nanoloop 0.1.4

Utility tool for detecting R-loops with Nanopore data

nanoloop: Identify R-loops with Nanopore Reads

• Introduction
• Core Functions
  • bam_to_tsv
  • tsv_to_plot
  • tsv_to_bed
  • tsv_to_peak
• Installation
• Parameters
• Examples

Introduction

The nanoloop library enables the identification of R-loop regions using nanopore sequencing data.

R-loops are three-stranded nucleic acid structures formed when nascent RNA hybridizes with its DNA template, displacing the non-template DNA strand. They play critical roles in transcription regulation, DNA replication, and repair. Dysregulated R-loops are associated with genomic instability and disease.

In A3A-treated samples, unprotected single-stranded DNA in R-loops undergoes cytosine (C) deamination to uracil (dU). When using standard Dorado basecalling models, these dU sites are frequently miscalled as C or T with lower basecalling quality. nanoloop leverages this signature to:

• Quantify and visualize basecalling quality over specified genomic regions

• Quantify and visualize C-to-T conversion frequencies over specified genomic regions

• Simulate MACS3-compatible BED files for peak calling

• Call R-loop peaks using a rolling average approach

Core Functions

bam_to_tsv

Converts a BAM file to a bgzipped TSV file. The output reports per-position statistics: base count distribution (if --type nt_count) or base quality distribution (if --type nt_qual).

tsv_to_plot

Generates plots from TSV files for a given genomic region:

• --type nt_qual: uses TSVs from nanoloop bam_to_tsv --type nt_qual and generates plot depicting base quality distribution
• --type nt_count: uses TSVs from nanoloop bam_to_tsv --type nt_count and generates plot depicting base count distribution

Note that when --type nt_count, the TSV input must be from bam_to_tsv with --type nt_count; and when --type nt_qual, the TSV input must be from bam_to_tsv with --type nt_qual.

tsv_to_bed

Simulates a MACS3-compatible BED file:

• --type nt_qual: uses TSVs from nanoloop bam_to_tsv --type nt_qual, lower average quality yields more tags
• --type nt_count: uses TSVs from nanoloop bam_to_tsv --type nt_count, more tags where C-to-T conversion is higher

Note that when --type nt_count, the TSV input must be from bam_to_tsv with --type nt_count; and when --type nt_qual, the TSV input must be from bam_to_tsv with --type nt_qual.

tsv_to_peak

Calls R-loop peaks using a rolling average approach.

• Supports both --type nt_qual and --type nt_count

Note that when --type nt_count, the TSV input must be from bam_to_tsv with --type nt_count; and when --type nt_qual, the TSV input must be from bam_to_tsv with --type nt_qual.

Installation

pip install nanoloop

Parameters

nanoloop -h

usage: nanoloop [-h] {bam_to_tsv,tsv_to_plot,tsv_to_bed,tsv_to_peak} ...

nanoloop

positional arguments:
  {bam_to_tsv,tsv_to_plot,tsv_to_bed,tsv_to_peak}
    bam_to_tsv          Parse BAM file and output TSV file
    tsv_to_plot         Parse TSV file and output plot
    tsv_to_bed          Convert TSV file to BED format for MACS3 peak calling. For "--type nt_qual": regions
                        with lower quality scores will generate more simulated read tags, creating peaks in
                        those regions. For "--type nt_count": the number of simulated read tags is proportional
                        to the fraction of C converted to T at each reference C position.
    tsv_to_peak         Call peaks using a sliding window approach

options:
  -h, --help            show this help message and exit

You can also run subcommand-specific help, e.g.nanoloop bam_to_tsv -h, etc.

Examples

The following examples use a downsampled BAM examples/bam/p1214_no_pcr.bam, derived from an A3A-treated plasmid sample expected to contain R-loops around its middle region. "no_pcr" indicates sequencing was performed directly after A3A treatment (without PCR), preserving dU signatures.

nanoloop supports both --type nt_qual and --type nt_count options. Their usage is demosntrated below.

• --type nt_qual: uses the average read quality score at each nucleotide position for peak calling. Regions exhibiting decreased quality scores are identified as potential R-loop candidates.
• --type nt_count: uses the C-to-T conversion frequency at each position for peak calling. Regions with elevated conversion rates are identified as potential R-loop candidates.

Using base quality: --type nt_qual

This approach leverages the decreased basecalling quality observed at R-loop regions, an artifact caused by the presence of the unconventional deoxyuridien (dU) base.

Step 1: generate TSV

nanoloop bam_to_tsv \
  --bam examples/bam/p1214_no_pcr.bam \
  --ref examples/ref/p1214.fa \
  --type nt_qual \
  --output examples/res/p1214_no_pcr_nt_qual.tsv.gz

zcat < examples/res/p1214_no_pcr_nt_qual.tsv.gz | head

#chr    start   end     ref_nt  qual_0_10       qual_10_20      qual_20_30      qual_30_40      qual_40_above   qual_avg
p1214   0       1       A       7       275     287     234     166     27.856553147574818
p1214   1       2       A       11      337     307     269     185     27.488728584310188
p1214   2       3       A       13      392     332     283     178     26.808848080133554
p1214   3       4       A       16      408     411     331     173     26.53846153846154
p1214   4       5       A       19      441     523     336     166     26.004040404040403
p1214   5       6       A       41      450     642     373     118     25.34975369458128
p1214   6       7       A       58      441     711     414     89      24.907180385288967
p1214   7       8       A       49      387     702     492     94      25.59570765661253
p1214   8       9       A       43      389     583     546     197     27.357792946530147

