Oral follicular lymphoma: a clinicopathologic and molecular study.

Follicular lymphoma is a hematolymphoid neoplasm that originates from germinal center B cells. It is made up of a combination of small cleaved centrocytes and a varying quantity of larger non-cleaved centroblasts to describe the clinical, microscopic, immunohistochemical, and molecular features of oral follicular lymphomas. Follicular lymphomas affecting the oral cavity were retrieved from pathology files. Immunohistochemistry was performed to confirm the diagnosis, and fluorescence in situ hybridization (FISH) was employed to detect rearrangements in BCL2, BCL6, and MYC genes. Clinical and follow-up data were obtained from the patient's medical and pathology files. Twenty cases were obtained. There was an equal sex distribution (10 males: 10 females) and a mean age of 60.9 years (range: 10-83 years-old). Lesions presented as asymptomatic swellings, usually in the palate (10 cases) and the buccal mucosa (7 cases). Five patients presented with concomitant nodal involvement. Microscopic evaluation depicted the follicular growth pattern with diffuse areas in six cases. Grades 1 and 2 follicular lymphomas represented 12 cases, while grade 3A neoplasms accounted for other 8 cases. Two cases showed rearrangements in MYC, BCL2, and BCL6 genes, while single BCL2 translocation was found in eight cases. Two cases had no translocation. Three patients deceased and the 2-year overall survival achieved 88%. Follicular lymphoma affecting the oral cavity is uncommon, usually affects the palate as a non-ulcerated swelling and the presence of a systemic disease most always be ruled out.

Biased Clonal Evolution in Acute Promyelocytic Leukemia through Imbalances Affecting the der(17) but Not the der(15) Chromosome: Report of Two Cases.

Acute promyelocytic leukemia (APL) is characterized by the chromosomal translocation t(15;17)(q24;q21), raising two hybrid genes: PML::RARA and RARA::PML. There is a biased clonal evolution in APL since imbalances affecting the der(15) chromosome (the one that carries the transforming PML::RARA gene) have never been reported; instead, imbalances of the der(17), mainly in form of an ider(17)(q10), have been repeatedly documented. We here present two cases with APL who acquired an ider(17)(q10) as a secondary chromosomal change. The presence of the ider(17)(q10) implies several genomic consequences with potential to fuel tumor progression: (1) a duplication of the hybrid gene RARA::PML; (2) a cumulative haploinsufficiency for tumor suppressor genes located in the 17p arm; and (3) a cumulative triplosensitivity of genes located in 17q10→RARA::PML→15qter. Both our patients were treated following the PETHEMA LPA 2012 protocol with ATRA plus idarubicin and they have had a long event-free survival.

Molecular and clinical insights into complex genomic rearrangements related to MECP2 duplication syndrome.

MECP2 duplication syndrome (MDS) is caused by copy number variation (CNV) spanning the MECP2 gene at Xq28 and is a major cause of intellectual disability (ID) in males. Herein, we describe two unrelated males harboring non-recurrent complex Xq28 rearrangements associated with MDS. Copy number gains were initially detected by quantitative real-time polymerase chain reaction and further delineated by high-resolution array comparative genomic hybridization, familial segregation, expression analysis and X-chromosome inactivation (XCI) evaluation in a carrier mother. SNVs within the rearrangements and/or fluorescent in situ hybridization (FISH) were used to assess the parental origin of the rearrangements. Patient 1 exhibited an intrachromosomal rearrangement, whose structure is consistent with a triplicated segment presumably embedded in an inverted orientation between two duplicated sequences (DUP-TRP/INV-DUP). The rearrangement was inherited from the carrier mother, who exhibits extreme XCI skewing and subtle psychiatric symptoms. Patient 2 presented a de novo (X;Y) unbalanced translocation resulting in duplication of Xq28 and deletion of Yp, originated in the paternal gametogenesis. Neurodevelopmental trajectory and non-neurological symptoms were consistent with previous reports, with the exception of cerebellar vermis hypoplasia in patient 2. Although both patients share the core MDS phenotype, patient 1 showed MECP2 transcript levels in blood similar to controls. Understanding the molecular mechanisms related to MDS is essential for designing targeted therapeutic strategies.

Coexistence of Fragile-X Syndrome, 8p23.1 Deletion, and Balanced Translocation t(7;10)(p10;q24) in a Single Family.

