Human Chromosome 18 and Acrocentrics: A Dangerous Liaison.

The presence of thousands of repetitive sequences makes the centromere a fragile region subject to breakage. In this study we collected 31 cases of rearrangements of chromosome 18, of which 16 involved an acrocentric chromosome, during genetic screening done in three centers. We noticed a significant enrichment of reciprocal translocations between the centromere of chromosome 18 and the centromeric or pericentromeric regions of the acrocentrics. We describe five cases with translocation between chromosome 18 and an acrocentric chromosome, and one case involving the common telomere regions of chromosomes 18p and 22p. In addition, we bring evidence to support the hypothesis that chromosome 18 preferentially recombines with acrocentrics: (i) the presence on 18p11.21 of segmental duplications highly homologous to acrocentrics, that can justify a NAHR mechanism; (ii) the observation by 2D-FISH of the behavior of the centromeric regions of 18 respect to the centromeric regions of acrocentrics in the nuclei of normal subjects; (iii) the contact analysis among these regions on published Hi-C data from the human lymphoblastoid cell line (GM12878).

Olfactory receptor genes and chromosome 11 structural aberrations: Players or spectators?

The largest multi-gene family in metazoans is the family of olfactory receptor (OR) genes. Human ORs are organized in clusters over most chromosomes and seem to include >0.1% the human genome. Because 369 out of 856 OR genes are mapped on chromosome 11 (HSA11), we sought to determine whether they mediate structural rearrangements involving this chromosome. To this aim, we analyzed 220 specimens collected during diagnostic procedures involving structural rearrangements of chromosome 11. A total of 222 chromosomal abnormalities were included, consisting of inversions, deletions, translocations, duplications, and one insertion, detected by conventional chromosome analysis and/or fluorescence in situ hybridization (FISH) and array comparative genomic hybridization (array-CGH). We verified by bioinformatics and statistical approaches the occurrence of breakpoints in cytobands with or without OR genes. We found that OR genes are not involved in chromosome 11 reciprocal translocations, suggesting that different DNA motifs and mechanisms based on homology or non-homology recombination can cause chromosome 11 structural alterations. We also considered the proximity between the chromosomal territories of chromosome 11 and its partner chromosomes involved in the translocations by using the deposited Hi-C data concerning the possible occurrence of chromosome interactions. Interestingly, most of the breakpoints are located in regions highly involved in chromosome interactions. Further studies should be carried out to confirm the potential role of chromosome territories' proximity in promoting genome structural variation, so fundamental in our understanding of the molecular basis of medical genetics and evolutionary genetics.

Ten new cases of Balanced Reciprocal Translocation Mosaicism (BRTM): Reproductive implications, frequency and mechanism.

Chromosomal anomalies are well known to be an important cause of infertility, sterility and pregnancy loss. Balanced Reciprocal Translocation Mosaicism (BRTM) is an extremely rare phenomenon, mainly observed in subjects with a normal phenotype accompanied by reproductive failure. To date the mechanism of origin and the incidence of BRTM are poorly defined. Here we describe 10 new cases of BRTM. In 9 cases chromosome analysis revealed the presence of two different cell lines, one with a normal karyotype and the second with an apparently balanced reciprocal translocation. In the remaining case, both cell lines showed two different, but apparently balanced, reciprocal translocations. We document the clinical implications of BRTM, discuss its frequency in our referred population and suggest that carrier individuals might be more frequent than expected.

The High Mobility Group A2 gene is amplified and overexpressed in human prolactinomas.

Trisomy of chromosome 12 is a nonrandom chromosomal change in pituitary adenomas, particularly prolactinomas. This and the finding that prolactin-secreting pituitary adenomas develop in transgenic mice overexpressing the wild-type HMGA2 gene (which maps to 12q14-15) prompted us to investigate HMGA2 rearrangements and expression in human prolactinomas. By dual-color interphase fluorescence in situ hybridization analysis using HMGA2-specific PACs and BACs, we found that the HMGA2 locus was amplified in seven of the eight prolactinoma samples examined. The cytogenetic manifestations of elevated HMGA2 concentrations ranged from simple trisomy to tetrasomy 12 and der(12) chromosomes to marker chromosomes bearing 12q14-15-derived regions. Reverse transcription-PCR, Western blot and immunohistochemical analysis showed HMGA2 overexpression in a number of prolactinomas bearing rearrangement of regions 12q14-15. These data suggest a critical role of the HMGA2 overexpression in the generation of prolactin-secreting pituitary adenomas in humans.

High-mobility group A2 gene expression is frequently induced in non-functioning pituitary adenomas (NFPAs), even in the absence of chromosome 12 polysomy.

