Memnune Yüksel Apak

Mutations in a newly identified GTPase gene cause autosomal dominant hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs; Strümpell-Lorrain syndrome, MIM number 18260) are a diverse class of disorders characterized by insidiously progressive lower-extremity spastic weakness (reviewed in refs. 1–3). Eight autosomal dominant HSP (ADHSP) loci have been identified, the most frequent of which is that linked to the SPG4 locus on chromosome 2p22 (found in ∼42%), followed by that linked to the SPG3A locus on chromosome 14q11–q21 (in ∼9%). Only SPG4 has been identified as a causative gene in ADHSP. Its protein (spastin) is predicted to participate in the assembly or function of nuclear protein complexes. Here we report the identification of mutations in a newly identified GTPase gene, SPG3A, in ADHSP affected individuals.

Genotype and phenotype in patients with dihydropyrimidine dehydrogenase deficiency

Abstract Dihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal recessive disease characterised by thymine-uraciluria in homozygous deficient patients and has been associated with a variable clinical phenotype. In order to understand the genetic and phenotypic basis for DPD deficiency, we have reviewed 17 families presenting 22 patients with complete deficiency of DPD. In this group of patients, 7 different mutations have been identified, including 2 deletions [295–298delTCAT, 1897delC], 1 splice-site mutation [IVS14+1G>A)] and 4 missense mutations (85T>C, 703C>T, 2658G>A, 2983G>T). Analysis of the prevalence of the various mutations among DPD patients has shown that the G→A point mutation in the invariant splice donor site is by far the most common (52%), whereas the other six mutations are less frequently observed. A large phenotypic variability has been observed, with convulsive disorders, motor retardation and mental retardation being the most abundant manifestations. A clear correlation between the genotype and phenotype has not been established. An altered β-alanine, uracil and thymine homeostasis might underlie the various clinical abnormalities encountered in patients with DPD deficiency.

Frequency of renal malformations in Turner syndrome: analysis of 82 Turkish children

Abstract We evaluated the frequency of renal malformations in relation to nonmosaic 45,X (group A, 45 patients, 54.9%) and mosaic/structural abnormalities of X (group B, 37 patients, 45.1%) in 82 Turkish patients with Turner syndrome (TS). Ultrasonography of the kidneys and collecting system was performed in all patients. Of the 82 patients, 31 had different renal malformations (37.8%). Horse-shoe kidney was observed in 9 (29.0%) of the 31 patients, and 17 patients (54.8%) had various collecting system malformations, while 5 (16.2%) had malrotation and other positional abnormalities. The prevalence of renal malformations was significantly higher in group A (51.1%) than group B (21.6%) (2:7.94, P<0.05). Although 8 of the 9 patients with horse-shoe kidney had the 45,X karyotype, collecting system malformations were observed more frequently in group B. Recurrent urinary tract infections (UTIs) were detected during follow-up in 7 patients, and hypertension developed in 3 patients. In patients who had a normal baseline nephrological evaluation, no problem suggesting renal disease developed during follow-up. We conclude that all forms of TS should have routine nephrological screening on diagnosis, since structural malformations of the kidney occur more frequently in nonmosaic 45,X TS, while collecting system malformations are mostly seen in mosaic/structural X forms. Those included in the group for nephrological follow-up had an increased risk for hypertension and/or UTI.

Unusual molecular findings in autosomal recessive spinal muscular atrophy.

All three types of autosomal recessive spinal muscular atrophy map to chromosome 5q11.2-q13.3 and are associated with deletions or mutations of the SMN (survival motor neurone) gene. The availability of a test to distinguish between the SMN gene and its nearly identical centromeric copy cBCD541 allows molecular diagnosis. We have analysed patients from 24 Belgian and 34 Turkish families for the presence or absence of a deletion in the SMN gene. A homozygous deletion in the SMN gene was seen in 90% of unrelated SMA patients. A non-radioactive SSCP assay allows for a semiquantitative analysis of the copy number of the centromeric and SMN genes. Hence, direct carrier detection has become feasible under certain conditions. We observed a phenotypically normal male, father of an SMA type I patient, presenting with only a single copy of the SMN gene and lacking both copies of the cBCD541 gene. This illustrates that a reduction of the total number of SMN and cBCD541 genes to a single SMN copy is compatible with normal life. In another SMA type I family, there is evidence for a de novo deletion of the centromeric gene in a normal sib. This observation illustrates the susceptibility of the SMA locus to de novo deletions and rearrangements.