Aims: Fragile-X syndrome (FXS) is the most common inherited form of intellectual disability; it is caused by an abnormal CGG-repeat expansion at the FMR1 gene. However, a few cases of girls with mutations in the FMR1 gene have been reported in the literature. In this study, we describe the clinical and genetic assessment of a family who exhibits the unusual coexistence of FXS, an 8p23.1 deletion, and balanced translocation t(7;10)(p10;q24) in multiple members, including a symptomatic girl with FXS. Materials and Methods: All of the family members underwent comprehensive clinical and neurological examinations. All members of the family were also molecularly diagnosed using a combination of fluorescent-polymerase chain reaction (PCR), Triplet Repeat Primed-PCR, capillary electrophoresis, and karyotyping. Results: We identified a male proband and a female patient that presented with the craniofacial characteristics of FXS, neuropsychomotor developmental delay, speech delay, intellectual deficit, and a positive molecular diagnosis of FXS. Interestingly, the female patient presented with a severe phenotype also associated with the presence of 8p23.1 deletion, while the proband patient presented a balanced translocation t(7;10)(p10;q24). Moreover, we detected multiple carriers of the FXS premutation in the family. Conclusions: To our knowledge, we describe for the first time the simultaneous occurrence of FXS and an 8p23.1 deletion and their possible synergistic effects on the phenotype of a female patient. Moreover, we describe the coexistence of FXS, an 8p23.1 deletion, and a balanced translocation t(7;10)(p10;q24) in the same family.

EZH2 expression is dependent on MYC and TP53 regulation in diffuse large B-cell lymphoma.

EZH2 is an important epigenetic regulator, but its role in diffuse large B-cell lymphoma (DLBCL) pathogenesis and its relationship with MYC, BCL2, and TP53 expression, chromosomal rearrangements, and clinical features are still poorly understood. So, we investigated EZH2 expression and its associations with the immunophenotypic presentations, including MYC, BCL2, and TP53 expression, MYC, BCL2, and BCL6 translocation status, clinicopathological features, and therapeutic response to R-CHOP in a series of 139 DLBCL cases. EZH2 positivity was associated with MYC and TP53 expression (p = 0.0002 and p = 0.0000, respectively) and to high proliferative index (Ki67>70%, p = 0.0082). No associations were found among EZH2 expression and chromosomal translocation status. The non-germinal center (nGC) DLBCL presented most of associations observed in the general sample; however, only TP53 immunodetection showed associations with EZH2 expression in the germinal center (GC) DLBCL. EZH2 expression had no impact on therapeutic efficacy in R-CHOP-treated patients. In conclusion, EZH2 seems to be upregulated by MYC, to rely on TP53 alterations, and is associated with high proliferative tumors in DLBCL, which might be dependent on GC or nGC subclassifications. Furthermore, it is not a therapeutic efficacy marker to R-CHOP in our series.

Identification of a novel long non-coding RNA within RUNX1 intron 5.

BACKGROUND: RUNX1 gene, a master regulator of the hematopoietic process, participates in pathological conditions as a partner for several genes in chromosomal translocations. One of the most frequent chromosomal translocations found in acute myeloid leukemia patients is the t(8;21), in which RUNX1 and ETO genes recombine. In RUNX1 gene, the DNA double-strand breaks that originate the t(8;21) are generated in the intron 5, specifically within three regions designated as BCR1, BCR2, and BCR3. To date, what determines that these regions are more susceptible to DNA double-strand breaks is not completely clear. In this report, we characterized RUNX1 intron 5, by analyzing DNase-seq and ChIP-seq data, available in the ENCODE Project server, to evaluate DNaseI hypersensitivity and the presence of the epigenetic mark H3K4me3 in 124 and 51 cell types, respectively. RESULTS: Our results show that intron 5 exhibits an epigenetic mark distribution similar to known promoter regions. Moreover, using the online tool YAPP and available CAGE data from the ENCODE Project server, we identified several putative transcription start sites within intron 5 in regions BCR2 and BCR3. Finally, available EST data was analyzed, identifying a novel uncharacterized long non-coding RNA, which is expressed in hematopoietic cell lines as shown by RT-PCR. Our data suggests that the core promoter of the novel long non-coding RNA locates within the region BCR3. CONCLUSION: We identified a novel long non-coding RNA within RUNX1 intron 5, transcribed from a promoter located in the region BCR3, one of the chromosomal breakpoints of RUNX1 gene.

CCAAT/enhancer-binding protein alpha (CEBPA) gene haploinsufficiency does not alter hematopoiesis or induce leukemia in Lck-CALM/AF10 transgenic mice.

