Are microsatellites prone to slippage?

Long microsatellites are highly unstable and likely to mutate. When slippage mutations happen, expansions occur more frequently if the number of repeat units is small, and contractions occur more frequently if the number of repeat units is large.

Why does polymerase slippage occur?

A slippage event normally occurs when a sequence of repetitive nucleotides (tandem repeats) are found at the site of replication. 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 processes.

What is a polymerase slippage?

Polymerase slippage sometimes occurs during replication; DNA polymerase III falls off of the DNA template at the repeat region, then reattaches at a more distant site. Polymerase slippage can cause the newly created DNA strand to contain an expanded section of DNA.

Why do microsatellites have high mutation rates?

Wierdl et al. [32] suggested microsatellite stability to be related to the length of the stretch of tandem repeats, and postulated that large microsatellites have an increased probability to realign in a misaligned confirmation during replication resulting in a higher mutation rate.

Why does forward slippage cause deletion?

Strand slippage occurs by mispairing of the template strand and the newly synthesized strand during DNA replication. If the template strand loops out, then a deletion will result. In contrast, if the newly synthesized strand loops out, then a deletion will result.

How is replication slippage fixed?

When strand slippage occurs during DNA replication, a DNA strand may loop out, resulting in the addition or deletion of a nucleotide on the newly-synthesized strand. Again, most of these spontaneous errors are corrected by DNA repair processes.

Are microsatellites polymorphic?

Microsatellites are polymorphic repeating units of 1–6 base pairs in length, found in human DNA. Microsatellites can be amplified for identification by PCR and can be used as molecular markers. Microsatellite analysis (MSA) is a PCR analysis of DNA in exfoliated urine cells.

Why are microsatellites mutation prone?

Because microsatellites consist of such repetitive sequences, DNA polymerase may make errors at a higher rate in these sequence regions. Several studies have found evidence that slippage is the cause of microsatellite mutations. Typically, slippage in each microsatellite occurs about once per 1,000 generations.

What happens to microsatellite when slip mutation occurs?

When slip mutation occurs, MS with small number of repeat units expands more frequently, while MS with large number of repeat units contracts more frequently. The mismatch repair (MMR), can repair errors during DNA replication.

What is the mechanism of microsatellite instability?

The mechanism of MS generation is generally believed to be DNA slippage in the process of replication, or mismatch of the basic group of slippage strand and complementary strand in the process of DNA replication and repair, resulting in one or more of the repeating units missing or insert.

Which is the best marker for microsatellite instability?

And the National Cancer Institute recommended two single nucleotide repeat loci BAT-25 and BAT-26 and three multi-nucleotide repeat loci D2S123, D5S346 and D17S250 as microsatellite markers to determine the status of MSI [ 9 ].

How is NGS used to detect microsatellite instability?

NGS detection is directly targeted to one hundred known genes for genome sequencing, to test microsatellite instability in tumor tissues. In 2017, MSK’s IMPACT products were approved to detect microsatellite instability in cancer tissues.