Somatic and Germinal Mosaicism for the Steroid Sulfatase Gene Deletion in a Steroid Sulfatase Deficiency Carrier.ppt


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Slide 1 : Somatic and Germinal Mosaicism for the Steroid Sulfatase Gene Deletion in a Steroid Sulfatase Deficiency Carrier MD Sergio Alberto Cuevas-Covarrubias, Hospital General de Mexico, Facultad de Medicina, Universidad Nacional Autónoma de Mëxico, Mexico THE JOURNAL OF INVESTIGATIVE DERMATOLOGY. VOL. 119, NO. 4 OCTOBER 2002
Slide 2 : Steroid sulfatase (STS) deficiency clinically results in the inborn error of metabolism known as X-linked ichthyosis (XLI) XLI is characterized by dark, adherent and polygonal scales of skin and has an incidence of 1: 2000–6000 males BACKGROUND
Slide 3 : The STS gene is located on the short arm of the X-chromosome (Xp22.1), close to the pseudoautosomal region Most XLI patients have large deletions of the STS gene and flanking markers BACKGROUND
Slide 4 : It appears that STS deletion is due to the presence of CRI-S232S sequences at either side of the VCXA and VCXB genes due to nonallelic homologous recombination (NAHR) VCX-A HDHD1A STS VCX-B1 VCX-B Xptel Xcen genes BACKGROUND
Slide 5 : The family consisted of the proband, grandmother, mother, and sister. The patient was referred to the Genetic Department of Hospital General de Mexico as having ichthyosis BACKGROUND
Slide 6 : STS activity was determined in leukocytes using 7-[3H]-dehydroepiandrosterone sulfate as a substrate Genomic DNA was obtained from leukocytes The STS gene was analyzed by polymerase chain reaction (PCR) and fluorescence in situ hybridization analysis (FISH) Telomeric--DXS89--DXS996--DXS1139--DXS1130--5’STS—3’STS--DXS1131--DXS1133--DXS237--DXS1132--DXF22S1--DXS278--DXS1134—centromeric regions were amplified by PCR Segregation analysis was performed through GeneScan using the ABI Prism Linkage Mapping Set MATERIAL AND METHODS
Slide 7 : STS activity was undetectable in the proband, very low in the mother as an XLI carrier and normal in the grandmother and sister Polymerase chain reaction failed to amplify the 2 Mb region that included the STS gene and flanking markers 5’-DXS1139, DXS1130 and 3’-DXS1131, DXS1133, DXS237, DXS1132, and DXS22S1 RESULTS
Slide 8 : Fluorescence in situ hybridization analysis showed one copy of the STS gene in 80% of leukocytes and in 98.5% of oral cells of the patient's mother and two copies of the gene in leukocytes of the grandmother and sister. Red mark corresponds to the STS gene (white arrows), whereas the green mark is a centromeric X-chromosome probe used as an internal control. 80% 20% RESULTS
Slide 9 : Segregation analysis showed that allele DXS987 208 bp long (located on X-chromosome) was present in the proband and his mother but not in the grandmother, indicating that the STS gene deletion occurred in the X-chromosome of the grandfather RESULTS
Slide 10 : In XLI, the major molecular defect is complete deletion of the STS gene and flanking markers Nevertheless, some partial deletions or point mutations of the STS gene have also been reported This is the first case of XLI in which the mother is carrying two line cells that differ genetically, one harbored two copies of the STS gene and the other only had one copy of the STS gene DISCUSION
Slide 11 : A great variety of diseases with different inherited patterns and somatic and/or germinal mosaicism have been reported Point mutations have been reported in X-linked chondrodysplasia punctata, ornithine transcarbamylase deficiency, Hunter disease, myotubular myopathy, and X-linked deafness type 3 Minor intragenic deletions have found to be present in X-linked agammaglobulinemia, dyskeratosis congenita, and X-linked chondrodysplasia punctata DISCUSION
Slide 12 : Insertions have been observed in the CYBB gene of chronic granulomatous disease and severe combined immunodefyciency The presence of triple mosaicism has been reported in the CYBB gene Reversion of an initial molecular defect has been observed in the Wiskott-Aldrich syndrome DISCUSION
Slide 13 : In most cases, genetic mosaicism is due to different aberrant mechanisms as: Unequal homologous crossovers Unequal sister or nonsister chromatid exchanges Excision of intrachromatid loops Inadequate incorporation of nucleotides in the replication process Reversion of an initial molecular defect Events of duplication-deletion due to the formation of an overlying loop followed by an uneven crossover at the level of the DNA strand DISCUSION
Slide 14 : In our XLI patient, considering the presence of VNTR on either side of the STS gene, mosaicism could be the consequence of an homologous recombination during the first mitotic divisions in the embryogenesis period Nevertheless, absence of a second copy of the STS gene in the sister or nonsister chromatid excludes this mechanism DISCUSION
Slide 15 : The STS gene deletion could be the result of a DNA slippage mechanism. In this mechanism the mutation arises by slipped strand mispairing that originates a single-stranded loop, followed by DNA elongation, strand breaking and the formation of a mismatch bubble DISCUSION
Slide 16 : DNA slippage mechanism Initial DNA replication (A, A’, B and B’ are all similar repetitive sequences) Slipped strand mispairing ( B with A’) originates a single-stranded loop Strand breaking with the repetitive sequence (A) Final of replication, one daughter double strand lacks the A-A’ repetitive sequence

 


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