Step 2: visualize quality distribution

nanoloop tsv_to_plot \
  --tsv examples/res/p1214_no_pcr_nt_qual.tsv.gz \
  --type nt_qual \
  --range p1214:0-10000 \
  --add_qual_avg true \
  --output examples/res/p1214_no_pcr_nt_qual.jpg

Plot shows a base quality drop near 5000–6200, suggesting an R-loop

Step 3: call peaks

Use nanoloop tsv_to_peak to call peaks (potential R-loops) using a rolling average approach:

nanoloop tsv_to_peak \
  --tsv examples/res/p1214_no_pcr_nt_qual.tsv.gz \
  --type nt_qual \
  --output examples/res/p1214_no_pcr_nt_qual_peak.bed.gz

The resulting BED file successfully captures the potential R-loop regions:

zcat < examples/res/p1214_no_pcr_nt_qual_peak.bed.gz | head

p1214	0	24
p1214	5469	5830
p1214	6018	6035

Alternatively, we can convert the TSV file into a MACS3-compatible BED file for peak calling. In this approach:

# Convert TSV to BED
nanoloop tsv_to_bed \
  --tsv examples/res/p1214_no_pcr_nt_qual.tsv.gz \
  --type nt_qual \
  --output examples/res/p1214_no_pcr_nt_qual.bed.gz

# Call peaks with MACS3
macs3 callpeak -f BED \
  -t examples/res/p1214_no_pcr_nt_qual.bed.gz \
  -n p1214_no_pcr_nt_qual_macs3 \
  -g 8779 \
  --keep-dup all \
  --nomodel \
  --extsize 200 \
  --outdir examples/res/p1214_no_pcr_nt_qual_macs3_peaks

Note that in the simulated BED file, each reference position is represented by multiple single-nucleotide read tags. The number of tags at each position reflects:

• The inverse of the average quality score (for --type nt_qual)
• The C-to-T conversion frequency (for --type nt_count)

To accurately capture these signals, we use --keep-dup all to retain all duplicate tags and --nomodel to skip fragment length estimation, since our tags are already single-nucleotide in length. The results produced are consistent with the rolling average approach described above.

cat examples/res/p1214_no_pcr_nt_qual_macs3_peaks/p1214_no_pcr_nt_qual_macs3_peaks.narrowPeak

p1214	4722	6364	p1214_no_pcr_nt_qual_macs3_peak_1	2414	.	1.57326	245.385	241.431	1044

Using base count: --type nt_count:

This approach focuses on the frequency of C-to-T conversions, another signature of R-loops in A3A-treated samples.

Step 1: generate TSV

nanoloop bam_to_tsv \
  --bam examples/bam/p1214_no_pcr.bam \
  --ref examples/ref/p1214.fa \
  --type nt_count \
  --output examples/res/p1214_no_pcr_nt_count.tsv.gz

zcat < examples/res/p1214_no_pcr_nt_count.tsv.gz | head

#chr    start   end     ref_nt  A       T       C       G       N
p1214   0       1       A       969     0       0       0       0
p1214   1       2       A       1109    0       0       0       0
p1214   2       3       A       1198    0       0       0       0
p1214   3       4       A       1339    0       0       0       0
p1214   4       5       A       1483    0       0       2       0
p1214   5       6       A       1620    0       0       4       0
p1214   6       7       A       1713    0       0       0       0
p1214   7       8       A       1722    0       1       1       0
p1214   8       9       A       1754    0       0       4       0

Step 2: visualize conversion frequencies

nanoloop tsv_to_plot \
  --tsv examples/res/p1214_no_pcr_nt_count.tsv.gz \
  --type nt_count \
  --range p1214:0-10000 \
  --add_gc true \
  --output examples/res/p1214_no_pcr_nt_count.jpg

The plot shows elevated C-to-T conversion frequencies around 5000-6200, indicating potential R-loop regions:

Step 3: call peaks

Use the rolling average approach to identify regions with significant C-to-T conversion:

nanoloop tsv_to_peak \
  --tsv examples/res/p1214_no_pcr_nt_count.tsv.gz \
  --type nt_count \
  --conversion_cutoff 0.03 \
  --output examples/res/p1214_no_pcr_nt_count_peak.bed.gz

The --conversion_cutoff 0.03 threshold was determined from the visualization in Step 2. The resulting peaks capture the potential R-loop regions:

zcat < examples/res/p1214_no_pcr_nt_count_peak.bed.gz | head

p1214   0       24
p1214   5469    5830
p1214   6018    6035

Alternatively, we can use MACS3 for peak calling:

# Convert TSV to BED
nanoloop tsv_to_bed \
  --tsv examples/res/p1214_no_pcr_nt_count.tsv.gz \
  --type nt_count \
  --output examples/res/p1214_no_pcr_nt_count.bed.gz

# Call peaks with MACS3
macs3 callpeak -f BED \
  -t examples/res/p1214_no_pcr_nt_count.bed.gz \
  -n p1214_no_pcr_nt_count_macs3 \
  -g 8779 \
  --keep-dup all \
  --nomodel \
  --extsize 200 \
  --outdir examples/res/p1214_no_pcr_nt_count_macs3_peaks

The MACS3 results are consistent with the rolling average approach as well:

cat examples/res/p1214_no_pcr_nt_count_macs3_peaks/p1214_no_pcr_nt_count_macs3_peaks.narrowPeak

p1214   5093    6061    p1214_no_pcr_nt_count_macs3_peak_1      2187    .       11.9622 222.658 218.707 671

Below is an IGV snapshot comparing peaks called with different approaches: