Massive contractions of myotonic dystrophy type 2-associated CCTG tetranucleotide repeats occur via double-strand break repair with distinct requirements for DNA helicases.

Myotonic dystrophy type 2 (DM2) is a genetic disease caused by expanded CCTG DNA repeats in the first intron of CNBP. The number of CCTG repeats in DM2 patients ranges from 75 to 11,000, yet little is known about the molecular mechanisms responsible for repeat expansions or contractions. We developed an experimental system in Saccharomyces cerevisiae that enables the selection of large-scale contractions of (CCTG)100 within the intron of a reporter gene and subsequent genetic analysis. Contractions exceeded 80 repeat units, causing the final repetitive tract to be well below the threshold for disease. We found that Rad51 and Rad52 are involved in these massive contractions, indicating a mechanism that uses homologous recombination. Srs2 helicase was shown previously to stabilize CTG, CAG, and CGG repeats. Loss of Srs2 did not significantly affect CCTG contraction rates in unperturbed conditions. In contrast, loss of the RecQ helicase Sgs1 resulted in a 6-fold decrease in contraction rate with specific evidence that helicase activity is required for large-scale contractions. Using a genetic assay to evaluate chromosome arm loss, we determined that CCTG and reverse complementary CAGG repeats elevate the rate of chromosomal fragility compared to a short-track control. Overall, our results demonstrate that the genetic control of CCTG repeat contractions is notably distinct among disease-causing microsatellite repeat sequences.

Massive contractions of Myotonic Dystrophy Type 2-associated CCTG tetranucleotide repeats occur via double strand break repair with distinct requirements for helicases.

Myotonic Dystrophy Type 2 (DM2) is a genetic disease caused by expanded CCTG DNA repeats in the first intron of CNBP. The number of CCTG repeats in DM2 patients ranges from 75-11,000, yet little is known about the molecular mechanisms responsible for repeat expansions or contractions. We developed an experimental system in Saccharomyces cerevisiae that enables selection of large-scale contractions of (CCTG)100 within the intron of a reporter gene and subsequent genetic analysis. Contractions exceeded 80 repeat units, causing the final repetitive tract to be well below the threshold for disease. We found that Rad51 and Rad52 are required for these massive contractions, indicating a mechanism that involves homologous recombination. Srs2 helicase was shown previously to stabilize CTG, CAG, and CGG repeats. Loss of Srs2 did not significantly affect CCTG contraction rates in unperturbed conditions. In contrast, loss of the RecQ helicase Sgs1 resulted in a 6-fold decrease in contraction rate with specific evidence that helicase activity is required for large-scale contractions. Using a genetic assay to evaluate chromosome arm loss, we determined that CCTG and reverse complementary CAGG repeats elevate the rate of chromosomal fragility compared to a low-repeat control. Overall, our results demonstrate that the genetic control of CCTG repeat contractions is notably distinct among disease-causing microsatellite repeat sequences.

The ACMG SF v3.0 gene list increases returnable variant detection by 22% when compared with v2.0 in the ClinSeq cohort.

PURPOSE: The American College of Medical Genetics and Genomics (ACMG) recommends the return of pathogenic and likely pathogenic (P/LP) secondary findings from exome and genome sequencing. The latest version (ACMG secondary finding [SF] v3.0) includes 14 additional genes. We interrogated the ClinSeq cohort for variants in these genes to determine the additional yield in unselected individuals. METHODS: Exome data from 1473 individuals (60% White, 34% African American or Black, 6% other) were analyzed. We restricted our analyses to coding variants; +1,+2,-1, and -2 splice site variants; and the pathogenic GAA variant, NM_000152.5:c.-32-13T>G. Variants were assessed with slightly modified ACMG/Association of Molecular Pathology guidelines. RESULTS: A total of 25 P/LP variants were identified. In total, 7 individuals had P/LP variants in genes recommended for return of heterozygous variants, namely HNF1A (1), PALB2 (3), TMEM127 (1), and TTN (2). In total, 4 individuals had a homozygous variant in a gene recommended for biallelic variant return, namely HFE, NM_000410.3(HFE):c.845G>A p.Cys282Tyr. A total of 17 P/LP variants were identified in the heterozygous state in genes recommended only for biallelic variant reporting and were not returned. The frequency of returnable P/LP variants did not significantly differ by race. CONCLUSION: Using the ACMG SF v3.0, the returnable P/LP variant frequency increased in the ClinSeq cohort by 22%, from 3.4% (n = 50, ACMG SF v2.0) to 4.1% (n = 61, ACMG SF v3.0).

Draft Genome Sequences of Bacillus glennii V44-8, Bacillus saganii V47-23a, Bacillus sp. Strain V59.32b, Bacillus sp. Strain MER_TA_151, and Paenibacillus sp. Strain MER_111, Isolated from Cleanrooms Where the Viking and Mars Exploration Rover Spacecraft Were Assembled.

We report the draft genome sequences of Bacillus glennii V44-8, Bacillus saganii V47-23a, and Bacillus sp. strain V59.32b, isolated from the Viking spacecraft assembly cleanroom, and Bacillus sp. strain MER_TA_151 and Paenibacillus sp. strain MER_111, isolated from the Mars Exploration Rover (MER) assembly cleanroom.