Although rare, CALM/AF10 is a chromosomal rearrangement found in immature T-cell acute lymphoblastic leukemia (T-ALL), acute myeloid leukemia, and mixed phenotype acute leukemia of T/myeloid lineages with poor prognosis. Moreover, this translocation is detected in 50% of T-ALL patients with gamma/delta T cell receptor rearrangement, frequently associated with low expression of transcription factor CCAAT/enhancer-binding protein alpha (CEBPA). However, the relevance of CEBPA low expression for CALM/AF10 leukemogenesis has not yet been evaluated. We generated double mutant mice, which express the Lck-CALM/AF10 fusion gene and are haploinsufficient for the Cebpa gene. To characterize the hematopoiesis, we quantified hematopoietic stem cells, myeloid progenitor cells, megakaryocyte-erythrocyte progenitor cells, common myeloid progenitor cells, and granulocyte-macrophage progenitor cells. No significant difference was detected in any of the progenitor subsets. Finally, we tested if Cebpa haploinsufficiency would lead to the expansion of Mac-1+/B220+/c-Kit+ cells proposed as the CALM/AF10 leukemic progenitor. Less than 1% of bone marrow cells expressed Mac-1, B220, and c-Kit with no significant difference between groups. Our results showed that the reduction of Cebpa gene expression in Lck-CALM/AF10 mice did not affect their hematopoiesis or induce leukemia. Our data corroborated previous studies suggesting that the CALM/AF10 leukemia-initiating cells are early progenitors with lymphoid/myeloid differentiating potential.

Identification and Characterization of Novel Fusion Genes with Potential Clinical Applications in Mexican Children with Acute Lymphoblastic Leukemia.

Acute lymphoblastic leukemia is the most common type of childhood cancer worldwide. Mexico City has one of the highest incidences and mortality rates of this cancer. It has previously been recognized that chromosomal translocations are important in cancer etiology. Specific fusion genes have been considered as important treatment targets in childhood acute lymphoblastic leukemia (ALL). The present research aimed at the identification and characterization of novel fusion genes with potential clinical implications in Mexican children with acute lymphoblastic leukemia. The RNA-sequencing approach was used. Four fusion genes not previously reported were identified: CREBBP-SRGAP2B, DNAH14-IKZF1, ETV6-SNUPN, ETV6-NUFIP1. Although a fusion gene is not sufficient to cause leukemia, it could be involved in the pathogenesis of the disease. Notably, these new translocations were found in genes encoding for hematopoietic transcription factors which are known to play an important role in leukemogenesis and disease prognosis such as IKZF1, CREBBP, and ETV6. In addition, they may have an impact on the prognosis of Mexican pediatric patients with ALL, with the potential to be included in the current risk stratification schemes or used as therapeutic targets.

Identification of two independent X-autosome translocations in closely related mammalian (Proechimys) species.

Multiple sex chromosome systems have been described for several mammalian orders, with different species from the same genus sharing the same system (e.g., X1X2Y or XY1Y2). This is important because the translocated autosome may be influenced by the evolution of the recipient sex chromosome, and this may be related to speciation. It is often thought that the translocation of an autosome to a sex chromosome may share a common origin among phylogenetically related species. However, the neo-X chromosomes of Proechimys goeldii (2n = 24♀, 25♂/NFa = 42) and Proechimys gr. goeldii (2n = 16♀, 17♂/NFa = 14) have distinct sizes and morphologies that have made it difficult to determine whether they have the same or different origins. This study investigates the origins of the XY1Y2 sex chromosome determination system in P. goeldii (PGO) and P. gr. goeldii (PGG) and elucidates the chromosomal rearrangements in this low-diploid-number group of Proechimys species. Toward this end, we produced whole-chromosome probes for P. roberti (PRO; 2n = 30♂/NFa = 54) and P. goeldii (2n = 25♂/NFa = 42) and used them in comparative chromosomal mapping. Our analysis reveals that multiple translocations and inversions are responsible for the karyotype diversity of these species, with only three whole-chromosomes conserved between PRO and PGO and eight between PGO and PGG. Our data indicate that multiple sex chromosome systems have originated twice in Proechimys. As small populations are prone to the fixation of chromosomal rearrangements, we speculate that biological features of Rodentia contribute to this fixation. We also highlight the potential of these rodents as a model for studying sex chromosome evolution.

