Chromosome evolution in Lophyohylini (Amphibia, Anura, Hylinae).

The hyline tribe Lophyohylini includes 87 species of treefrogs, of which cytogenetics aspects have been studied in less than 20% of them. In order to evaluate the evolution of some of its chromosome characters (NOR position, C-bands, and DAPI/CMA3 bands), we studied the karyotypes of 21 lophyohylines, 16 of them for the first time, and analyzed them in a phylogenetic context. Most species showed similar karyotypes regarding chromosome number (2n = 24) and morphology (FN = 48), excepting Phyllodytes edelmoi and Osteocephalus buckleyi with 2n = 22 (FN = 44) and 2n = 28 (FN = 50), respectively. The NOR location was variable among species and provided valuable phylogenetic information. This marker was located in pair 11 in all species of Trachycephalus, Itapotihyla langsdorffii, and Nyctimantis arapapa, representing the plesiomorphic condition of Lophyohylini. Besides, other apomorphic states were recovered for the clades comprising N. rugiceps and N. siemersi (NOR in pair 5), and Dryaderces pearsoni, Osteocephalus, and Osteopilus (NOR in pair 9). Phyllodytes presented variation for NORs position; they were in pair 2 in P. edelmoi, pair 7 in P. melanomystax, and pair 8 in P. gyrinaethes and P. praeceptor. Polymorphisms in size, number, and activity of this marker were observed for N. siemersi, Osteocephalus fuscifacies, and some species of Trachycephalus. Remarkably, in N. siemersi NORs were detected on a single chromosome in the two specimens studied by this technique, raising the question of how this complex polymorphism is maintained. Interstitial telomeric sequences were found in P. edelmoi, P. melanomystax, and Osteocephalus buckleyi, and their presence seems to be not related to the chromosome reorganization events. Finally, some species showed spontaneous rearrangements, possibly as a consequence of an uncommon phenomenon in anuran cytogenetics: the presence of fragile sites or secondary constrictions not associated with NORs. We propose that this rare feature would have played an important role in the evolution of this group of frogs. From the evidence obtained in this and previous studies, we conclude that Lophyohylini presents a complex chromosome evolution.

The epicenter of chromosomal fragility of Fra14A2, the mouse ortholog of human FRA3B common fragile site, is largely attached to the nuclear matrix in lymphocytes but not in other cell types that do not express such a fragility.

Common fragile sites (CFSs) correspond to chromosomal regions susceptible to present breaks, discontinuities or constrictions in metaphase chromosomes from cells subjected to replication stress. They are considered as genomic regions intrinsically difficult to replicate and they are evolutionary conserved at least in mammals. However, the recent discovery that CFSs are cell-type specific indicates that DNA sequence by itself cannot account for CFS instability. Nevertheless, the large gene FHIT that includes FRA3B, the most highly expressed CFS in human lymphocytes, is commonly deleted in a variety of tumors suggesting a tumor suppressor role for its product. Here, we report that the epicenter of fragility of Fra14A2/Fhit, the mouse ortholog of human FRA3B/FHIT that like its human counterpart is the most highly expressed CFS in mouse lymphocytes, is largely attached to the nuclear matrix compartment in naive B lymphocytes but not in primary hepatocytes or cortical neurons that do not express such a CFS. Our results suggest a structural explanation for the difficult-to-replicate nature of such a region and so for its common fragility in lymphocytes, that is independent of the possible tumor suppressor role of the gene harboring such CFS.

45S rDNA sites in meiosis of Lolium multiflorum Lam.: variability, non-homologous associations and lack of fragility.

