Biology subjects

Frith, M. C.

Publications and source records attributed to Frith, M. C..

13 recordsLinked to original sources

bioRxiv2024

Challenges in Detecting Somatic Recombination of Repeat Elements: Insights from Short and Long Read Datasets

Non-allelic copies of the two major families of repeat elements in the human genome, Alu and L1, recombine somatically at high frequency. Tissue-specific recombination profiles are dynamic in cell differentiation and are altered in neurodegeneration, suggesting that somatic recombination of repeat elements can contribute to functional heterogeneity of cells in health and disease. The study of these genomic variants, however, presents several technical challenges related to their extremely low copy number and their sequence content. Here, we address key issues regarding detecting and annotating structural variants derived from recombining repeat elements in NGS data. We show that PCR introduces significant changes of recombination profiles in sequencing libraries and that recombination profiles are affected by the choice of sequencing platform. We refine previous estimates of recombination in single cells by analyzing recombination profiles in PCR-free HG002 datasets sequenced by Oxford Nanopore Technologies and PacBio sequencers while describing several platform-specific differences. We additionally provide evidence that recombination events annotated in state-of-the-art single-cell HG002 whole-genome sequencing datasets are likely molecular artifacts generated by PCR. By exploring the limits of current technologies, this work establishes essential requirements for future developments to enhance the reliability of detecting somatic recombination of repeat elements in genomic datasets.

genomics↗

bioRxiv2023

Finding related sequences by a simple sum over alignments

The main way of analyzing genetic sequences is by finding sequence regions that are related to each other. There are many methods to do that, usually based on this idea: find an alignment of two sequence regions, which would be unlikely to exist between unrelated sequences. Unfortunately, it is hard to tell if an alignment is likely to exist by chance. Also, the precise alignment of related regions is uncertain. One alignment does not hold all evidence that they are related. We should consider alternative alignments too. This is rarely done, because we lack a simple and fast method that fits easily into practical sequence-search software. Here is described a simplest-possible change to standard sequence alignment, which sums probabilities of alternative alignments. Remarkably, this makes it easier to tell if a similarity is likely to occur by chance. This approach is better than standard alignment at finding distant relationships, at least in a few tests. It can be used in practical sequence-search software, with minimal increase in implementation difficulty or run time. It generalizes to different kinds of alignment, e.g. DNA-versus-protein with frameshifts. Thus, it can widely contribute to finding subtle relationships between sequences.

bioinformatics↗

bioRxiv2023

DNA conserved across animals that controls genes for embryonic development

DNA that controls gene expression (e.g. enhancers, promoters) has seemed almost never to be conserved between distantly-related animals, like vertebrates and arthropods. This is mysterious, because development of such animals is partly organized by homologous genes with similar complex expression patterns, termed "deep homology". Here we report twenty-five regulatory DNA segments conserved across bilaterian animals, of which seven are also conserved in cnidaria (coral and sea anemone). They control developmental genes (e.g. Nr2f, Ptch, Rfx1/3, Sall, Smad6, Sp5, Tbx2/3), including six homeobox genes: Gsx, Hmx, Meis, Msx, Six1/2, and Zfhx3/4. The segments contain perfectly or near-perfectly conserved CCAAT boxes, E-boxes, and other sequences recognized by regulatory proteins. More such DNA conservation will surely be found soon, as more genomes are published and sequence comparison is optimized. This reveals a control system for animal development conserved since the Precambrian.

developmental biology↗

bioRxiv2022

An immune-suppressing protein in human endogenous retroviruses

Retroviruses are important contributors to disease and evolution in vertebrates. Sometimes, retrovirus DNA is heritably inserted in a vertebrate genome: an endogenous retrovirus (ERV). Vertebrate genomes have many such virus-derived fragments, usually with mutations disabling their original functions. Some primate ERVs appear to encode an overlooked protein. This protein is homologous to protein MC132 from Molluscum contagiosum virus, which is a human poxvirus, not a retrovirus. MC132 suppresses the immune system by targeting NF-{kappa}B, and it had no known homologs until now. The ERV homologs of MC132 in the human genome are mostly disrupted by mutations, but there is an intact copy on chromosome 4. We found homologs of MC132 in ERVs of apes, monkeys, and bushbaby, but not tarsiers, lemurs or non-primates. This suggests that some primate retroviruses had, or have, an extra immune-suppressing protein, which underwent horizontal genetic transfer between unrelated viruses.