The impact of balanced reciprocal translocation - 46,XX,t(7;17) (p13;q24) probably involving the SOX9 gene in the in vitro fertilization with own oocytes evaluated by preimplantation genetic testing or donated oocytes.

Preimplantation genetic testing (PGT) for in vitro fertilization (IVF) - also known as PGT for Structural Rearrangements (PGT-SR) - has emerged as an option for at-risk couples carrying balanced translocations. The female in the couple featured in this case report is a carrier of a balanced reciprocal translocation who underwent IVF. PGT showed all her embryos were aneuploid. She subsequently had two cycles using donor oocytes, which ended in miscarriages.

CCAAT/enhancer-binding protein alpha (CEBPA) gene haploinsufficiency does not alter hematopoiesis or induce leukemia in Lck-CALM/AF10 transgenic mice

Although rare, CALM/AF10 is a chromosomal rearrangement found in immature T-cell acute lymphoblastic leukemia (T-ALL), acute myeloid leukemia, and mixed phenotype acute leukemia of T/myeloid lineages with poor prognosis. Moreover, this translocation is detected in 50% of T-ALL patients with gamma/delta T cell receptor rearrangement, frequently associated with low expression of transcription factor CCAAT/enhancer-binding protein alpha (CEBPA). However, the relevance of CEBPA low expression for CALM/AF10 leukemogenesis has not yet been evaluated. We generated double mutant mice, which express the Lck-CALM/AF10 fusion gene and are haploinsufficient for the Cebpa gene. To characterize the hematopoiesis, we quantified hematopoietic stem cells, myeloid progenitor cells, megakaryocyte-erythrocyte progenitor cells, common myeloid progenitor cells, and granulocyte-macrophage progenitor cells. No significant difference was detected in any of the progenitor subsets. Finally, we tested if Cebpa haploinsufficiency would lead to the expansion of Mac-1+/B220+/c-Kit+ cells proposed as the CALM/AF10 leukemic progenitor. Less than 1% of bone marrow cells expressed Mac-1, B220, and c-Kit with no significant difference between groups. Our results showed that the reduction of Cebpa gene expression in Lck-CALM/AF10 mice did not affect their hematopoiesis or induce leukemia. Our data corroborated previous studies suggesting that the CALM/AF10 leukemia-initiating cells are early progenitors with lymphoid/myeloid differentiating potential.

Clinical features of carriers of reciprocal chromosomal translocations involving chromosome 2: report of nine cases and review of the literature

ABSTRACT Objective: To explore the clinical features of carriers of chromosome 2 translocations, enabling informed genetic counseling of these patients. Materials and Methods: Eighty-two male carriers of a translocation who were infertile or receiving fertility counseling were recruited. Cytogenetic analyses were performed using G-banding. A search of PubMed was performed to determine whether the identified translocations on chromosome 2 are involved in male infertility. The relationships of translocation breakpoints with male infertility and recurrent pregnancy loss were analyzed. Results: Of the 82 translocation carriers, 9 (11%) were carriers of a chromosome 2 translocation. Four cases had oligozoospermia or infertility, while five had normal semen. In an analysis of the literature, 55 patients who were carriers of chromosome 2 translocations were also reviewed. Breakpoints at 2p13 and 2q31 were observed in six patients each, and were the most common. Breakpoints at 2p23, 2p13, 2p11.2, 2q31, and 2q37 were associated to both pre-gestational and gestational infertility, while other breakpoints were associated with gestational infertility. Conclusions: All breakpoints at chromosome 2 were correlated with gestational infertility. Carriers of chromosome 2 translocations should therefore receive counseling to continue with natural conception and use of different technologies available via assisted reproductive technology, such as preimplantation genetic diagnosis.

Clinical features of carriers of reciprocal chromosomal translocations involving chromosome 2: report of nine cases and review of the literature.

OBJECTIVE: To explore the clinical features of carriers of chromosome 2 translocations, enabling informed genetic counseling of these patients. MATERIALS AND METHODS: Eighty-two male carriers of a translocation who were infertile or receiving fertility counseling were recruited. Cytogenetic analyses were performed using G-banding. A search of PubMed was performed to determine whether the identified translocations on chromosome 2 are involved in male infertility. The relationships of translocation breakpoints with male infertility and recurrent pregnancy loss were analyzed. RESULTS: Of the 82 translocation carriers, 9 (11%) were carriers of a chromosome 2 translocation. Four cases had oligozoospermia or infertility, while five had normal semen. In an analysis of the literature, 55 patients who were carriers of chromosome 2 translocations were also reviewed. Breakpoints at 2p13 and 2q31 were observed in six patients each, and were the most common. Breakpoints at 2p23, 2p13, 2p11.2, 2q31, and 2q37 were associated to both pre-gestational and gestational infertility, while other breakpoints were associated with gestational infertility. CONCLUSIONS: All breakpoints at chromosome 2 were correlated with gestational infertility. Carriers of chromosome 2 translocations should therefore receive counseling to continue with natural conception and use of different technologies available via assisted reproductive technology, such as preimplantation genetic diagnosis.