In this study, we evaluated the behavior of 45S ribosomal DNA (rDNA) sites during the meiosis of Lolium multiflorum. The reason to study it in this species is that 45S rDNA sites are usually visualized as gaps in mitotic metaphase chromosomes and were initially denominated fragile sites (FSs). In different species, FSs were related to rearrangements that alter the karyotype and affect the chromosome pairing in meiosis. However, our findings show that the chromosome pairing in L. multiflorum is regular and, as in mitosis, the number of sites is variable. In diakinesis with five sites, one of the bivalents was in hemizygous state while, in diakinesis with seven sites, one of the bivalents had three conspicuous signals, two in syntheny in one of the homologous. Only four cells had gaps in the region of the 45S rDNA. Owing to the lower number of signals observed at the initial stages of meiosis, it is assumed that they are involved both in homologous and non-homologous associations and that they might assist the chromosome pairing. Regarding segregation, only meiocytes with five and six 45S rDNA signals were observed, and they were characterized by the segregation of 2/3 signals in the poles of anaphases I up to metaphases II; 2/2 and 3/3 in anaphases II and telophases II; and also 2/2 and 4/4 in the nuclei of tetrads, unlike the number of 45S signals expected. The numerical non-equivalence of sites among nuclei at later stages of meiosis is explained by the presence of chromosomes with hemizygous sites.

Relationship between epigenetic marks and the behavior of 45S rDNA sites in chromosomes and interphase nuclei of Lolium-Festuca complex.

The grasses of the Lolium-Festuca complex show a prominent role in world agricultural scenario. Several studies have demonstrated that the plasticity of 45S rDNA sites has been recently associated with the possible fragility of the loci. Often, these fragile sites were observed as extended sites and gaps in metaphases. This organization can be evaluated in relation to their transcriptional activity/accessibility through epigenetic changes. Thus, this study aimed to investigate the relationship of the 5-methylcytosine and histone H3 lysine-9 dimethylation in different conformations of 45S rDNA sites in interphase nuclei and in metaphase chromosomes of L. perenne, L. multiflorum and F. arundinacea. The FISH technique using 45S rDNA probes was performed sequentially after the immunolocalization. The sites showed predominantly the following characteristics in the interphase nuclei: intra- and perinucleolar position, decondensed or partially condensed and hypomethylated and hyper/hypomethylated status. Extranucleolar sites were mainly hypermethylated for both epigenetic marks. The 45S rDNA sites with gaps identified in metaphases were always hypomethylated, which justifies it decondensed and transcriptional state. The frequency of sites with hypermethylated gaps was very low. The structural differences observed in these sites are directly related to the assessed epigenetic marks, justifying the different conformations throughout the cell cycle.

Dynamics of 45S rDNA sites in the cell cycle: fragile sites and chromosomal stability in Lolium and Festuca.

Analyses carried out with fluorescence in situ hybridization (FISH) in C-metaphases of the Lolium-Festuca complex have shown the occurrence of spontaneous fragile sites (FSs) in 45S rDNA regions. FSs are expressed as gaps but they do not result in breaks or chromosomal fragments in these species. These gaps have high DNA condensation observed as thin chromatin fibers that connect the apparent segments of the fragile chromosome, allowing for genomic stability. Assessing the behavior of these regions in the cell cycle of Lolium and Festuca species may lead to a better understanding of the dynamics that preserve stability during cell division. Furthermore, it is interesting to track the dynamics of chromosomes bearing 45S rDNA sites in the cell cycle as well as to observe the expression of FSs with no effect of the mitotic block. We observed variation in both the number and size of 45S FISH signals from the S/G2 phases of interphase and from prophase to anaphase where gaps in 45S rDNA sites also were observed. The change in the degree of condensation of the 45S site begins in the S/G2 phase and appears to be related to the transcriptional demand. Taking into account that the number of 45S rDNA sites tends to be re-established when cells reach telophase, we suggest that the chromatin fiber goes back to the normal condensation level to the anaphase (after segregation), allowing for the approximation of chromosome segments and ensuring dynamics that favor the genomic stability of these species.

Fragile sites, dysfunctional telomere and chromosome fusions: What is 5S rDNA role?