bioinformatics↗

bioRxiv2022

How to optimally sample a sequence for rapid analysis

We face an increasing flood of genetic sequence data, from diverse sources, requiring rapid computational analysis. Rapid analysis can be achieved by sampling a subset of positions in each sequence. Previous sequence-sampling methods, such as minimizers, syncmers, and minimally-overlapping words, were developed by heuristic intuition, and are not optimal. We present a sequence-sampling approach that provably optimizes sensitivity for a whole class of sequence comparison methods, for randomly-evolving sequences. It it likely near-optimal for a wide range of alignmentbased and alignment-free analyses. For real biological DNA, it increases specificity by avoiding simple repeats. Our approach generalizes universal hitting sets (which guarantee to sample a sequence at least once), and polar sets (which guarantee to sample a sequence at most once). This helps us understand how to do rapid sequence analysis as accurately as possible.

bioinformatics↗

bioRxiv2022

Finding rearrangements in nanopore DNA reads with LAST and dnarrange

Long-read DNA sequencing techniques such as nanopore are especially useful for characterizing complex sequence rearrangements, which occur in some genetic diseases and also during evolution. Analyzing the sequence data to understand such rearrangements is not trivial, due to sequencing error, rearrangement intricacy, and abundance of repeated similar sequences in genomes. The O_SCPLOWLASTC_SCPLOW and dnarrange software packages can resolve complex relationships between DNA sequences, and characterize changes such as gene conversion, processed pseudogene insertion, and chromosome shattering. They can filter out numerous rearrangements shared by controls, e.g. healthy humans versus a patient, to focus on rearrangements unique to the patient. One useful ingredient is last-train, which learns the rates (probabilities) of deletions, insertions, and each kind of base match and mismatch. These probabilities are then used to find the most likely sequence relationships/alignments, which is especially useful for DNA with unusual rates, such as DNA from Plasmodium falciparum (malaria) with [~] 80% a+t. This is also useful for less-studied species that lack reference genomes, so the DNA reads are compared to a different species genome. We also point out that a reference genome with ancestral alleles would be ideal.

bioinformatics↗

bioRxiv2021

Paleozoic Protein Fossils Illuminate the Evolution of Vertebrate Genomes and Transposable Elements

Genomes hold a treasure trove of protein fossils: fragments of formerly protein-coding DNA, which mainly come from transposable elements (TEs) or host genes. These fossils reveal ancient evolution of TEs and genomes, and many fossils have been exapted to perform diverse functions important for the hosts fitness. However, old and highly-degraded fossils are hard to identify, standard methods (e.g. BLAST) are not optimized for this task, and few Paleozoic protein fossils have been found. Here, a recently optimized method is used to find protein fossils in vertebrate genomes. It finds Paleozoic fossils predating the amphibian/amniote divergence from most major TE categories, including virus-related Polinton and Gypsy elements. It finds 10 fossils in the human genome (8 from TEs and 2 from host genes) that predate the last common ancestor of all jawed vertebrates, probably from the Ordovician period. It also finds types of transposon and retrotransposon not found in human before. These fossils have extreme sequence conservation, indicating exaptation: some have evidence of gene-regulatory function, and they tend to lie nearest to developmental genes. Some ancient fossils suggest "genome tectonics", where two fragments of one TE have drifted apart by up to megabases, possibly explaining gene deserts and large introns. This paints a picture of great TE diversity in our aquatic ancestors, with patchy TE inheritance by later vertebrates, producing new genes and regulatory elements on the way. Host-gene fossils too have contributed anciently-conserved DNA segments. This paves the way to further studies of ancient protein fossils.

bioinformatics↗

bioRxiv2020

Minimally-overlapping words for sequence similarity search

Analysis of genetic sequences is usually based on finding similar parts of sequences, e.g. DNA reads and/or genomes. For big data, this is typically done via "seeds": simple similarities (e.g. exact matches) that can be found quickly. For huge data, sparse seeding is useful, where we only consider seeds at a subset of positions in a sequence. Here we study a simple sparse-seeding method: using seeds at positions of certain "words" (e.g. ac, at, gc, or gt). Sensitivity is maximized by using words with minimal overlaps. That is because, in a random sequence, minimally-overlapping words are anti-clumped. We provide evidence that this is often superior to acclaimed "minimizer" sparse-seeding methods. Our approach can be unified with design of inexact (spaced and subset) seeds, further boosting sensitivity. Thus, we present a promising approach to sequence similarity search, with open questions on how to optimize it.

