development of biomedical health research activities and building capacities. Sixty percent of the reported disorders are autosomal recessive, which could be explained by the high prevalence of endogamous mating. Congenital malformations (29.54%) are the major disease group, followed by metabolic diseases (22%). Sixty percent of the genetic diseases have a known molecular etiology. We also reported additional cases of comorbidity that seem to be a common phenomenon in our population. We also noticed that epidemiological data are scarce. Newborn and carrier screening was only limited to pilot projects for a few genetic diseases. Collected data are being integrated into a database under construction that will be a valuable decision-making tool. This study provides the current situation of genetic diseases in Tunisia and highlights their particularities. Early detection of the disease is important to initiate critical intervention and to reduce morbidity and mortality.

Identification of two additional novel mutations in the AR gene associated with severe forms of androgen insensitivity syndrome.

The Androgen insensitivity syndrome (AIS) in its complete form (CAIS) is a disorder in abnormal male development characterized by a complete female phenotype in a 46,XY individual. The most frequent cause of this disorder is a hemizygous mutation in androgen receptor (AR) gene located in X chromosome. The first aim of this study was to confirm the clinical diagnosis in a series of Tunisian patients with a typical phenotype of CAIS by molecular genetic analysis. The second aim was to determine the AR mutational profile in the local population. The entire coding region and the exon-intron junctions of the AR gene were sequenced in a series of ten patients. AR defects were found in nine patients. Despite the small number of cases, two of the nine identified mutations were novel. The first novel mutation was an 8-bp deletion in exon 1 (c.862_869del) resulting in a frameshift (p.A288Qfs*14). The second was a splice site mutation c.1885 + 1G > T (IVS3 + 1G > T). In this study, genetic testing has confirmed the diagnosis of most CAIS patients and has revealed two novel mechanisms responsible for the pathogenesis of AIS, as well as seven other reported mutations.

WDR73-related galloway mowat syndrome with collapsing glomerulopathy.

Galloway-Mowat syndrome (GAMOS [MIM 251300]) is a rare autosomal recessive disorder that manifests as a combination of nephrotic syndrome, brain abnormalities and developmental delay. It is a clinically and genetically heterogeneous disease. The WDR73 variations are associated with GAMOS1. Here we report two consanguineous families affected by GAMOS1. In the first family, three sisters are affected and in the second family, only one index case is identified. They all show a nephrotic syndrome, a neurological involvement and a collapsing glomerulopathy. The analysis of mutations of WDR73 revealed a new homozygous missense mutation NM_032856.3 c.293T > C; p.(Leu98Pro) in two patients from the first family, and a new homozygous missense mutation NM_032856.3: c.767G > A; p.(Arg256Gln) in the second one. This study extended the clinical and molecular spectrum of GAMOS1 with other cases associated with collapsing glomerulopathy and two novel WDR73 variations that are most likely pathogenic.

TOP3B: A Novel Candidate Gene in Juvenile Myoclonic Epilepsy?

Juvenile myoclonic epilepsy (JME) is characterized by seizures, severe cognitive abnormalities, and behavior impairments. These features could evolve over time and get worse, especially when the encephalopathy is pharmacoresistant. Thus, genetic studies should provide a better understanding of infantile epilepsy syndromes. Herein, we investigate the genetics of JME in a consanguineous family analyzing the copy number variations detected using over 700 K SNP arrays. We identified a 254-kb deletion in the 22q11.2 region, including only the TOP3B gene, detected in the patient and her father. TOP3B encodes a topoisomerase DNA (III) ß protein and has been implicated in several neurological diseases such as schizophrenia and autism. In this study, we discuss the implication of the 22q11.2 region in neurodevelopmental disorders and the association of TOP3B with epilepsy.

Novel PAX3 mutations causing Waardenburg syndrome type 1 in Tunisian patients.