Repetitive DNA regions are known as fragile chromosomal sites which present a high flexibility and low stability. Our focus was characterize fragile sites in 5S rDNA regions. The Ancistrus sp. species shows a diploid number of 50 and an indicative Robertsonian fusion at chromosomal pair 1. Two sequences of 5S rDNA were identified: 5S.1 rDNA and 5S.2 rDNA. The first sequence gathers the necessary structures to gene expression and shows a functional secondary structure prediction. Otherwise, the 5S.2 rDNA sequence does not contain the upstream sequences that are required to expression, furthermore its structure prediction reveals a nonfunctional ribosomal RNA. The chromosomal mapping revealed several 5S.1 and 5S.2 rDNA clusters. In addition, the 5S.2 rDNA clusters were found in acrocentric and metacentric chromosomes proximal regions. The pair 1 5S.2 rDNA cluster is co-located with interstitial telomeric sites (ITS). Our results indicate that its clusters are hotspots to chromosomal breaks. During the meiotic prophase bouquet arrangement, double strand breaks (DSBs) at proximal 5S.2 rDNA of acrocentric chromosomes could lead to homologous and non-homologous repair mechanisms as Robertsonian fusions. Still, ITS sites provides chromosomal instability, resulting in telomeric recombination via TRF2 shelterin protein and a series of breakage-fusion-bridge cycles. Our proposal is that 5S rDNA derived sequences, act as chromosomal fragile sites in association with some chromosomal rearrangements of Loricariidae.

Decondensation of chromosomal 45S rDNA sites in Lolium and Festuca genotypes does not result in karyotype instability.

Fragile sites (FSs) in plants have been described for species like Lolium and other grasses. Whereas in humans FSs were shown to be involved in genome instabilities; the consequences of FSs expression in plants are not known yet. To evaluate whether FSs cause karyotype instabilities, we assessed the frequency of micronuclei and lagging chromosomes in meristematic cells, the stability of the DNA content, and the occurrence of neocentromeres in the presumed chromosomal fragments of Lolium perenne, Lolium multiflorum, Festuca arrundinacea, and two Festulolium hybrids. The cell cycle analysis along with flow cytometric genome size measurements showed high stability in all genomes evaluated. Neocentromeric activity was neither observed in the presumed fragments nor in any other chromosomal region, then this is not the mechanism responsible by the stability. However, Fluorescence in situ hybridization (FISH) with a 45S ribosomal DNA (rDNA) probe in combination with YOYO staining of metaphasic chromosomes showed that many extended nucleolus organizing region (NOR) form very thin YOYO-positive chromatin fibers connecting the acentric 'fragment' with the centromere-containing chromosome region. The obtained data indicate that the expression of FSs does not result in genome instabilities or neocentromere formation. The FS-containing 45S rDNA carrying chromatin fibers undergo a cell cycle and gene activity-dependent dynamic decondensation process.

Fragile sites of 45S rDNA of Lolium multiflorum are not hotspots for chromosomal breakages induced by X-ray.

Sites of 45S rDNA of Lolium are regions denominated fragile sites (FSs), constituting regions slightly stained with DAPI due to increased DNA unpacking in metaphasic chromosomes. Considered to be fragile regions in the genome, the FSs might be more responsive to induced breaks and result in chromosomal fragments and rearrangements, unless repairing mechanisms such as recombination or de novo telomere formation play a role at the break site of the DNA. Thus, this study aimed at investigating if SFs from Lolium are hotspots for the occurrence of breakages induced by X-ray and if they are regions favorable to synthesize new telomeres, using Hordeum vulgare as a comparative model. Lolium multiflorum and H. vulgare seedlings were irradiated with 20 and 50 Gy X-ray and evaluated one day following the irradiation and at 7-days intervals for a 28-days period, using FISH technique with 45S rDNA and Arabidopsis-type telomere probes in order to investigate the presence of chromosomal breakages and new telomere formation. H. vulgare did not survive after a few days of irradiation due to the increased rate of abnormalities. L. multiflorum also exhibited chromosomal abnormalities following the exposure, yet over the 28-days trial it had a decrease in the chromosomal damage rate and formation of de novo telomere has not been detected along this time. Despite being considered to be fragile regions in the genome, the 45S rDNA sites of Lolium are not hotspots to chromosomal breakages after the induction of breakages.

