Familial Pancreatic Cancer (FPC) is a cancer syndrome characterized by clustering of pancreatic cancer in families, but most FPC cases do not have a known genetic etiology. Understanding genetic predisposition to pancreatic cancer is important for improving screening as well as treatment. The central aim of this thesis is to identify candidate susceptibility genes for FPC, and I used three approaches of increasing resolution. First, based on a candidate-gene approach, I hypothesized that BRCA1 is inactivated by loss-of-heterozygosity in pancreatic adenocarcinoma of germline mutation carriers. I demonstrated that 5/7 pancreatic tumors from BRCA1-mutation carriers show LOH, compared to only 1/9 sporadic tumors, suggesting that BRCA1 inactivation is involved in tumorigenesis in germline mutation carriers. Second, I hypothesized that the germline genomes of FPC subjects differ in copy-number profile from healthy genomes, and that regions affected by rare deletions or duplications in FPC subjects overlap candidate tumor-suppressors or oncogenes. I found no significant difference in the global copy-number profile of FPC and control genomes, but I identified 93 copy-number variable genomic regions unique to FPC subjects, overlapping 88 genes of which several have functional roles in cancer development. I investigated one duplication to sequence the breakpoints, but I found that this duplication did not segregate with disease in the affected family. Third, I hypothesized that in a family with multiple pancreatic cancer patients, genes containing rare variants shared by the affected members constitute susceptibility genes. Using next-generation sequencing to capture most bases in coding regions of the genome, I interrogated the germline exome of three relatives who died of pancreatic cancer and a relative who is healthy at advanced age. I identified a short-list of nine candidate genes with unreported mutations shared by the three affected relatives and absent in the unaffected relative, of which a few had functional relevance to tumorigenesis. I performed Sanger sequencing to screen an unrelated cohort of approximately 70 FPC patients for mutations in the top two candidate genes, but I found no additional rare variants in those genes. In conclusion, I present a list of candidate FPC susceptibility genes for further validation and investigation in future studies.

Familial Pancreatic Cancer (FPC) is a cancer syndrome characterized by clustering of pancreatic cancer in families, but most FPC cases do not have a known genetic etiology. Understanding genetic predisposition to pancreatic cancer is important for improving screening as well as treatment. The central aim of this thesis is to identify candidate susceptibility genes for FPC, and I used three approaches of increasing resolution. First, based on a candidate-gene approach, I hypothesized that BRCA1 is inactivated by loss-of-heterozygosity in pancreatic adenocarcinoma of germline mutation carriers. I demonstrated that 5/7 pancreatic tumors from BRCA1-mutation carriers show LOH, compared to only 1/9 sporadic tumors, suggesting that BRCA1 inactivation is involved in tumorigenesis in germline mutation carriers. Second, I hypothesized that the germline genomes of FPC subjects differ in copy-number profile from healthy genomes, and that regions affected by rare deletions or duplications in FPC subjects overlap candidate tumor-suppressors or oncogenes. I found no significant difference in the global copy-number profile of FPC and control genomes, but I identified 93 copy-number variable genomic regions unique to FPC subjects, overlapping 88 genes of which several have functional roles in cancer development. I investigated one duplication to sequence the breakpoints, but I found that this duplication did not segregate with disease in the affected family. Third, I hypothesized that in a family with multiple pancreatic cancer patients, genes containing rare variants shared by the affected members constitute susceptibility genes. Using next-generation sequencing to capture most bases in coding regions of the genome, I interrogated the germline exome of three relatives who died of pancreatic cancer and a relative who is healthy at advanced age. I identified a short-list of nine candidate genes with unreported mutations shared by the three affected relatives and absent in the unaffected relative, of which a few had functional relevance to tumorigenesis. I performed Sanger sequencing to screen an unrelated cohort of approximately 70 FPC patients for mutations in the top two candidate genes, but I found no additional rare variants in those genes. In conclusion, I present a list of candidate FPC susceptibility genes for further validation and investigation in future studies.

