- Repeat expansions are a type of genetic mutation in which a short DNA sequence that is normally repeated a limited number of times becomes expanded to contain an unusually large number of repeated copies. These changes are an important form of genomic variation and are associated with numerous inherited neurological, muscular, developmental, and multisystem disorders. Repeat expansions can affect genes at different genomic locations and can interfere with gene expression, RNA function, protein production, or other cellular processes.
- DNA contains many repetitive sequences, and the presence of repeated DNA is not necessarily abnormal. In a healthy population, many repeat regions naturally vary in their number of copies. The number of repeats can become clinically important when it increases beyond a particular range or when the repeat occurs in a functionally important region of a gene. The distinction between normal variation and disease-associated expansion therefore depends on the specific repeat, its genomic location, and established population and clinical evidence.
- Several types of repetitive sequences can undergo expansion. Short tandem repeats (STRs), also known as microsatellites when they consist of very short repeated motifs, are particularly important. Some disease-associated repeats consist of three, four, or other numbers of nucleotides repeated consecutively. Expansions can also involve longer repetitive sequences and more complex repeat structures. The sequence motif, repeat length, genomic position, and surrounding DNA environment all influence the biological consequences of an expansion.
- Repeat expansions can occur in different regions of genes. Some are located in coding regions, where the expanded DNA can generate an abnormal number of repeated amino acids in the resulting protein. Others occur in introns, promoters, untranslated regions (UTRs), or other regulatory regions. The location of the repeat strongly influences the mechanism by which the mutation produces disease.
- When a repeat expansion occurs within a coding region, it can alter the resulting protein. Expanded nucleotide repeats may produce proteins containing excessive stretches of particular amino acids, potentially causing abnormal protein folding, aggregation, or interactions with other cellular molecules. These mechanisms are particularly important in several neurodegenerative disorders and other diseases involving abnormal protein function.
- Repeat expansions outside protein-coding regions can produce disease through different mechanisms. An expansion in a promoter or regulatory region may reduce or alter gene expression, sometimes by affecting chromatin structure or DNA methylation. Expansions within introns or UTRs may produce abnormal RNA molecules that interact with cellular proteins and interfere with RNA processing, splicing, transport, or translation.
- Some repeat expansions produce disease through toxic RNA mechanisms. In these cases, expanded RNA molecules can accumulate or bind abnormally to RNA-binding proteins, disrupting normal RNA metabolism. This can affect the processing of many other transcripts and lead to widespread cellular dysfunction. Other repeat expansions primarily produce disease through reduced gene expression or production of abnormal proteins.
- A particularly important characteristic of repeat expansions is genetic instability. The number of repeats can change when DNA is copied or transmitted between generations. Repeat sequences may expand or contract because of errors during DNA replication, DNA repair, or recombination. This instability can make repeat expansions behave differently from many other types of genetic mutations.
- One mechanism involved in repeat instability is replication slippage, in which repetitive DNA sequences can misalign during DNA replication. Because the repeated sequences are highly similar to one another, the newly synthesized DNA strand or template strand can form abnormal structures. This can result in the addition or removal of repeat units.
- DNA repair pathways can also influence repeat expansion. Repetitive DNA may form unusual structures such as hairpins or other secondary DNA structures, which can interfere with replication and repair. Proteins involved in DNA repair can therefore influence whether repeat regions remain stable or undergo expansion.
- Repeat expansions can exhibit anticipation, a genetic phenomenon in which a disorder may appear at an earlier age or with greater severity in successive generations. Anticipation is associated with repeat instability during transmission. However, the pattern varies between different disorders and depends on the particular repeat, its size, parental origin, and other genetic factors.
- The size of a repeat expansion can therefore be biologically important. Many repeat-associated disorders have ranges that can broadly be described as normal, intermediate, premutation, and full-expansion ranges, although the terminology and thresholds differ between diseases. An intermediate repeat length may not produce the classic disorder but can have implications for future generations because the repeat may expand during transmission.
- The concept of a premutation is especially important for some repeat-expansion disorders. A premutation contains more repeats than are usually observed in the general population but may not produce the full clinical phenotype associated with a larger expansion. During transmission, however, some premutation alleles can be unstable and expand to a larger size in the next generation.
- The sex of the transmitting parent can influence repeat instability in some disorders. Certain repeats are particularly prone to expansion during transmission through the maternal germline, while others show different patterns of instability. These parent-of-origin effects contribute to differences in inheritance and clinical presentation among repeat-expansion disorders.
- Repeat expansions can occur in germline DNA, allowing them to be inherited from a parent. They can also show somatic mosaicism, meaning that repeat sizes may differ among cells within the same individual. This can produce a mixture of cells containing different numbers of repeats and may contribute to differences in disease onset, severity, or tissue involvement.
- Some repeat expansions are particularly unstable in certain tissues. Repeat length may therefore vary between blood cells and affected tissues, making diagnosis more complicated. In some cases, the repeat size observed in a blood sample may not completely represent the repeat structure present in the nervous system or another clinically important tissue.