Functional repetitive sequences and fragile sites in chromosomes of Lolium perenne L.

Lolium perenne is considered a high-quality forage widely used in temperate regions to meet the shortage of forage during the winter. In this species, some peculiarities related to cytogenetic aspects have already been described, as the variability in number and position of 45S ribosomal DNA (rDNA) sites and the expression of fragile sites, which require further studies to support the understanding of their causes and consequences. In this way, this study aimed to evaluate the relationship between the expression of fragile sites and functional repetitive sequences (rDNA and telomeric) in chromosomes of diploid and polyploid cultivars of L. perenne. The techniques of FISH, Ag-NOR and fluorescence banding were used to assess the distribution of sites of 45S rDNA, 5S, telomeric sequences, and the transcriptional activity of the 45S ribosomal genes and the distribution of AT- and/or GC-rich sequences in L. perenne, respectively. There was variability in the number and location of 45S rDNA sites, which was not observed for 5S rDNA sites. One of the genotypes showed two 45S rDNA sites on the same chromosome, located in different chromosome arms. Breaks and gaps were found in 45S rDNA sites in most metaphases evaluated for both cultivars. Telomeric sequences were not detected at the end of the chromosomal fragments corresponding to the location of breaks at 45S sites. Apparently, the transcriptional activity was modified in fragile sites. Variation in the number and size of nucleoli, nucleolar fusions and dissociations were observed. All CMA(+) bands were colocalized with the 45S sites.

[Cytogenetic study in peripheral blood of melanoma patients]. / Estudio citogenético en sangre periférica de pacientes con melanoma.

Among all the skin diseases, melanoma is the main cause of death in Colombia (40%) and it represents 1% of all deaths by cancer. Due to the fast increase in the incidence of melanoma, it is necessary to carry out research on the mechanisms involved in its genesis and progression. This study determined chromosomal anomalies from peripheral blood samples on 30 patients with melanoma and on 23 control subjects using conventional cytogenetics (G Banded), where a high incidence in numerical anomalies and a low incidence in recurrent structural rearrangements were observed. Chromosomic losses were prevalent in all the tumor stages studied. The analysis showed that the chromosomes X, 9 and 17 were mainly affected. Among the numerical anomalies, monosomies in X and 17 chromosomes, as well as trisomies formed by a marker chromosome, were the most common in both early and late stages of the disease. Deletions and chromosomal crossovers appeared to be as isolated anomalies. In the control group no anomaly was identified, and a low percentage of fragility was observed when compared with the patients group. A high frequency in chromosomal anomalies was observed in patients, in contrast with the control subjects. This suggests the existence of heterogeneity and genetic predisposition during the illness development. To further research, these must be analyzed and validated as possible sources of molecular markers, which could be of use for the early diagnosis, treatment and follow up of the disease.

Molecular approach of the fragile chromosomal region Xq31-34 in cattle (Bos taurus) by microdissection and DOP-PCR / Aproximação molecular da região cromossômica frágil Xq31-34 em bovinos (Bos taurus) utilizando microdissecação cromossômica e DOP-PCR

Fragile sites (FS) are chromosomal regions where the normal compactation of chromatine is not observed. FRAXA (Fra Xq27.3, X sexual chromosome) is one of the most studied FS in humans. FRAXA is an expansion of the trinucleotide CGG located in the gene FMR-1. In cattle, sites of chromosomal fragility were reported in BTAX, associated with different pathologies and fertility impairment. Chromosomal microdissection has became a valuable tool for isolating chromatine fragments. In this work, it was combined the chromosomal microdissection technique with DOP-PCR in order to carry out a molecular analysis of the fragile chromosomal region BTAXq31-34. In that region, polymorphic DNA-RAPD sequences (GC rich) are present and sequences of the gene FMR-1 are missing. The results showed the usefulness of the microdissection-DOP-PCR technique for molecular characterization of fragile chromosomal sites in cattle.(AU)