Genome wide studies have uncovered the existence of large-scale copy number variation (CNV) in the human genome. The human genome of different individuals was initially estimated to be 99.9% similar, but population studies on CNV have revealed that it is 12-16% copy number variable. Abnormal genomic CNVs are frequently found in subtelomeres of patients with mental retardation (MR) and other neurological disorders. Rearrangements of chromosome subtelomeric regions represent a high proportion of cytogenetic abnormalities and account for approximately 30% of pathogenic CNVs. Although DNA double strand breaks (DSBs) are implicated as a major factor in chromosomal rearrangements, the causes of chromosome breakage in subtelomeric regions have not been elucidated. But due to the presence of repetitive sequences in subtelomeres, we hypothesized that chromosomal rearrangements in these regions are not stochastic but driven by specific sequence motifs. In a collaborative effort with Dr. Rudd (Department of human genetics at Emory University), we characterized subtelomeric breakpoints on different chromosome ends in search of common motifs that cause double-strand breaks. Using a yeast-based gross chromosomal rearrangement (GCR) system, we have identified a subtelomeric breakage motif from chromosome 2 (2q SBM) with a GCR rate that is 340 fold higher than background levels. To determine if the fragility of 2q SBM was driven by the formation of secondary structures, the helicase activities of Sgs1 and Pif1 were disrupted. These helicases have been shown to destabilize DNA secondary structures such as G-quadruplex structures. Disruption of these helicases augmented chromosomal rearrangements induced by 2q SBM, indicating that these helicases are required for maintenance of this sequence. We also donwregulated replication fork components to determine if 2q SBM was imposing any problems to the replication fork machinery. Downregulation of replication fork components increased chromosomal rearrangements, indicating that intact replication fork was a critical determinant of 2q SBM fragility. Using a yeast-based functional assay, these experiments have linked human subtelomeric repetitive sequences to chromosomal breakage that could give rise to human CNV in subtelomeric regions.

Post-zygotic genetic variation has previously been shown in healthy individuals and linked to various disorders. The definition of post-zygotic or somatic variation is the existence of genetically distinct populations of cells in a subject derived from a single zygote. Structural changes in the human genome are a major type of inter-individual genetic variation and copy number variation (CNV), involving changes in the copy number of genes, are one of the best studied category of structural genetic changes. In paper I we reported a pair of healthy female monozygotic (MZ) twins discordant for aneuploidy of chromosomes X and Y, contributing to the delineation of the frequency of somatic variation in MZ twins. It also illustrates the plasticity of the genome for tolerating large aberrations in healthy subjects. In paper II we showed age-related accumulation of copy number variation in the nuclear genomes in vivo for both megabase- and kilobase-range variants. Using age-stratified MZ twins and single-born subjects, we detected megabase-range aberrations in 3.4% of people ≥60 years old but not in individuals younger than 55 years. Moreover, the longitudinal analysis of subjects with aberrations suggests that the aberrant cell clones are not immortalized and disappear from circulation. We also showed that sorted blood cells display different genomic profiles.  The detected recurrent rearrangements are candidates for common age-related defects in blood cells. This work might help to describe the cause of an age-related decline in the number of cell clones in the blood, which is one of the hallmarks of immunosenescence. In paper III we described a variable number tandem repeat (VNTR) ~4 kb upstream of the IFNAR1 gene, which was somatically variable.  We detected 14 alleles displaying inter- and intra-individual variation. Further analyses indicated strong clustering of transcription factor binding sites within this region, suggesting an enhancer. This putative VNTR-based enhancer might influence the transcriptional regulation of neighboring cytokine receptor genes and the pathways they are involved in. These three studies stress the importance of research on post-zygotic variation in genetics. Furthermore, they emphasize that biobanks should consider sampling of multiple tissues to better address this issue in the genetic studies.