- Repeat expansions are associated with numerous genetic disorders. Examples include Huntington disease, myotonic dystrophy, fragile X-associated disorders, several spinocerebellar ataxias, Friedreich ataxia, and other neurological or multisystem conditions. Different disorders involve different repeat sequences, genomic locations, inheritance patterns, and molecular mechanisms.
- Huntington disease is associated with expansion of a CAG repeat in the coding region of the HTT gene. The expanded repeat produces an abnormal polyglutamine tract in the huntingtin protein. The repeat length is related to aspects of disease onset, although other genetic and biological factors also contribute to the clinical course.
- Myotonic dystrophy provides an example of a repeat expansion outside a conventional protein-coding region. Different forms of the disease involve different repeat sequences, and expanded RNA can contribute to abnormal interactions with RNA-binding proteins and widespread changes in RNA processing.
- Fragile X syndrome is associated with expansion of a CGG repeat in the FMR1 gene. A sufficiently large expansion can lead to epigenetic changes and reduced expression of the gene. The distinction between normal, intermediate, premutation, and full-expansion repeat ranges is particularly important in understanding this condition and its inheritance.
- Friedreich ataxia is associated with expansion of a GAA repeat within an intronic region of the FXN gene. The expansion can reduce production of the frataxin protein and interfere with normal mitochondrial function. This illustrates how an intronic repeat expansion can cause disease without directly changing the protein-coding sequence.
- Repeat expansions can also contribute to spinocerebellar ataxias and other neurological disorders. Different forms of spinocerebellar ataxia can involve different repeat motifs and molecular mechanisms. Some affect coding sequences and produce abnormal proteins, while others affect noncoding regions and alter RNA or gene expression.
- The diagnosis of repeat-expansion disorders can be challenging because standard sequencing methods do not always detect large or highly repetitive DNA expansions accurately. Conventional Sanger sequencing and many short-read next-generation sequencing (NGS) approaches may have limitations when repeats are very long, highly repetitive, or difficult to align computationally.
- Specialized genetic tests are therefore frequently used. Depending on the disorder, laboratories may use PCR, repeat-primed PCR, fragment analysis, Southern blotting, or other targeted approaches to determine whether an expansion is present and estimate its size. The appropriate method depends on the repeat sequence, expected expansion range, and clinical question.
- Long-read sequencing has become an important research and diagnostic technology for studying repeat expansions. Long DNA reads can span repetitive regions more effectively than many short-read methods, potentially allowing researchers to determine repeat length, sequence interruptions, and complex repeat structures. Long-read approaches may also help identify expansions that are difficult to characterize using conventional methods.
- Bioinformatics plays an increasingly important role in repeat analysis. Computational tools can identify unusual repeat structures, estimate repeat sizes, analyze sequencing data, and investigate sequence interruptions or mosaicism. However, highly repetitive genomic regions remain technically challenging, and test performance depends on the technology and analytical method used.
- The interpretation of a repeat expansion requires consideration of the exact repeat sequence, repeat size, genomic location, inheritance pattern, population frequency, and established disease-associated thresholds. Genetic counseling can be particularly important because repeat instability may affect the risk of transmission and the repeat size observed in future generations.
- Repeat expansions also provide important insights into population genetics and human evolution. Repeat lengths naturally vary among individuals and populations, and the stability of these sequences can influence how genetic variation changes across generations. Studying repeat variation can therefore reveal information about mutation mechanisms, ancestry, and the evolution of the human genome.
- Repeat expansions can interact with other genetic factors. A person’s clinical phenotype may depend not only on repeat length but also on other genetic variants, modifier genes, epigenetic changes, environmental influences, and tissue-specific mechanisms. This contributes to the considerable variability observed among individuals with some repeat-expansion disorders.
- Research into repeat expansions is also relevant to precision medicine. Understanding the molecular mechanism of a particular expansion can help researchers develop targeted therapies. Potential strategies include reducing toxic RNA, modifying abnormal protein production, correcting gene expression, or using genome and RNA-based technologies to interfere with the disease mechanism.
- Emerging approaches include antisense oligonucleotides, RNA-targeting technologies, gene-silencing strategies, and genome editing. Researchers are investigating whether these approaches can reduce the production of toxic RNA or proteins, restore normal gene expression, or directly modify expanded DNA. Many such approaches remain under investigation, and their applicability varies among different repeat-expansion disorders.
- Overall, Repeat Expansions are an important class of genetic mutations in which repetitive DNA sequences increase beyond their normal range. They can affect coding and noncoding regions and produce disease through abnormal proteins, toxic RNA, reduced gene expression, epigenetic changes, and other molecular mechanisms. Their unusual instability, anticipation, parent-of-origin effects, mosaicism, and technical challenges in genetic testing make repeat expansions a distinctive and important area of human genetics.
