splicedrift 0.1.1

Age-related splicing drift analysis using Bayesian Beta-Binomial regression

SpliceDrift

Bayesian Beta-Binomial regression for age-related splicing drift analysis.

SpliceDrift fits a probabilistic model to RNA-seq junction counts in order to detect age-associated changes in both the mean and variability of exon inclusion (PSI) across tissues and species.

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Motivation

Most available RNA-seq datasets (< 50 million reads per sample) provide limited coverage for individual splicing events. Percent Spliced In (PSI) estimates derived from junction counts can therefore be imprecise, and direct correlation or regression analyses on PSI point estimates are often insufficiently sensitive.

SpliceDrift addresses this limitation by adopting a probabilistic modelling framework that explicitly accounts for read counts supporting each splice junction, rather than collapsing them into a single PSI value.

Model

For each exon-skipping event, splicing is quantified by the number of inclusion reads k and the total number of informative reads N. Inclusion counts are modelled with a Beta-Binomial distribution parameterised by the mean exon inclusion level (PSI) μ and a precision (inverse-dispersion) parameter φ, both modelled as functions of normalised age:

k_i  ~  BetaBinomial(N_i,  μ_i · φ_i,  (1 − μ_i) · φ_i)

logit(μ_i) = α₀  + α′ · age_z_i
 log(φ_i)  = β₀  + β′ · age_z_i

where age_z is age standardised to zero mean and unit variance.

Age-related effects are captured through two key parameters:

Parameter	Name in code	Interpretation
α′	alpha_prime	Whether mean PSI increases (α′ > 0) or decreases (α′ < 0) with age.
β′	beta_prime	Whether splicing variability increases (β′ < 0, precision drops) or decreases (β′ > 0, stabilises) with age.

Together these parameters allow age to influence both the average exon inclusion level and the stability of splicing regulation.

Beta and beta-binomial formulations are standard in alternative splicing analysis (e.g. rMATS and MAJIQ); however, to our knowledge this is the first approach to explicitly introduce linear dependence on age for both the mean and dispersion components of the model.

— Chervontseva, Interim Report #1 (December 2025)

The model is fitted using Markov Chain Monte Carlo (MCMC) via PyMC. Convergence is checked automatically (R̂, ESS, divergences) and the sampler is re-run with more conservative settings if diagnostics fail.

Installation

pip install splicedrift

Or install from source:

git clone https://github.com/zchervontseva/SpliceDrift.git
cd SpliceDrift
pip install -e .

Requirements

• Python ≥ 3.10
• PyMC ≥ 5.0, ArviZ ≥ 0.15, NumPy, Pandas, Matplotlib, Seaborn, statsmodels, tqdm

Quick start

Python API

import splicedrift

# 1. Load data
data = splicedrift.load_data("input.tsv")

# 2. Fit all (event, group) pairs
results = splicedrift.fit(data)

# 3. Save results
results.to_csv("summary.tsv", sep="\t", index=False)

# 4. Plot α′ vs β′ scatter (one panel per group)
fig = splicedrift.plot_scatter(results, save="scatter.png")

# 5. Plot PSI vs age for a specific event
fig = splicedrift.plot_event(
    data,
    event_id="HsaEX0000244",
    group="Brain - Cortex",
    save="event_plot.png",
)

Command line

# Fit the model
splicedrift fit -i input.tsv -o results/summary.tsv -v

# Scatter plot (α′ vs β′)
splicedrift plot-scatter -i results/summary.tsv -o scatter.png

# Single-event plot (PSI vs age with posterior band)
splicedrift plot-event -i input.tsv -e HsaEX0000244 -g "Brain - Cortex" -o event.png

Input format

SpliceDrift expects a tab-separated file with the following columns:

Column	Required	Description
event_id	✅	Splicing event identifier (e.g. HsaEX0000244).
sample_id	✅	Sample identifier.
k	✅	Inclusion junction read count.
N	✅	Total informative read count (k + 2 × exclusion_count).
age	✅	Continuous covariate (typically donor age in years).
group	optional	Tissue / condition / species label. If absent, all rows are treated as one group.

Example:

event_id	group	sample_id	k	N	age
HsaEX0000244	Brain - Cortex	GTEX-1117F	8	21	66
HsaEX0000244	Brain - Cortex	GTEX-111CU	4	15	57
HsaEX0000244	Lung	GTEX-1117F	12	30	66

Computing k and N from raw junction counts

If your data contains raw junction counts rather than pre-computed k/N:

k = inclusion_junction_1 + inclusion_junction_2
N = k + 2 × exclusion_junction

For VastDB / GTEx formatted files, use the convenience loader:

data = splicedrift.load_vastdb(
    "counts_psi__HsaEX0000244.tsv",
    "age_info.tsv",
)

Output format

splicedrift fit produces a TSV with one row per (event, group) pair:

Column	Description
event_id	Event identifier.
group	Group / tissue label.
n_samples	Number of samples used in the fit.
age_mean	Mean age (for de-standardising).
age_std	Std of age (for de-standardising).
alpha_prime_mean	Posterior mean of α′ (age effect on mean PSI).
alpha_prime_hdi_lo	Lower bound of 95% HDI for α′.
alpha_prime_hdi_hi	Upper bound of 95% HDI for α′.
P_alpha_prime_gt_0	Posterior probability that α′ > 0.
beta_prime_mean	Posterior mean of β′ (age effect on variability).
beta_prime_hdi_lo	Lower bound of 95% HDI for β′.
beta_prime_hdi_hi	Upper bound of 95% HDI for β′.
P_beta_prime_lt_0	Posterior probability that β′ < 0 (increased variability).
diagnostics_ok	Whether MCMC convergence diagnostics passed.

Parameters

All MCMC parameters can be tuned via the Python API or CLI flags:

Parameter	Default	CLI flag	Description
draws	1000	--draws	Posterior draws per chain.
tune	1000	--tune	Warm-up steps per chain.
chains	4	--chains	Number of MCMC chains.
cores	1	--cores	CPU cores for chain sampling.
target_accept	0.9	--target-accept	NUTS target acceptance probability.
random_seed	42	--random-seed	Random seed for reproducibility.
min_total	5	--min-total	Minimum N per sample.
rhat_max	1.01	--rhat-max	Maximum acceptable R̂.
ess_min	500	--ess-min	Minimum acceptable bulk ESS.
max_refits	2	--max-refits	Auto-refits on convergence failure.

Plots

α′ vs β′ scatter

One panel per group. Each point is a splicing event. Significance is determined by converting posterior probabilities to two-sided p-values and applying Benjamini–Hochberg FDR correction.

• Red — α′ significantly > 0 (inclusion increases with age)
• Blue — α′ significantly < 0 (inclusion decreases with age)
• Gray — not significant
• Orange × — β′ significant (variability changes with age)

PSI vs age event plot

Observed PSI values (scatter) overlaid with the posterior mean μ(age) curve and 90% credible band.

Example

Generate a synthetic dataset and run the full pipeline:

cd SpliceDrift
python examples/generate_example_data.py
splicedrift fit -i examples/example_input.tsv -o results.tsv -v
splicedrift plot-scatter -i results.tsv -o scatter.png

License

MIT