Nance-Horan syndrome in females due to a balanced X;1 translocation that disrupts the NHS gene: Familial case report and review of the literature.

The Nance-Horan syndrome is an X-linked disorder characterized by congenital cataract, facial features, microcornea, microphthalmia, and dental anomalies; most of the cases are due to NHS gene mutations on Xp22.13. Heterozygous carrier females generally present less severe features, and up to 30% of the affected males have intellectual disability. We describe two patients, mother and daughter, manifesting Nance-Horan syndrome. The cytogenetic and molecular analyses demonstrated a 46,X,t(X;1)(p22.13;q22) karyotype in each of them. No copy-number genomic imbalances were detected by high-density microarray analysis. The mother had a preferential inactivation of the normal X chromosome; expression analysis did not detect any mRNA isoform of NHS. This is the first report of Nance-Horan syndrome due to a skewed X chromosome inactivation resulting from a balanced translocation t(X;1) that disrupts the NHS gene expression, with important implications for clinical presentation and genetic counseling.

Hipercalcemia, peak monoclonal y leucemización como presentación de un linfoma de células B de alto grado, con reordenamiento de MYC, BCL-2 y BCL-6 (Triple Hit). Caso clínico / Hypercalcemia, monoclonal peak and leukemic presentation of a High-grade B-cell lymphoma with MYC, BCL-2 and BCL-6 translocations (triple hit). Report of one case

High-grade B-cell lymphomas with rearrangement of MYC, BCL-2 and/or BCL-6 were introduced by the update of the WHO classification of lymphoid neoplasms. They usually present unique morphological and molecular characteristics, with an aggressive clinical outcome and worse prognosis. We report a 48 year-old female patient presenting with B symptoms and enlarged lymph nodes. Blood count showed pancytopenia and peripheral blood smears showed large lymphoid cells, some with nuclei and vacuoles. LDH was 3524 g/L and serum calcium was 11.5 mg/dL. Flow cytometry immunophenotyping showed pathological mature B lymphocytes. Protein electrophoresis showed a slight monoclonal peak. The biopsy disclosed a triple expressor diffuse large B-cell lymphoma, arising from germinal center. FISH was positive for MYC, BCL-2 and BCL-6 (triple hit) with a clonal evolution. Conventional cytogenetics showed a complex karyotype. Chemotherapy was started with R-CHOP (Rituximab/cyclophosphamide/doxorubicin/vincristine/prednisone). She developed impaired consciousness; the brain CT scan showed a large brain mass. The patient died within 3 weeks.

A rare case of TFE-related pigmented renal tumor with overlapping features between melanotic Xp11 translocation renal cancer and Xp11 renal cell carcinoma with melanotic features.

In recent years, an increasing number of TFE3 rearrangement-associated tumors with melanotic features have been reported as primary neoplasm in different anatomical sites, including the kidney. Melanotic Xp11 translocation renal cancer (MXTRC) and Xp11 renal cell carcinoma with melanotic features (XRCCM) have been proposed to be main categories for pigmented lesions in the microophthalmia-associated transcription factor (MiTF/TFE3) family of renal tumors that may show variable degrees of melanocytic differentiation. Herein we report a rare case of TFE3-related pigmented renal tumor showing unusual immunoexpression of cytokeratins (AE1/AE3) and renal cell carcinoma markers (RCC, CD10). Cathepsin-K and Vimentin were diffusely positive whereas melanocytic markers (HMB-45 and Melan-A) displayed weak and patchy expression. We found no labelling for PAX-8, muscle markers (desmin, smooth muscle actin, muscle-specific actin and caldesmon) and S-100. TFE3 fusion was confirmed by break-apart fluorescence in situ hybridization (FISH). This case corroborates previous evidence for overlap in the TFE3-associated cancer family and illustrates that it may not be possible to set a clear cutoff between epithelial (XRCCM) and mesenchymal (MXTRC) subgroups.