Analysis of the CFTR gene in Turkish cystic fibrosis patients: identification of three novel mutations (3172delAC, P1013L and M1028I)

Abstract In order to determine the spectrum of cystic fibrosis (CF) mutations in the Turkish population, a complete coding region of the cystic fibrosis transmembrane conductance regulator (CFTR) gene including exon-intron boundaries, on 122 unrelated CF chromosomes from 73 Turkish CF families was analysed by denaturing gradient gel electrophoresis and multiplex heteroduplex analysis on MDE gel matrix. In addition to 15 previously reported mutations and 12 polymorphisms, three novel mutations, namely 3172delAC, P1013L and M1028I, were detected. ΔF508 was found to be present on 18.8% of CF chromosomes. The second most common mutation was 1677delTA, with a frequency of 7.3%, followed by G542X and 2183AA→G mutations, with frequencies of 4.9%. These four most common mutations in Turkish CF population account for approximately 36% of mutations. This study could only detect 52.5% of disease-causing mutations in this population; 47.5% of CF alleles remain to be identified, reflecting the high molecular heterogeneity of the Turkish population.

A new locus for autosomal recessive non‐syndromic mental retardation maps to 1p21.1–p13.3

Autosomal recessive inheritance of non‐syndromic mental retardation (ARNSMR) may account for approximately 25% of all patients with non‐specific mental retardation (NSMR). Although many X‐linked genes have been identified as a cause of NSMR, only three autosomal genes are known to cause ARNSMR. We present here a large consanguineous Turkish family with four mentally retarded individuals from different branches of the family. Clinical tests showed cognitive impairment but no neurological, skeletal, and biochemical involvements. Genome‐wide mapping using Human Mapping 10K Array showed a single positive locus with a parametric LOD score of 4.92 in a region on chromosome 1p21.1–p13.3. Further analyses using polymorphic microsatellite markers defined a 6.6‐Mb critical region containing approximately 130 known genes. This locus is the fourth one linked to ARNSMR.

Further delineation of Frank–ter Haar syndrome

The combination of the symptoms megalocornea, multiple skeletal anomalies, and developmental delay was first recognized as a separate entity by Frank et al. [ 1973 ] and subsequently confirmed by ter Haar et al. [ 1982 ]. The main characteristics are brachycephaly, wide fontanels, prominent forehead, hypertelorism, prominent eyes, macrocornea with or without glaucoma, full cheeks, small chin, bowing of the long bones, and flexion deformity of the fingers. Protruding, simple ears, and prominent coccyx bone can be also regarded as important diagnostic signs. Inheritance most likely is autosomal recessive. Several manifestations such as progressive “coarsening” of the face, hirsutism, gallstones, lingual papillomatosis, and cardiac valve anomalies all point to a possible metabolic basis of the disorder. Here we describe four patients, including three sibs of Turkish descent, with this entity.

CYP21A2 gene mutations in congenital adrenal hyperplasia: genotype-phenotype correlation in Turkish children.

Background: Congenital adrenal hyperplasia (CAH) due 21−hydroxylase deficiency (21−OHD) is a common autosomal recessive disorder. It is caused by defects in the CYP21A2 gene. Objective: Our aim was to determine the frequency of common gene mutations and to evaluate genotype−phenotype correlations in Turkish 21−OHD patients. Methods: Molecular analysis of the CYP21A2 gene was performed for the detection of the eight most common point mutations [p.P30L, IVS2−13C>G (IVS−2), p.I172N, exon 6 mutation cluster (p.I236N, p.V237E, p.M239K), p.V281L, p.Q318X, p.R356W, 8−bp−deletion], of large deletion and conversion by southern blotting, allele specific semi−quantitative PCR/enzyme restriction method and sequencing, in 56 patients with 21−OHD, from 52 families. Results: Disease−causing mutations were identified in 77 out of 91 alleles (84.6%) of the patients. Mutations were found in 34 of 43 alleles (79.1%) in salt wasting (SW; n=26), 32 of 36 alleles (88.8%) in simple virilizing (SV; n=24) and 11 of 12 alleles (91.6%) in non−classical (NC; n=6) form of CAH. The most frequent mutations were IVS−2 (22.0%), large conversion (14.3%), p.I172N (9.9%) p.R356W (8.8%), and large deletion (6.6%). In the SW form, the most frequent genotypes were homozygous for IVS−2 (11.5%) and homozygous for large conversion of the gene (11.5%). In the SV form, the most frequent genotype was homozygous for IVS−2 (20%), followed by compound heterozygous for p.I172N/8−bp del (10%). Homozygous for p.V281L (16.7%) was most common in NC. In most cases there was good correlation between genotype and phenotype. In the SW and NC forms, genotypes of all the patients correlated with their phenotypes. Conclusions: This is the first comprehensive study on the molecular basis of CAH patients in the Turkish population. Based on these results, we propose a modified screening strategy to facilitate molecular testing of CAH patients in our population. Conflict of interest:None declared.

Cystic fibrosis mutations and associated haplotypes in Turkish cystic fibrosis patients.

AbstractIdentification of mutations causing cystic fibrosis (CF) in the Turkish population is essential for assessment of the molecular basis of CF in Turkey and the development of strategies for prenatal diagnosis and genetic counseling. Here, we present an updated report of mutations found in the Turkish CF population from an extensive screening study of the entire coding region, including exon-intron boundaries and the promoter region. Cases for which mutations could not be identified were also screened for previously defined large alterations and (TG)mTn-M470V loci. This study revealed a total of 27 different mutations accounting for almost 60% of disease genes in the Turkish population. In this study, we also identified the haplotypes associated with 17 mutations and those associated with unknown mutations. The mutation spectrum of CF in Turkey and its associated haplotypes indicated the presence of a major Mediterranean component in the contemporary Turkish population.

An Insulin Receptor Mutant (Asp707 → Ala), Involved in Leprechaunism, Is Processed and Transported to the Cell Surface but Unable to Bind Insulin

We have identified a homozygous mutation near the carboxyl terminus of the insulin receptor (IR) α subunit from a leprechaun patient, changing Asp707 into Ala. Fibroblasts from this patient had no high affinity insulin binding sites. To examine the effect of the mutation on IR properties, the mutant IR was stably expressed in Chinese hamster ovary cells. Western blot analysis and metabolic labeling showed a normal processing of the mutant receptor to α and β subunits. No increase in high affinity insulin binding sites was observed on Chinese hamster ovary cells expressing the mutant receptor, and also, affinity cross-linking of 125I-labeled insulin by disuccinimidyl suberate to these cells failed to label the mutant α subunit. Biotinylation of cell surface proteins by biotin succinimidyl ester resulted in efficient biotinylation of the mutant IR α and β subunits, showing its presence on the cell surface. On solubilization of the mutant insulin receptor in Triton X-100-containing buffers, 125I-insulin was efficiently cross-linked to the receptor α subunit by disuccinimidyl suberate. These studies demonstrate that Ala707 IR is normally processed and transported to the cell surface and that the mutation distorts the insulin binding site. Detergent restores this site. This is an example of a naturally occurring mutation in the insulin receptor that affects insulin binding without affecting receptor transport and processing. This mutation points to a major contribution of the α subunit carboxyl terminus to insulin binding.