The high-mobility group A2 (HMGA2) gene has a critical role in benign tumors where it is frequently rearranged, and in malignant tumors, where it is overexpressed in the absence of structural modification of the HMGA2 locus. By previous fluorescence in situ hybridization (FISH) and reverse transcriptase PCR analyses on human prolactin-secreting pituitary adenomas we detected rearrangement of the HMGA2 gene and amplification of its native region associated with activated expression. These data indicated a role for the HMGA2 gene in the development of human pituitary prolactinomas, since they are consistent with the appearance of prolactin/growth hormone adenomas in transgenic mice overexpressing the HMGA2 gene. To assess a more general role for HMGA2 in pituitary oncogenesis, we investigated HMGA2 amplification and expression in a panel of non-functioning pituitary adenomas (NFPAs) which account for 25% of all pituitary adenomas. We provide evidence that out of 18 NFPA tumors tested, 12 expressed HMGA2, but, different from prolactinomas, only in two cases the upregulation of the gene could be associated with amplification and/or rearrangement of the HMGA2 locus. Increased dosage of chromosome 12 was found in the expressing and non-expressing NFPAs, confirming that this sole event is insufficient to drive up activation of the HMGA2 gene. A role for chromosome 12 polysomy to promote structural instability of HMGA2 is confirmed, but the mechanism via trisomy is less prevalent in the frequently diploid NFPAs than in the usually hyperdiploid prolactinomas. Micro-rearrangements of HMGA2 gene not detectable by FISH analysis and/or sequence alterations could contribute to upregulation of HMGA2 gene in pituitary adenomas of the NFPA subtype. However, it cannot be excluded that the HMGA2 overexpression may be due, in some NFPA patients, to the same, still mainly unknown, mechanisms responsible for HMGA2 overexpression in malignant neoplasias.

Small familial supernumerary ring chromosome 2: FISH characterization and genotype-phenotype correlation.

A child and his mother were found to be mosaic for a small supernumerary marker chromosome (SMC) that was identified and characterized by means of fluorescent in situ hybridization. The marker chromosome was derived from the pericentromeric region of chromosome 2; the involvement of proximal 2q was determined by YAC probes. The proband was referred because of psychotic illness and mild mental retardation, whereas his mother presented only minor dysmorphisms. There are only a few published reports concerning SMC(2) or proximal 2q trisomy. We reviewed the previously reported cases in an attempt to establish genotype-phenotype correlations, which are particularly important when SMCs are identified in prenatal diagnosis.

Combined characterization of a pituitary adenoma and a subcutaneous lipoma in a MEN1 patient with a whole gene deletion.

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant hereditary disorder associated with mutations of the MEN1 gene, which is characterized by combined tumors of the parathyroid glands, pancreatic islet cells, and the anterior pituitary. A significant number of patients with the clinical features of MEN1, however, do not show MEN1 mutations upon direct sequencing. We describe a young woman who fulfilled the clinical and biochemical criteria for MEN1 syndrome, but DNA sequencing did not indicate any MEN1 mutations. She developed a prolactin-secreting pituitary macroadenoma, primary hyperparathyroidism with parathyroid hyperplasia, pancreatic lesions, and two subcutaneous lipomas. Array comparative genomic hybridization (aCGH) analysis of peripheral blood DNA revealed a heterozygous germline deletion at 11q13.1 that spanned at least 22.23 kilobases and contained the entire MEN1 gene. Integrated aCGH and cytogenetic analyses of the adenoma and lipoma tissues revealed somatic inactivation of the wild-type MEN1 allele by different routes: the second hit of MEN1 recessive oncogenesis leading to adenoma implied a loss of heterozygosity, whereas a balanced translocation deleting the wild-type MEN1 allele primed the lipoma development. These findings show that aCGH is a valuable means of optimizing genetic testing in MEN1 patients which complements other technologic approaches to elucidating the pathologic mechanisms of MEN1 tumors.

Fluorescence in situ hybridization dissection of a chronic myeloid leukemia case bearing the apparently balanced translocations (9;22)(q34;q11.2) and (11;11)(p15;q13).

We report on a patient with chronic myeloid leukemia (CML), which was detected by conventional cytogenetic analysis, to carry two different acquired and apparently balanced translocations, (9;22)(q34;q11.2) and (11;11)(p15;q13). By fluorescence in situ hybridization characterization, we were able to finely map the genomic regions involved in the translocation breakpoints and to disclose concomitant deletions adjacent to the breakpoints on the two derivative chromosomes 11 and the derivative chromosome 22, and the insertion of a segment from chromosome band 11q12.2 into the derivative chromosome 9. We discuss the putative mechanism that could have led to the formation of this complex rearrangement and speculate on the role in leukemogenesis played by the genes mapping at the breakpoints and within the deleted regions.

De novo balanced chromosome rearrangements in prenatal diagnosis.

OBJECTIVE: We surveyed the datasheets of 29 laboratories concerning prenatal diagnosis of de novo apparently balanced chromosome rearrangements to assess the involvement of specific chromosomes, the breakpoints distribution and the impact on the pregnancy outcome. METHOD: By means of a questionnaire, data on 269.371 analyses performed from 1983 to 2006 on amniotic fluid, chorionic villus and fetal blood samples were collected. RESULTS: A total of 246 balanced anomalies were detected at frequencies of 72% for reciprocal translocations, 18% for Robertsonian translocations, 7% for inversions and 3% for complex chromosome rearrangements. The total frequencies of balanced rearrangements were 0.09%, 0.08% and 0.05% on amniotic fluid, chorionic villus and fetal blood samples. CONCLUSION: A preferential involvement of chromosomes 22, 7, 21, 3, 9 and 11 and a less involvement of chromosomes X, 19, 12, 6 and 1 was observed. A nonrandom distribution of the breakpoints across chromosomes was noticed. Association in the location of recurrent breakpoints and fragile sites was observed for chromosomes 11, 7, 10 and 22, while it was not recorded for chromosome 3. The rate of pregnancy termination was about 20%, with frequencies decreasing from complex chromosomal rearrangements (33%), reciprocal translocations (24%) to inversions (11%) and Robertsonian translocations (3%).