bioinformatics↗

bioRxiv2020

Significant non-existence of sequences in genomes and proteomes

Nullomers are minimal-length oligomers absent from a genome or proteome. Although research has shown that artificially synthesized nullomers have deleterious effects, there is still a lack of a strategy for the prioritisation and classification of non-occurring sequences as potentially malicious or benign. In this work, by using Markovian models with multiple-testing correction, we reveal significant absent oligomers which are statistically expected to exist. This strongly suggests that their absence is due to negative selection. We survey genomes and proteomes covering the diversity of life, and find thousands of significant absent sequences. Common significant nullomers are often mono- or dinucleotide tracts, or palindromic. Significant viral nullomers are often restriction sites, and may indicate unknown restriction motifs. Surprisingly, significant mammal genome nullomers are often present, but rare, in other mammals, suggesting that they are suppressed but not completely forbidden. Significant human nullomers are rarely present in human viruses, indicating viral mimicry of the host. More than 1/4 of human proteins are one substitution away from containing a significant nullomer. We provide a web-based, interactive database of significant nullomers across genomes and proteomes.

bioinformatics↗

bioRxiv2019

Genome-wide survey of tandem repeats by nanopore sequencing shows that disease-associated repeats are more polymorphic in the general population

Tandem repeats are highly mutable and contribute to the development of human disease by a variety of mechanisms. However, it is difficult to predict which tandem repeats may cause a disease. We performed a genome-wide survey of the millions of human tandem repeats using long read genome sequencing data from 16 humans. We found that known Mendelian disease-causing or disease-associated repeats, especially coding CAG and 5UTR GGC repeats, are relatively long and polymorphic in the general population. This method, especially if used in GWAS, may indicate possible new candidates of pathogenic or biologically important tandem repeats in human genomes.

genomics↗

bioRxiv2019

Hallmarks of slow translation initiation revealed in mitochondrially localizing mRNA sequences

The mRNA of some, but not all, nuclear encoded mitochondrial proteins localize to the periphery of mitochondria. Previous studies have shown that both the nascent polypeptide chain and an mRNA binding protein play a role in this phenomenon, and have noted a positive correlation between mRNA length and mitochondrial localization. Here, we report the first investigation into the relationship between mRNA translation initiation rate and mRNA mitochondrial localization. Our results indicate that translation initiation promoting factors such as Kozak sequences are associated with cytosolic localization, while inhibiting factors such as 5' UTR secondary structure correlate with mitochondrial localization. Moreover, the frequencies of nucleotides in various positions of the 5' UTR show higher correlation with localization than the 3' UTR. These results indicate that mitochondrial localization is associated with slow translation initiation. Interestingly this may help explain why short mRNAs, which are thought to initiate translation rapidly, seldom localize to mitochondria. We propose a model in which translating mRNA has reduced mobility and tends not to reach mitochondria. Finally, we explore this model with a simulation of mRNA diffusion using previously estimated translation initiation probabilities, confirming that our model can produce localization values similar to those measured in experimental studies.

molecular biology↗

bioRxiv2019

Long-read sequencing identifies GGC repeat expansion in human-specific NOTCH2NLC associated with neuronal intranuclear inclusion disease

Neuronal intranuclear inclusion disease (NIID) is a progressive neurodegenerative disease characterized by eosinophilic hyaline intranuclear inclusions in neuronal and somatic cells. The wide range of clinical manifestations in NIID makes ante-mortem diagnosis difficult 1-8, but skin biopsy realized its ante-mortem diagnosis 9,10 and many NIID cases have been diagnosed by skin biopsy11,12. Most cases of NIID are sporadic, but several familial cases are known. Using a large NIID family, we conducted linkage mapping, found a 58.1-Mb linked-region at 1p22.1-q21.3 with a maximum logarithm of odds (LOD) score of 4.21, and successfully identified a GGC repeat expansion in the 5 portion of NOTCH2NLC in all affected members by long-read sequencing, but not in unaffected members. We further found the similar expansions in additional 8 unrelated families with NIID as well as 39 sporadic NIID patients. Repeat-primed PCR consistently detected the GGC repeat expansion in all the familial and sporadic patients diagnosed by skin biopsy, but never in unaffected family members nor 200 controls. This shows that pathogenic changes in a human-specific gene evolutionarily generated by segmental duplication indeed causes a human disease.

genetics↗

bioRxiv2018

Robust detection of tandem repeat expansions from long DNA reads

Tandemly repeated sequences are highly mutable and variable features of genomes. Tandem repeat expansions are responsible for a growing list of human diseases, even though it is hard to determine tandem repeat sequences with current DNA sequencing technology. Recent long-read technologies are promising, because the DNA reads are often longer than the repetitive regions, but are hampered by high error rates. Here, we report robust detection of human repeat expansions from careful alignments of long (PacBio and nanopore) reads to a reference genome. Our method (tandem-genotypes) is robust to systematic sequencing errors, inexact repeats with fuzzy boundaries, and low sequencing coverage. By comparing to healthy controls, we can prioritize pathological expansions within the top 10 out of 700000 tandem repeats in the genome. This may help to elucidate the many genetic diseases whose causes remain unknown.

genetics↗