Double-hit lymphomas: clinical, morphological, immunohistochemical and cytogenetic study in a series of Brazilian patients with high-grade non-Hodgkin lymphoma.

BACKGROUND: Double-hit lymphomas (DHL) are rare high-grade neoplasms characterized by two translocations: one involving the gene MYC and another involving genes BCL2 or BCL6, whose diagnosis depends on cytogenetic examination. This research studied DHL and morphological and/or immunophenotypic factors associated with the detection of these translocations in a group of high-grade non-Hodgkin lymphoma cases. METHOD: Clinical and morphological reviews of 120 cases diagnosed with diffuse large B-cell lymphoma and Burkitt lymphoma were conducted. Immunohistochemistry (CD20, CD79a, PAX5, CD10, Bcl6, Bcl2, MUM1, TDT and Myc) and fluorescence in situ hybridization for detection of MYC, BCL2 and BCL6 gene translocations were performed in a tissue microarray platform. RESULTS: Three cases of DHL were detected: two with translocations of MYC and BCL2 and one with translocations of MYC and BCL6, all leading to death in less than six months. Among 90 cytogenetically evaluable biopsies, associations were determined between immunohistochemistry and fluorescence in situ hybridization for MYC (p = 0.036) and BCL2 (p = 0.001). However, these showed only regular agreement, indicated by Kappa values of 0.23 [0.0;0.49] and 0.35 [0.13;0.56], respectively. "Starry sky" morphology was strongly associated with MYC positivity (p = 0.01). The detection of three cases of DHL, all resulting in death, confirms the rarity and aggressiveness of this neoplasm. CONCLUSIONS: The "starry sky" morphological pattern and immunohistochemical expression of Myc and Bcl2 represent possible selection factors for additional cytogenetic diagnostic testing.

Wolf-Hirschhorn Syndrome with Epibulbar Dermoid: An Unusual Association in a Patient with 4p Deletion and Functional Xp Disomy.

Wolf-Hirschhorn syndrome (WHS) is a contiguous gene and multiple malformation syndrome that results from a deletion in the 4p16.3 region. We describe here a 6-month-old girl that presented with WHS features but also displayed unusual findings, such as epibulbar dermoid in the left eye, ear tags, and left microtia. Although on G-banding her karyotype appeared to be normal, chromosomal microarray analysis revealed an ∼13-Mb 4p16.3p15.33 deletion and an ∼9-Mb Xp22.33p22.31 duplication, resulting from a balanced maternal t(X;4)(p22.31;p15.33) translocation. The patient presented with functional Xp disomy due to an unbalanced X-autosome translocation, a rare cytogenetic finding in females with unbalanced rearrangements. Sequencing of both chromosome breakpoints detected no gene disruption. To the best of our knowledge, this is the first patient described in the literature with WHS and epibulbar dermoid, a typical characteristic of the oculoauriculovertebral spectrum (OAVS). Our data suggest that possible candidate genes for OAVS may have been deleted along with the WHS critical region.

Cytogenetics of the Thorny Catfish Trachydoras paraguayensis (Eigenmann & Ward, 1907), (Siluriformes, Doradidae): Evidence of Pericentric Inversions and Chromosomal Fusion.

The karyotype and chromosomal characteristics of Trachydorasparaguayensis, a representative of the South American catfish family Doradidae, were analyzed by conventional (Giemsa staining, silver staining, C-banding) and molecular (FISH with rDNA and telomeric probes) cytogenetic techniques. The diploid chromosome number was 2n = 56, with 36 metacentric, 16 submetacentric, and 4 subtelocentric chromosomes in both sexes; however, a remarkable heteromorphism in pair 22 (submetacentric and metacentric elements) was detected in 6 individuals. Compared to other representatives of Doradidae which mostly have 58 chromosomes, the karyotype of T. paraguayensis suggests a reduction in 2n due to chromosomal fusion, as could be deduced from the presence of an interstitial telomere sequence in the submetacentric pair 19. Pale heterochromatic blocks were present in the terminal regions of some chromosomes, very similar to other species of Doradidae. The interstitial position of the NORs observed in the karyotype of T. paraguayensis differs from those reported for most Doradidae species, indicating that it is a derived character. FISH with 5S rDNA revealed 2 interstitial fluorescent signals in the submetacentric pair 22, and the polymorphism of these sites likely resulted from a pericentric inversion.