Breakpoint analysis of the recurrent constitutional t(8;22)(q24.13;q11.21) translocation

Non-B DNA structures as a booster of genome instability

Non-canonical secondary structures (NCSs) are alternative nucleic acid structures that differ from the canonical B-DNA conformation. NCSs often occur in repetitive DNA sequences and can adopt different conformations depending on the sequence. The majority of these structures form in the context of physiological processes, such as transcription-associated R-loops, G4s, as well as hairpins and slipped-strand DNA, whose formation can be dependent on DNA replication. It is therefore not surprising that NCSs play important roles in the regulation of key biological processes. In the last years, increasing published data have supported their biological role thanks to genome-wide studies and the development of bioinformatic prediction tools. Data have also highlighted the pathological role of these secondary structures. Indeed, the alteration or stabilization of NCSs can cause the impairment of transcription and DNA replication, modification in chromatin structure and DNA damage. These events lead to a wide range of recombination events, deletions, mutations and chromosomal aberrations, well-known hallmarks of genome instability which are strongly associated with human diseases. In this review, we summarize molecular processes through which NCSs trigger genome instability, with a focus on G-quadruplex, i-motif, R-loop, Z-DNA, hairpin, cruciform and multi-stranded structures known as triplexes.

Auricular fistula: a review of its clinical manifestations, genetics, and treatments

Auricular fistula is a common congenital auricular malformation, characterized as a small opening in the skin and a subcutaneous cyst. It can be classified in different ways according to positions of pits and directions of fistula tracts. The term preauricular fistula and variant type of preauricular fistula (postauricular fistula) are used. Auricular fistula prevalence varies in countries and populations, and its actual prevalence is presently unknown. The most accepted and widely cited theory of auricular fistula etiopathogenesis is an incorrect or incomplete fusion of six auricular hillocks that are mesenchymal proliferations. Auricular fistula can occur either sporadically or genetically. The pattern in inherited cases is thought to be incomplete autosomal dominant, with variable expressions, reduced penetrance, and inapparent gender differences. Auricular fistula has several forms and is reported as being a component of many syndromes. In the field of genetics, currently, there is no related review to comprehensively summarize the genetic basis of auricular fistula and related disorders. This article provides a comprehensive review of auricular fistula, especially congenital preauricular fistula, which accounts for the majority of auricular fistula, by summarizing the clinical manifestations, histological and embryological development, genetics, examinations, and treatments, as well as syndromes with auricular fistula.

Double strand breaks (DSBs) as indicators of genomic instability in PATRR-mediated translocations

Genomic instability contributes to a variety of potentially damaging conditions, including DNA-based rearrangements. Breakage in the form of double strand breaks (DSBs) increases the likelihood of DNA damage, mutations and translocations. Certain human DNA regions are known to be involved in recurrent translocations, such as the palindrome-mediated rearrangements that have been identified at the breakpoints of several recurrent constitutional translocations: t(11;22)(q23;q11), t(17;22)(q11;q11) and t(8;22) (q24;q11). These breakpoints occur at the center of palindromic AT-rich repeats (PATRRs), which suggests that the structure of the DNA may play a contributory role, potentially through the formation of secondary cruciform structures. The current study analyzed the DSB propensity of these PATRR regions in both lymphoblastoid (mitotic) and spermatogenic cells (meiotic). Initial results found an increased association of sister chromatid exchanges (SCEs) at PATRR regions in experiments that used SCEs to assay DSBs, combining SCE staining with fluorescence in situ hybridization (FISH). Additional experiments used chromatin immunoprecipitation (ChIP) with antibodies for either markers of DSBs or proteins involved in DSB repair along with quantitative polymerase chain reaction to quantify the frequency of DSBs occurring at PATRR regions. The results indicate an increased rate of DSBs at PATRR regions. Additional ChIP experiments with the cruciform binding 2D3 antibody indicate an increased rate of cruciform structures at PATRR regions in both mitotic and meiotic samples. Overall, these experiments demonstrate an elevated rate of DSBs at PATRR regions, an indication that the structure of PATRR containing DNA may lead to increased breakage in multiple cellular environments.