Waardenburg syndrome (WS) is an auditory-pigmentary disease characterized by a clinical and genetic variability. WS is classified into four types depending on the presence or absence of additional symptoms: WS1, WS2, WS3 and WS4. Type 1 and 3 are mostly caused by PAX3 mutations, while type 2 and type 4 are genetically heterogeneous. The aims of this study are to confirm the diagnostic of WS1 by the sequencing of PAX3 gene and to evaluate the genotype phenotype correlation. A clinical classification was established for 14 patients WS, as proposed by the Waardenburg Consortium, and noted a predominance of type 1 and type 2 with 6 patients WS1, 7 patients WS2 and 1 patient WS3. A significant inter and intra-familial clinical heterogeneity was also observed. A sequencing of PAX3 gene in the 6 patients WS1 confirmed the diagnosis in 4 of them by revealing three novel mutations that modify two functional domains of the protein: the c.942delC; the c.933_936dupTTAC and the c.164delTCCGCCACA. These three variations are most likely responsible for the phenotype, however their pathogenic effects need to be confirmed by functional studies. The MLPA analysis of the 2 patients who were sequence negative for PAX3 gene revealed, in one of them, a heterozygous deletion of exons 5 to 9 confirming the WS1 diagnosis. Both clinical and molecular approaches led to the conclusion that there is a lack of genotype-phenotype correlation in WS1, an element that must be taken into account in genetic counseling. The absence of PAX3 mutation in one patient WS1 highlights the fact that the clinical classification is sometimes insufficient to distinguish WS1 from other types WS hence the interest of sequencing the other WS genes in this patient.

Functional Analysis of Mutations at Codon 127 of the SRY Gene Associated with 46,XY Complete Gonadal Dysgenesis.

Complete gonadal dysgenesis (CGD) is characterized by an incomplete differentiation of the genital organs in a patient with a 46,XY karyotype. It is induced by mutations in the sex-determining region Y (SRY) gene which plays a key role in testis-determining pathways. The aim of this study was to investigate the possible pathogenic nature of a novel SRY mutation (p.Y127H) identified in a 46,XY female patient. To determine the effect of this mutation on SRY function, we studied its impact on DNA interaction by electrophoretic mobility shift assays. Since tyrosine 127 is close to the C-terminal nuclear localization signal of SRY, we conducted HA-SRY protein expression to observe the impact of the mutation on the nuclear import function in transfected cells. Our results showed that the Y127H mutation nearly abolishes the DNA-binding capacity of SRY and strongly impairs the nuclear localization of the mutated protein. Together with a previously described mutation analyzed in parallel in this paper (p.Y127C), our results highlight this tyrosine residue as a crucial structural determinant of the high mobility group box domain. This is the first report to explain the molecular mechanism of the Y127H mutation causing sex reversal and gives new insights for clinical practice to benefit patients with disorders of sex development.

Distinctive findings in a boy with Simpson-Golabi-Behmel syndrome.

Simpson-Golabi-Behmel syndrome (SGBS) is an X-linked condition characterized by pre and post natal overgrowth, facial malformations, and visceral, skeletal, and neurological anomalies. The physical characteristics of SGBS have been well documented; however there is a lack of description regarding the behavioral phenotype. We report the case of a 6-year-old boy, with confirmed deletion of 6-8 exons of the glypican-3 gene (GPC3) who presents three distinctive findings: the persistence of the craniopharyngeal canal, an immune-allergic specificity, and a scarcely behavioral phenotype consisting in the association of Austim Spectrum Disorder with accompanying mild intellectual disability and language impairments. He also fulfilled the criteria of Attention Deficit Hyperactivity Disorder and Oppositional Defiant Disorder according to DSM 5 criteria. The specificities of the case are discussed in the light of recent pathophysiological data.

RGS6: a novel gene associated with congenital cataract, mental retardation, and microcephaly in a Tunisian family.