Os sítios frágeis (FS) são regiões de cromossomo onde a compactação normal da cromatina não é realizada. O FRAXA (Fra Xq27.3, cromossomo sexual X) é um dos FS mais estudados em seres humanos. O FRAXA apresenta expansão do trinucleotídeo CGG localizado no gene FMR-1. Em bovinos, existem estudos informando sobre fragilidade cromossômica em BTAX associada com diversas patologias e alterações na fertilidade. A microdissecação cromossômica é uma valiosa técnica para isolar fragmentos de cromatina. Neste trabalho, combinou-se a técnica de microdissecação de cromossomo com DOP-PCR para executar a análise molecular da região do sitio frágil cromossômico BTAXq31-34. Naquela região estão presentes seqüências do polimorfo DNA-RAPD (rico em GC), em que as seqüências do gene FMR-1 estão ausentes. Os resultados mostram a utilidade da técnica de microdissecação-DOP-PCR para a caracterização molecular de sítios frágeis cromossômicos em bovinos.(AU)

Molecular approach of the fragile chromosomal region Xq31-34 in cattle (Bos taurus) by microdissection and DOP-PCR / Aproximação molecular da região cromossômica frágil Xq31-34 em bovinos (Bos taurus) utilizando microdissecação cromossômica e DOP-PCR

Fragile sites (FS) are chromosomal regions where the normal compactation of chromatine is not observed. FRAXA (Fra Xq27.3, X sexual chromosome) is one of the most studied FS in humans. FRAXA is an expansion of the trinucleotide CGG located in the gene FMR-1. In cattle, sites of chromosomal fragility were reported in BTAX, associated with different pathologies and fertility impairment. Chromosomal microdissection has became a valuable tool for isolating chromatine fragments. In this work, it was combined the chromosomal microdissection technique with DOP-PCR in order to carry out a molecular analysis of the fragile chromosomal region BTAXq31-34. In that region, polymorphic DNA-RAPD sequences (GC rich) are present and sequences of the gene FMR-1 are missing. The results showed the usefulness of the microdissection-DOP-PCR technique for molecular characterization of fragile chromosomal sites in cattle.

Os sítios frágeis (FS) são regiões de cromossomo onde a compactação normal da cromatina não é realizada. O FRAXA (Fra Xq27.3, cromossomo sexual X) é um dos FS mais estudados em seres humanos. O FRAXA apresenta expansão do trinucleotídeo CGG localizado no gene FMR-1. Em bovinos, existem estudos informando sobre fragilidade cromossômica em BTAX associada com diversas patologias e alterações na fertilidade. A microdissecação cromossômica é uma valiosa técnica para isolar fragmentos de cromatina. Neste trabalho, combinou-se a técnica de microdissecação de cromossomo com DOP-PCR para executar a análise molecular da região do sitio frágil cromossômico BTAXq31-34. Naquela região estão presentes seqüências do polimorfo DNA-RAPD (rico em GC), em que as seqüências do gene FMR-1 estão ausentes. Os resultados mostram a utilidade da técnica de microdissecação-DOP-PCR para a caracterização molecular de sítios frágeis cromossômicos em bovinos.

Molecular alterations in the tumor suppressor gene WWOX in oral leukoplakias.

Oral leukoplakia is the most prevalent and potentially malignant disorder of the oral mucosa. Previous studies have demonstrated that molecular changes of the WWOX gene (WW-domain containing oxidoreductase), a candidate tumor suppressor gene located at 16q23.3-24.1 that spans FRA16D, the second most common fragile site, are present in several malignant neoplasias, including oral squamous cell carcinoma. In this report, the role of the WWOX gene was investigated in 23 cases of oral leukoplakias. Using nested RT-PCR and immunohistochemistry, altered mRNA transcription and/or reduced Wwox protein expression was observed in 35% of the lesions when compared with normal mucosa. The majority of lesions (4/6) with altered transcripts had a reduction in the expression of Wwox protein. Although normal WWOX expression was found in some lesions with dysplasia, all lesions with WWOX mRNA and/or protein expression showed histological evidence of dysplasia and none of the cases without dysplasia presented this alteration. These results show that the WWOX gene alteration is an early genetic alteration and may contribute to oral carcinogenesis.