Understanding the mechanism of recombination has important implications for genome evolution and genomic variability. The work presented in this thesis studies the properties of recombination by investigating the effects it has on genome evolution in humans and dogs. Using alignments of human genes with chimpanzee and macaque orthologues we studied substitution patterns along the human lineage and scanned for evidence of positive selection. The properties mirror the situation in human non-coding sequences with the fixation bias ‘GC-biased gene conversion’ (gBGC) as a driving force in the most rapidly evolving regions. By assigning candidate genes to distinct classes of evolutionary forces we quantified the extent of those genes affected by gBGC to 20%. This suggests that human-specific characters can be prompted by the fixation bias of gBGC, which can be mistaken for selection. The gene PRDM9 controls recombination in most mammals, but is lacking in dogs. Using whole-genome alignments of dog with related species we examined the effects of PRDM9 inactivation. Additionally, we analyzed genomic variation in the genomes of several dog breeds. We identified that non-allelic homologous recombination (NAHR) via sequence identity, often GC-rich, creates structural variants of genomic regions. We show that these regions, which are also found in dog recombination hotspots, are a subset of unmethylated CpG-islands (CGIs). We inferred that CGIs have experienced a drastic increase in biased substitution rates, concurrent with a shift of recombination to target these regions. This enables recurrent episodes of gBGC to shape their distribution. The work presented in this thesis demonstrates the importance of meiotic recombination on patterns of molecular evolution and genomic variability in humans and dogs. Bioinformatic analyses identified mechanisms that regulate genome composition. gBGC is presented as an alternative to positive selection and is revealed as a major factor affecting allele configuration and the emergence of accelerated evolution on the human lineage. Characterization of recombination-induced sequence patterns highlights the potential of non-methylation and establishes unmethylated CGIs as targets of meiotic recombination in dogs. These observations describe recombination as an interesting process in genome evolution and provide further insights into the mechanisms of genomic variability.

Developmental disorders including: autism, intellectual disability, and congenital abnormalities are present in 3-8% of live births and display a huge amount of phenotypic and genetic heterogeneity. Traditionally, geneticists have identified individual monogenic diseases among these patients but a majority of patients fail to receive a clinical diagnosis. However, the genomes of these patients frequently harbour large copynumber variants (CNVs) but their interpretation remains challenging. Using pathway analysis I found significant functional associations for 329 individual phenotypes and show that 39% of these could explain the patients’ multiple co-morbid phenotypes; and multiple associated genes clustered within individual CNVs. I showed there was significantly more such clustering than expected by chance. In addition, the presence of a multiple functionally-related genes is a significant predictor of CNV pathogenicity beyond the presence of known disease genes and size of the CNV. This clustering of functionally-related genes was part of a broader pattern of functional clusters across the human genome. These genome-wide functional clusters showed tissuespecific expression and some evidence of chromatin-domain level regulation. Furthermore, many genome-wide functional clusters were enriched in segmental duplications making them prone to CNV-causing mutations and were frequently seen disrupted in healthy individuals. However, the majority of the time a pathogenic CNV affected the entire functional cluster, where as benign CNVs tended to affect only one or two genes. I also showed that patients with CNVs affecting the same functional cluster are significantly more phenotypically similar to each other than expected even if their CNVs do not affect any of the same genes. Lastly, I considered one of the major limitations in pathway analysis, namely ascertainment biases in functional information due to the prioritization of genes linked to human disease, and show how the modular nature of gene-networks can be used to identify and prioritize understudied genes.

This master thesis deals with analysis of structural variation of genome and with methods of its sequencing across all generations. Subsequently it contains a description of copy number variation and methods of its detection. The experimental part focuses on algorithm proposal for CNV detection according analysis and testing of uneven coverage in genome, variable representation of GC content and distance of sequence reads. Finally, the algorithm for detecting copy number variation is tested on genomic data of bacteria Klebsiella pneumoniae.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第21300号 / 医博第4389号 / 新制||医||1030(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 戸井 雅和, 教授 小川 修, 教授 武田 俊一 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

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