PURPOSE: The object of this study is to identify the underlying genetic defect in a consanguineous Tunisian family affected with autosomal recessive congenital cataract associated with mental retardation and microcephaly. METHODS: A whole-genome scan was performed with polymorphic microsatellites in the axiom data for the screened members. Homozygous regions were analyzed with integrated Systems Tool for Eye gene Discovery (iSyTE), to identify candidate genes with lens-enriched expression that were potentially associated with cataract. Then we screened for mutations by direct sequencing. Structure and function of the mutant gene were analyzed by bioinformatics analysis. RESULTS: Using whole-genome scanning, we identified six runs of homozygosity shared among affected members of the studied family. Analysis of these regions by iSyTE allowed us to select 3 genes (RGS6, PCNX, and P4HA1) according to their expression in 3 critical stages of lens development. Upon screening for mutations by sequencing analysis, we found a novel mutation in RGS6, the splice-acceptor variant c.1369-1G>C that was not previously reported in congenital cataract phenotypes. CONCLUSIONS: Our study identified a new gene to be included in the large spectrum of cataract-associated genes. Importantly, the study demonstrated that, in addition to lens-enriched genes that exhibit high expression levels, genes identified by iSyTE that are highly lens-enriched but have lower absolute expression may also represent candidates for potential function in the lens.

GJB2 and GJB6 screening in Tunisian patients with autosomal recessive deafness.

UNLABELLED: Autosomal recessive nonsyndromic deafness (ARNSD or DFNB) is a very common genetically heterogenous disorder. Although DFNB1 mutations are known to be the most frequent cause of this disorder, they are largely dependent on ethnic groups. The aims of our study are to specify the prevalence and the spectrum of GJB2 mutations as well as the prevalence of GJB6 large deletion in Tunisian population. PATIENTS AND METHODS: 95 unrelated patients with moderate to severe sensorineural hearing loss have been tested. The GJB2 coding region has been studied by PCR/Sequencing and the del(GJB6-D13S1830) mutation has been screened by fluorescent PCR multiplex. RESULTS: 27.36% of patients present mutations on both alleles of GJB2 gene and no one has the del(GJB6-D13S1830) mutation. The c.35delG mutation represents 86.5% of GJB2 deafness alleles and is found in homozygous state in 22 patients and in heterozygous state in one patient. Four other mutations are detected in four probands: two are compound heterozygous for the p.V37I/p.E47X and the c.35delG/p.R184P mutations, and two are homozygous for the p.E47X and the c.333-334delAA mutations. CONCLUSION: Our results showed that c.35delG is the most common but not the only GJB2 mutation and that the del(GJB6-del D13S1830) is absent in our cohort. Consequently, we propose a systematic sequencing of GJB2 coding region for ARNSD Tunisian patients and we suggest additional studies to specify the real prevalence of del(GJB6-D13S1830) in our population.

Epidemiologic and clinical characteristics of 458 Tunisian patients with intellectual deficiency and a reconsidered diagnostic strategy.

Intellectual Deficiency (ID) is a common neuropsychiatric disorder whose etiopathogenesis still insufficiently understood. In the last decade, several surveys, assessing epidemiologic, clinical and etiologic parameters of ID, have been performed but none of them is realized in a Tunisian population. In this retrospective survey, we propose to study these parameters, in a Tunisian cohort of 458 patients with constitutional ID, and to assess our diagnostic strategy. Data analyses, by the SPSS program, reveal a male predominance, a high level of consanguinity, an advanced mean age of patients, a rare frequentation of specialized institutions by the severely affected patients, and a high frequency of familial forms with predominance of the recessive autosomal ones. The study of clinical parameters and investigations' results shows that 72.1% of our patients present a syndromic ID. For these patients, chromosomal anomalies are rarely described, EEG anomalies were usually non-specific in patients without clinical evidence of epilepsy, and brain anomalies are common in patients with severe ID, neurological symptoms or history of seizures. Aetiology is identified in 13.1% of them whereas it is still unknown in 100% of patients with non-specific ID. This study allows us to better characterize, epidemiologically and clinically, the first large Tunisian cohort of patients with ID and to assess our diagnostic strategy in order to propose a revised one that will improve the diagnostic lead, the care chain and the preventive resources of ID.

Prevalence of Bardet-Biedl syndrome in Tunisia.

Bardet-Biedl syndrome (BBS, OMIM 209900) is a ciliopathy causing multivisceral abnormalities. This disease is mainly characterized by obesity, post-axial polydactyly, hypogenitalism, intellectual disabilities, pigmentary retinopathy, and renal deficiency. The prevalence of BBS has been estimated in different populations, ranging from 1 in 160,000 in European populations to 1 in 13,000 in Bedouins from Kuwait. In the present report, we present the first epidemiological study of Bardet-Biedl syndrome in Tunisia. From 1984 to 2009, 46 Tunisian families, including 67 affected members, were diagnosed as BBS. The patients' ages ranged between 6 months and 37 years, with median age of 10.4 years. High level of consanguinity was noted in our cohort (93.47%). The overall minimum prevalence in our population was estimated to be approximately 1 in 156,000 individuals. Our study reflects the actual frequency of BBS in North Africa and showed that this disease seems uncommon.

Absence of mutations in four genes encoding for congenital cataract and expressed in the human brain in Tunisian families with cataract and mental retardation.

BACKGROUND: To identify the genetic defect associated with autosomal recessive congenital cataract (ARCC), mental retardation (MR) and ARCC, MR and microcephaly present in most patients in four Tunisian consanguineous families. METHODS: We screened four genes implicated in congenital cataract by direct sequencing in two groups of patients; those affected by ARCC associated to MR and those who presented also microcephaly. Among its three genes PAX6, PITX3 and HSF4 are expressed in human brain and one gene LIM2 encodes for the protein MP20 that interact with the protein galectin-3 expressed in human brain and plays a crucial role in its development. All genes were screened by direct sequencing in two groups of patients; those affected by ARCC associated to MR and those who presented also microcephaly. RESULTS: We report no mutation in the four genes of congenital cataract and its flanking regions. Only variations that did not segregate with the studied phenotypes (ARCC associated to MR, ARCC associated with MR and microcephaly) are reported. We detected three intronic variations in PAX6 gene: IVS4 -274insG (intron 4), IVS12 -174G>A (intron12) in the four studied families and IVS4 -195G>A (intron 4) in two families. Two substitutions polymorphisms in PITX3 gene: c.439 C>T (exon 3) and c.930 C>A (exon4) in one family. One intronic variation in HSF4 gene: IVS7 +93C>T (intron 7) identified in one family. And three intronic substitutions in LIM2 gene identified in all four studied families: IVS2 -24A>G (intron 2), IVS4 +32C>T (intron 4) and c.*15A>C (3'-downstream sequence). CONCLUSION: Although the role of the four studied genes: PAX6, PITX3, HSF4 and LIM2 in both ocular and central nervous system development, we report the absence of mutations in all studied genes in four families with phenotypes associating cataract, MR and microcephaly.

Hexasomy of the Prader-Willi/Angelman critical region, including the OCA2 gene, in a patient with pigmentary dysplasia: case report.

Derivatives of chromosome 15, often referred to as inv dup(15), represent the most common supernumerary marker chromosome (SMC). SMC(15)s can be classified into two major groups according to their length: small SMC(15) and large ones. Depending on the amount of euchromatin, the carriers may either present with a normal phenotype or with a recognizable syndrome. Here we describe a patient with severe mental retardation, epilepsy, dysmorphic features and pigmentary dysplasia. His karyotype was 47,XY,+mar[41]/46,XY[9]. Chromosomal fluorescence in situ hybridization (FISH) showed the SMC to be originating from chromosome 15, dicentric and containing four copies of the Prader-Willi/Angelman Syndrome Critical Region (PWACR), including the OCA2 gene. Molecular studies indicated that it is maternally derived. This report supports the previous observations assuming that severity of phenotype in patients with SMC(15) depends on the dosage of the PWACR and that skin pigmentation is correlated to OCA2 gene copy number.