Association between decreased WWOX protein expression and thyroid cancer development.

CONTEXT: Chromosomal fragile sites are often related to cancer development. The WW domain-containing oxidoreductase gene (WWOX) spans the second most common chromosomal fragile site (FRA16D) and encodes an important proapoptotic protein. OBJECTIVE: To verify our hypothesis that underexpression of WWOX could contribute to malignant transformation of the thyroid cells. METHOD: We compared WWOX expression among follicular adenomas (FAs) and differentiated thyroid carcinomas [follicular thyroid carcinomas (FTCs) and papillary thyroid carcinomas (PTCs)] in 53 thyroid tumors resected from patients submitted to total thyroidectomy. DESIGN: Multiple fields of tumor areas of FAs, FTCs, and PTCs as well as normal thyroid tissue were stained with WWOX antiserum, and classified by the extent of staining (percentage of cells staining) and staining intensity. MAIN OUTCOME: PTCs showed a significantly decreased expression of WWOX when compared to FAs and FTCs. Further, using a unique model of comparison in patients in whom FAs and PTCs were concomitantly present, we detected the same result (i.e., no expression in PTCs). CONCLUSION: We conclude that WWOX underexpression is an important step that might increase the vulnerability to the carcinogenesis process in PTCs.

Laboratorial diagnosis of fragile-X syndrome: experience in a sample of individuals with pervasive developmental disorders.

Fragile X syndrome is a frequent genetic disease associated to developmental disorders, including learning disability, mental retardation, behavioral problems and pervasive developmental disorders (autism and related conditions). We studied a sample of 82 individuals (69 males and 13 females) presenting with pervasive developmental disorders using three techniques for the diagnosis of fragile X syndrome (FXS). Cytogenetic analysis detected the fragile site in four males, but only one showed a consistent positive rate. Molecular study based on the PCR technique was inconclusive for most females (92.3%), which where latter submitted to Southern blotting analysis, and for one male (1.4%), excluding the FRAXA mutation in the remaining male individuals (98.6%). Molecular tests using the Southern blotting technique confirmed only one positive case (1.2%) in a male subject. These results showed that Southern blotting analysis of the FRAXA mutation has the best sensitivity and specificity for the diagnosis of FXS but also validated the PCR technique as a confinable screening test.

Laboratorial diagnosis of fragile-X syndrome: experience in a sample of individuals with pervasive developmental disorders

A síndrome do cromossomo X frágil (SXF) é uma doença genética freqüente associada a distúrbios do desenvolvimento neurológico, incluindo dificuldades de aprendizagem, retardo mental, problemas comportamentais e distúrbios invasivos do desenvolvimento (autismo e correlatos). Estudamos uma amostra de 82 indivíduos (69 homens e 13 mulheres) apresentando distúrbios invasivos do desenvolvimento, utilizando três técnicas para o diagnóstico da SXF. A análise citogenética detectou a presença do sítio frágil em quatro homens, porém apenas um deles com percentagem consistente. O estudo molecular baseado na técnica da PCR foi inconclusivo para a maioria das mulheres (92,3%), as quais foram posteriormente submetidas a análise por Southern blotting, e para um homem (1,4%), excluindo a mutação FRAXA nos demais homens (98,6%). O teste molecular usando a técnica de Southern blotting confirmou apenas um caso positivo (1,2%) em um indivíduo do sexo masculino. Tais resultados mostraram que a técnica de Southern blotting para análise da mutação FRAXA apresenta a melhor sensibilidade e especificidade para o diagnóstico da SXF, mas também valida a técnica da PCR como um teste confiável para seu rastreamento.

