Schematic representation of the slippage process at a replication fork

Slipped strand mispairing (SSM), (also known as replication slippage), is a mutation process which occurs during

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritan ...

. It involves denaturation and displacement of the DNA strands, resulting in mispairing of the complementary bases. Slipped strand mispairing is one explanation for the origin and evolution of repetitive DNA sequences. It is a form of

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

that leads to either a trinucleotide or dinucleotide expansion, or sometimes contraction, during

.Hartl L.D and Ruvolo M, 2012, Genetic Analysis of Genes and Genomes, Jones & Bartlett Learning, Burlington, pg. 529 A slippage event normally occurs when a sequence of repetitive nucleotides ( tandem repeats) are found at the site of replication. Tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place, resulting in genome rearrangements.

DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to crea ...

, the main enzyme to catalyze the polymerization of free deoxyribonucleotides into a newly forming DNA strand, plays a significant role in the occurrence of this mutation. When DNA polymerase encounters a direct repeat, it can undergo a replication slippage. Strand slippage may also occur during the DNA synthesis step of

DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...

processes. Within DNA trinucleotide repeat sequences, the repair of

DNA damage DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...

by the processes of

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...

non-homologous end joining Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. NHEJ is referred to as "non-homologous" because the break ends are directly ligated without the need for a homologous template, in contrast to homology direc ...

DNA mismatch repair DNA mismatch repair (MMR) is a system for recognizing and repairing erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination, as well as repairing some forms of DNA damage. Mismatch ...

base excision repair Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry and genetics, that repairs damaged DNA throughout the cell cycle. It is responsible primarily for removing small, non-helix-distorting base lesions from ...

may involve strand slippage mispairing leading to trinucleotide repeat expansion when the repair is completed. Slipped strand mispairing has also been shown to function as a

phase variation In biology, phase variation is a method for dealing with rapidly varying environments without requiring random mutation. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial populati ...

mechanism in certain bacteria.

Stages

Slippage occurs through five main stages: # In the first step, DNA polymerase encounters the direct repeat during the replication process. # The polymerase complex suspends replication and is temporarily released from the template strand. # The newly synthesized strand then detaches from the template strand and pairs with another direct repeat upstream. # DNA polymerase reassembles its position on the template strand and resumes normal replication, but during the course of reassembling, the polymerase complex backtracks and repeats the insertion of deoxyribonucleotides that were previously added. This results in some repeats found in the template strand being replicated twice into the daughter strand. This expands the replication region with newly inserted nucleotides. The template and the daughter strand can no longer pair correctly. # Nucleotide excision repair proteins are mobilized to this area where one likely outcome is the expansion of nucleotides in the template strand while the other is the absence of nucleotides. Although trinucleotide contraction is possible, trinucleotide expansion occurs more frequently.

Effects

Tandem repeats (the main influence for slippage replication) can be found in coding and non-coding regions. If these repeats are found in coding regions then the variations to the polynucleotide sequence can result in the formation of abnormal proteins in eukaryotes. Many human diseases have been reported to be associated with trinucleotide repeat expansions including

Huntington's disease Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an uns ...

.Brown TA. Genomes. 2nd edition. Oxford: Wiley-Liss; 2002. Chapter 14, Mutation, Repair and Recombination. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21114/ Accessed November 3, 2012 The HD gene is found in all human genomes. In the event that a slippage event occurs there can be a large expansion in the tandem repeats of the HD gene. An individual who is not affected by Huntington’s disease will have 6-35 tandem repeats at the HD locus. However, an affected individual will have 36- 121 repeats present. The expansion of the HD locus results in a dysfunctional protein leading to Huntington’s disease.

Disease associations

Huntington disease is normally progressive and results in movement, cognitive and psychiatric disorders. These disorders can lead to a severe impact on an individual’s daily activities, making it hard for proper communication and independent actions to take place. Huntington's Disease Replication slippage can also lead to other neurodegenerative diseases in humans. These include

spinal and bulbar muscular atrophy Spinal and bulbar muscular atrophy (SBMA), popularly known as Kennedy's disease, is a debilitating neurodegenerative disorder resulting in muscle cramps and progressive weakness due to degeneration of motor neurons in the brainstem and spinal cor ...

( trinucleotide expansion in the AR gene), dentatorubral–pallidoluysian atrophy ( trinucleotide expansion in the DRPLA gene),

spinocerebellar ataxia type 1 Spinocerebellar ataxia type 1 (SCA1) is a rare autosomal dominant disorder, which, like other spinocerebellar ataxias, is characterized by neurological symptoms including dysarthria, hypermetric saccades, and ataxia of gait and stance. This cere ...

( trinucleotide expansion in the SCA1gene), Machado-Joseph disease ( trinucleotide expansion in the SCA3 gene),

myotonic dystrophy Myotonic dystrophy (DM) is a type of muscular dystrophy, a group of genetic disorders that cause progressive muscle loss and weakness. In DM, muscles are often unable to relax after contraction. Other manifestations may include cataracts, int ...

( trinucleotide expansion in the DMPK gene), and

Friedreich's ataxia Friedreich's ataxia (FRDA or FA) is an autosomal-recessive genetic disease that causes difficulty walking, a loss of sensation in the arms and legs, and impaired speech that worsens over time. Symptoms generally start between 5 and 20 year ...

( a trinuncleotide expansion in the X25 gene). Therefore, replication slippage leads to a form of trinucleotide expansion which results in serious changes to protein structure.

Self-acceleration

SSM events can result in either insertions or deletions. Insertions are thought to be self-accelerating: as repeats grow longer, the probability of subsequent mispairing events increases. Insertions can expand simple tandem repeats by one or more units. In long repeats, expansions may involve two or more units. For example, insertion of a single repeat unit in GAGAGA expands the sequence to GAGAGAGA, while insertion of two repeat units in Asub>6 would produce Asub>8. Genomic regions with a high proportion of repeated DNA sequences ( tandem repeats,

microsatellite A microsatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from one to six or more base pairs) are repeated, typically 5–50 times. Microsatellites occur at thousands of locations within an organism's genome. ...

s) are prone to strand slippage during

and

. Trinucleotide repeat expansion is a cause of a number of human diseases including

fragile X syndrome Fragile X syndrome (FXS) is a genetic disorder characterized by mild-to-moderate intellectual disability. The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled. Physical features ma ...

, Huntington’s disease, several spinocerebellar ataxias,

and Friedrich ataxia.

Evolution of diverse adjacent repeats

The combination of SSM events with

point mutation A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequence ...

is thought to account for the evolution of more complex repeat units. Mutations followed by expansion would result in the formation of new types of adjacent

short tandem repeat A microsatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from one to six or more base pairs) are repeated, typically 5–50 times. Microsatellites occur at thousands of locations within an organism's genome. ...

units. For example, a

transversion Transversion, in molecular biology, refers to a point mutation in DNA in which a single (two ring) purine ( A or G) is changed for a (one ring) pyrimidine ( T or C), or vice versa. A transversion can be spontaneous, or it can be caused by i ...

could change the simple two- base repeat Asub>10 to Asub>4GATA Asub>2. This could then be expanded to Asub>4 ATAsub>3 Asub>2 by two subsequent SSM events. Simple repetitive DNA sequences containing a variety of adjacent short tandem repeats are commonly observed in non-protein coding regions of

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

genomes.

References