Non-syndromic autosomal recessive mental retardation in Tunisian families : exclusion of GRIK2 and TUSC3 genes.

BACKGROUND: Mental retardation is one of the most frequent major handicap, with a 1-3 % frequency in the general population, it appear a major problem of public health. The recent progress of molecular biology and cytogenetic allowed to identify new genes for non syndromic autosomal recessive mental retardation (NSAR-MR). AIM: Genetic analysis of NSAR-MR: the GRIK2 gene (6q16.3-q21) and the TUSC3 gene (8p22). METHODS: Four Tunisian families with NSAR-MR were included in this study. Genotyping was made using polymorphic microsatellite markers and statistical analysis was validated using the Fast Link programme of the Easy linkage software (V4:00beta). RESULTS: Genotyping and linkage analysis excluded linkage of the GRIK2 gene and TUSC3 gene. CONCLUSION: Our results confirm the extreme genetic heterogeneity of NSAR-MR.

Detection of a frequent duplicated CYP21A2 gene carrying a Q318X mutation in a general population with quantitative PCR methods.

Earlier we had reported a large prevalence of the Q318X mutation in the CYP21A2 gene with 35.3% in Tunisian patients with a classical form of 21-hydroxylase deficiency, in contrast with 0.5% to 13.8% as described in other populations. Here we present the analysis of the Q318X mutation in a healthy Tunisian population. We screened 136 individuals by the polymerase chain reaction (PCR)/random fragment length polymorphism method, which was confirmed by direct sequencing. Surprisingly, 17 Q318X carriers were identified, for a carrier frequency of 12.5% (95% confidence interval: 7.86-19.20). To explain this unexpectedly high rate we suggest that the haplotype with Q318X mutation and duplicated CYP21A2 gene could be very frequent in the Tunisian population. To test our hypothesis, we used 2 different quantitative PCR methods, that is, multiplex ligation-dependent probe amplification and real-time PCR. The molecular studies showed the presence of a duplicated CYP21A2 gene in all 17 heterozygous Q318X mutation carriers. In addition, both quantitative PCR methods used in this study represent a sensitive and useful approach to detecting copy number variations of the CYP21A2 gene. We have identified a very high frequency of carriers with duplicated CYP21A2 gene haplotype in a healthy Tunisian population. This finding complicates the molecular diagnosis of 21-hydroxylase deficiency and we recommend that, whenever a Q318X is identified, the structure of the CYP21A2 region should be determined to discriminate between the severe Q318X mutation and the normal Q318X variant.

A novel frame shift mutation in the PQBP1 gene identified in a Tunisian family with X-linked mental retardation.

Mental retardation (MR) is the most frequent cause of serious handicap in children and young adults. Despite recent progress, in most cases the molecular defects underlying this disorder remain unknown. Linkage studies followed by mutational analysis of known X-chromosomal genes related to mental retardation (MRX genes) localized within defined genetic intervals represent a rational strategy to identify a genetic cause of the disorder. Here, we report a Tunisian family including 3 males with severe to mild mental retardation, short stature, lean body and microcephaly; we mapped the disease to a unique interval encompassing Xp21.1-Xq21.33 (with a maximum LOD score of 0.90). Subsequent mutation analysis of genes located in this interval allowed us to identify a truncating mutation in the PQBP1 gene. This mutation is an insertion of an adenosine residue in exon 5 (c.631insA). This frameshift insertion causes premature stop codon at amino acid position 226. The observed mutation was found in all males with MR in this family. Together with previously reported observations, our data further confirm that PQBP1 gene should be tested for males showing mental retardation, short stature, lean body and microcephaly.