Size-dependent antirecombinogenic effect of short spacers on palindrome recombinogenicity

Palindromic sequences in DNA can instigate genetic recombination and genome instability, which can result in devastating conditions such as the Emmanuel syndrome. Palindrome recombinogenicity increases with its size and sequence similarity between palindrome arms, while quasipalindromes with long spacers are less recombinogenic. However, the minimal spacer length, which could reduce or abolish palindrome recombinogenicity in the eukaryotic genome, was never determined. Therefore, we constructed a series of palindromes containing spacers of lengths ranging from 0 (perfect palindrome) to 10 bp and tested their recombinogenicity in yeast Saccharomyces cerevisiae. We found that a 7 bp spacer significantly reduces 126 bp palindrome recombinogenicity, while a 10 bp spacer completely stabilizes palindromes up to 150 bp long. Additionally, we showed that palindrome stimulated recombination rate is not dependent on Mus81 and Yen1 endonucleases. We also compared the recombinogenicity of a perfect 126 bp palindrome and a corresponding quasipalindrome consisting of the same palindrome arms with a stabilising 10 bp spacer in sgs1Δ and rad27Δ backgrounds, since both Sgs1 helicase and Rad27 endonuclease are implicated in preventing hairpin formation at palindromic sequences during replication.

A Genome Model to Explain Major Features of Neurodevelopmental Disorders in Newborns

The purpose of this study was to test the hypothesis that infections are linked to chromosomal anomalies that cause neurodevelopmental disorders. In children with disorders in the development of their nervous systems, chromosome anomalies known to cause these disorders were compared with foreign DNAs, including known teratogens. Genes essential for neurons, lymphatic drainage, immunity, circulation, angiogenesis, cell barriers, structure, epigenetic and chromatin modifications were all found close together in polyfunctional clusters that were deleted or rearranged in neurodevelopmental disorders. In some patients, epigenetic driver mutations also changed access to large chromosome segments. These changes account for immune, circulatory, and structural deficits that accompany neurologic deficits. Specific and repetitive human DNA encompassing large deletions matched infections and passed rigorous artifact tests. Deletions of up to millions of bases accompanied infection-matching sequences and caused massive changes in human homologies to foreign DNAs. In data from 3 independent studies of private, familial, and recurrent chromosomal rearrangements, massive changes in homologous microbiomes were found and may drive rearrangements and encourage pathogens. At least 1 chromosomal anomaly was found to consist of human DNA fragments with a gap that corresponded to a piece of integrated foreign DNA. Microbial DNAs that match repetitive or specific human DNA segments are thus proposed to interfere with the epigenome and highly active recombination during meiosis, driven by massive changes in human DNA-foreign DNA homologies. Abnormal recombination in gametes produces zygotes containing rare chromosome anomalies that cause neurologic disorders and nonneurologic signs. Neurodevelopmental disorders may be examples of assault on the human genome by foreign DNAs at a critical stage. Some infections may be more likely tolerated because they resemble human DNA segments. Even rare developmental disorders can be screened for homology to infections within altered epigenomes and chromatin structures. Considering effects of foreign DNAs can assist prenatal and genetic counseling, diagnosis, prevention, and early intervention.

A clinical and molecular analysis of a patient with Emanuel syndrome

Emanuel syndrome (ES) is the most frequent type of recurrent non‑Robertsonian translocation that is characterized by numerous anomalies. Over 100 patients with ES have been described in the literature. The phenotype of this syndrome varies but often consists of facial dysmorphism, microcephaly, severe intellectual disability, developmental retardation, congenital heart disease and genital anomalies. The present study describes a 2‑year‑old boy with multiple malformations, including facial dysmorphism, severe intellectual disability, growth retardation, congenital heart disease, cleft lip and palate, genital malformation (micropenis), amblyopia, thymic dysplasia and hearing impairment. The karyotype of the patient was 47,XY,+del(22)(q13), and the maternal karyotype was 46,XX,t(11;22)(q25;q13),9qh‑,15p+. Single‑nucleotide polymorphism‑array analysis of the proband indicated a partial duplication of chromosomes 22 and 11 at 22q11.1‑q11.21 and 11q23.3‑q25, respectively, which confirmed the diagnosis of ES. To date, no cases of ES have been reported in mainland China. The present case further emphasizes the necessity and importance of high‑resolution techniques for genetic diagnosis and for subsequent genetic counseling. The present study contributed to the phenotypic delineation of ES and confirmed the first ES patient in mainland China.