More on geographic hematology: the breakpoint cluster regions of the PML/RARalpha fusion gene in Mexican Mestizo patients with promyelocytic leukemia are different from those in Caucasians.

Acute promyelocytic leukemia is characterized the PML/RARalpha chimeric fusion gene, transcript and protein; the breakpoint cluster regions (bcr) in the PML gene may occur in 3 different sites: Intron 6 (bcr1), exon 6 (bcr2) or intron 3 (bcr3). In a 10-year period in a single institution, we studied prospectively the breakpoint cluster regions of the PML/RARalpha fusion gene in 43 Mexican Mestizo patients with APL, and found that the bcr1 represented 62.7%, the bcr2 9.3% whereas the bcr3 27.9%. The prevalence of the bcr1 subtype is significantly higher than that informed in Caucasians and similar to that in Asians; these data are consonant with those described in other Latin-American patients with APL. Since other Asian genetic markers have been found in the Indian component of the Mexican mestizos, it is possible that the Asian immigration into the Americas through the strait of Behring 12,000 years ago may account for a possible genetic susceptibility to suffer certain forms of APL.

Chromosomal studies on sphaerodactyl lizards of genera Gonatodes and Coleodactylus (Squamata, Gekkonidae) using differential staining and fragile sites analyses.

The karyotypes of three species of sphaerodactyl gekkonid lizards are described after conventional and differential staining. Karyotypes of Gonatodeshumeralis and G. hasemani are formed by a gradual series of 32 acrocentric chromosomes, similar to those already published for other species of the genus. G. humeralis shows multiple Ag-NORs with intra-individual variability, and positive C-bands located at centromeric and telomeric regions of several chromosome pairs. Coleodactylus amazonicus, the first non-Gonatodes sphaerodactyl studied so far karyologically, exhibits 36 acrocentric/subtelocentric chromosomes and a single pair of Ag- NORs. Fragile sites were detected on two medium-sized chromosome pairs in the karyotype of G. humeralis, most of them obtained in BrdU-treated culture preparations. These sites may represent a putative fission/fusion spot involved in the differentiation of G. humeralis-like 2n = 32 and C. amazonicus-like 2n = 36 karyotypes. Our results, especially on the location of Ag-NORs and the description of fragile sites, are relevant in improving our knowledge about the events of chromosome evolution in this extremely variable and poorly known group of lizards.

Localisation of aphidicolin-induced break points in Holstein-Friesian cattle (Bos taurus) using RBG-banding.

Fragile sites (FS) seem to play a role in genome instability and may be involved in karyotype evolution and chromosome aberrations. The majority of common fragile sites are induced by aphidicolin. Aphidicolin was used at two different concentrations (0.15 and 0.30 microM) to study the occurrence of FS in the cattle karyotype. In this paper, a map of aphidicolin induced break points and fragile sites in cattle chromosomes was constructed. The statistical analysis indicated that any band with three or more breaks was significantly damaged (P<0.05). According to this result, 30 of the 72 different break points observed were scored as fragile sites. The Pearson correlation test showed a positive association between chromosome length and the number of fragile sites (r=0.54). On the contrary, 21 FS were identified on negative R bands while 9 FS were located on positive R bands.

[Medical pathology due to trinucleotide repeats]. / Patología médica ocasionada por repetidos de trinucleótidos.

Trinucleotide repeat expansion is responsible for ten human diseases described so far. Four types of repeats are involved in these expansions, with type, number and position in the gene varying from one disease to another. In some fragile sites, the trinucleotide repeat is found to be enlarged to 200 or more. Smaller expansions have been found within coding regions of some genes that are associated with neurodegenerative diseases, such as Huntington's disease. The continuous expansion of the trinucleotide repeats in subsequent generations explains the genetic anticipation, peculiar to these disorders. Recently, it was shown that two expanded minisatellite sequences are also involved in both progressive myoclonus epilepsy type 1 and distamycin A-sensitive fragile site, FRA16B. This form of peculiar heredity is very important because of its relationship with some of the common human degenerative diseases.