Constitutional t(8;22)(q24;q11.2) that mimics the variant Burkitt-type translocation in Philadelphia chromosome-positive chronic myeloid leukemia

Constitutional translocations that coincide with t(9;22)(q34;q11.2) may lead to unnecessary treatments in chronic myeloid leukemia (CML) patients, as, under the standard criteria, a diagnosis of CML with additional chromosomal abnormalities indicates an accelerated phase (AP). In the present report, a 47-year-old male had pain in the right foot due to gout. Peripheral blood examination showed leukocytosis with left shift. Bone marrow aspiration revealed myeloid hyperplasia with megakaryocytosis. RT-PCR revealed the major BCR-ABL fusion transcript, and CML in the chronic phase was diagnosed, followed by nilotinib treatment. Although WBC counts decreased immediately, G-banding analysis showed 46,XY,t(8;22)(q24;q11.2),t(9;22)(q34;q11.2) [20]. The t(8;22)(q24;q11.2) translocation is known to be recurrent in Burkitt's lymphoma. The diagnosis was changed to CML in AP, leading to B-lymphoid crisis. Unexpectedly, the karyotype was 46,XY,t(8;22)(q24;q11.2) [20] in hematological complete remission, even after 3 months. Fluorescence in situ hybridization on metaphase spreads revealed the MYC signal on the der(22)t(8;22), indicating that the 8q24 breakpoint was centromeric to MYC at 8q24.21. G-banding analysis of phytohemagglutinin-stimulated peripheral blood T-lymphocytes also indicated 46,XY,t(8;22)(q24.1;q11.2). We conclude that the t(8;22) is constitutional in this patient. As the tumor suppressor gene TRC8/RNF139 is disrupted by constitutional t(8;22)(q24.13;q11.21) in dysgerminoma, it may be associated with the onset of CML.

Palindrome-Mediated Translocations in Humans: A New Mechanistic Model for Gross Chromosomal Rearrangements

Palindromic DNA sequences, which can form secondary structures, are widely distributed in the human genome. Although the nature of the secondary structure-single-stranded "hairpin" or double-stranded "cruciform"-has been extensively investigated in vitro, the existence of such unusual non-B DNA in vivo remains controversial. Here, we review palindrome-mediated gross chromosomal rearrangements possibly induced by non-B DNA in humans. Recent advances in next-generation sequencing have not yet overcome the difficulty of palindromic sequence analysis. However, a dozen palindromic AT-rich repeat (PATRR) sequences have been identified at the breakpoints of recurrent or non-recurrent chromosomal translocations in humans. The breakages always occur at the center of the palindrome. Analyses of polymorphisms within the palindromes indicate that the symmetry and length of the palindrome affect the frequency of the de novo occurrence of these palindrome-mediated translocations, suggesting the involvement of non-B DNA. Indeed, experiments using a plasmid-based model system showed that the formation of non-B DNA is likely the key to palindrome-mediated genomic rearrangements. Some evidence implies a new mechanism that cruciform DNAs may come close together first in nucleus and illegitimately joined. Analysis of PATRR-mediated translocations in humans will provide further understanding of gross chromosomal rearrangements in many organisms.

Translocation and deletion breakpoints in cancer genomes are associated with potential non-B DNA-forming sequences

Gross chromosomal rearrangements (including translocations, deletions, insertions and duplications) are a hallmark of cancer genomes and often create oncogenic fusion genes. An obligate step in the generation of such gross rearrangements is the formation of DNA double-strand breaks (DSBs). Since the genomic distribution of rearrangement breakpoints is non-random, intrinsic cellular factors may predispose certain genomic regions to breakage. Notably, certain DNA sequences with the potential to fold into secondary structures [potential non-B DNA structures (PONDS); e.g. triplexes, quadruplexes, hairpin/cruciforms, Z-DNA and single-stranded looped-out structures with implications in DNA replication and transcription] can stimulate the formation of DNA DSBs. Here, we tested the postulate that these DNA sequences might be found at, or in close proximity to, rearrangement breakpoints. By analyzing the distribution of PONDS-forming sequences within ±500 bases of 19 947 translocation and 46 365 sequence-characterized deletion breakpoints in cancer genomes, we find significant association between PONDS-forming repeats and cancer breakpoints. Specifically, (AT)_n, (GAA)_n and (GAAA)_n constitute the most frequent repeats at translocation breakpoints, whereas A-tracts occur preferentially at deletion breakpoints. Translocation breakpoints near PONDS-forming repeats also recur in different individuals and patient tumor samples. Hence, PONDS-forming sequences represent an intrinsic risk factor for genomic rearrangements in cancer genomes.