Posted in Data visualisation, python

Using Seaborn library to plot clustergrams

Unsupervised hierachical clustering can also allow direct visualisation of clusters without the need for dimensionality reduction. The full article can be found here.

There are two common strategies to for data normalisation. One way is to plot the heatmap or clustergram based on log2 fold-change instead of absolute abundance. The comparisons can be performed against time-point = 0 (baseline) for temporal studies, or against a control/placebo experiment (for static studies). However, the disadvantage of using this method is that the distribution of baseline or controls cannot be easily visualised.

Another alternative is to perform a Z-score transformation and plot the Z-scores in the heatmap. The Z-score transformation converts each gene or protein across all conditions with mean = 0 and standard deviation = 1, enabling users to easily compare expression values across multiple genes and protein. Values that are above 0 means the gene expression level is above mean expression, whereas values below 0 means that the gene expression level is below mean expression. However, the limitation is that Z-scores can only be applied for experiments that are obtained within a single experiment. Hence, the choice of either method for data normalisation is highly dependent on the research question and objectives.

Before jumping straight into plotting heatmap or clustergrams for your data, it is generally a good idea to first filter the dataset, especially when you have a large number of genes. The reason is because in most scenarios, the large majority of the genes or proteins remain unchanged, which will consequently impact the ability for the unsupervised hierarchical clustering to separate gene expression profiles based on your experimental conditions. To circumvent this limitation, you can choose to filter the dataset based on differentially expressed genes or enriched biological pathways before plot the heatmaps or clustergrams.

To plot heatmaps and clustergrams using Python, we first load the required packages. In this blog entry, we will be using the Seaborn library to plot the heatmaps and clustergrams:

import numpy as np
import pandas as pd
import seaborn as sns

Similar to my previous blog entries, we will use the transcriptomics dataset published by Zak et al., PNAS, 2012, examining how seropositive and seronegative subjects respond to the Ad5 vaccine across various time points. The summary of the study can be found here, and the processed dataset, which was analysed by Partek Genomics Suite can be found in GitHub. The fold change, ratio, p-value and adjusted p-values (q-value) are calculated with respect to baseline (timepoint = 0).

We will load and inspect the processed dataframe from GitHub. It is important to label the gene column as the index column so that gene names can be referred to in the clustergram or heatmap. The commands are as follows:

df = pd.read_csv('https://raw.githubusercontent.com/kuanrongchan/vaccine-studies/main/Ad5_seroneg.csv',index_col=0)
df.head()

The output file shows the values of the p-value (pval), adjusted p-values (qval), ratio, and fold change (fc) for 6 hours, 1-day, 3-day and 7-day time points compared to baseline (timepoint = 0):

As described above, it is important to normalise the dataset to ensure that the relative expression is comparable between different genes. Here, we will use log2 fold-change for normalisation and create log2 fold-change (log2FC) columns in the dataframe:

df['log2FC_6h'] = np.log2(df['ratio_6h'])
df['log2FC_1d'] = np.log2(df['ratio_1d'])
df['log2FC_3d'] = np.log2(df['ratio_3d'])
df['log2FC_7d'] = np.log2(df['ratio_7d'])

There are a total of 17,623 genes that are measured. To visualise the comparisons between time-points better, we will filter the dataset. Since we have previously ascertained that day 1 has the most differentially expressed genes (DEGs), we could filter the dataset based on upregulated DEGs (with fold-change > 1.5, adjusted p-value < 0.05). The filtered dataframe is saved under DEGs_up_1d. Since we are interested in plotting the log2-fold change values, we will select the log2FC columns and remove all other columns. The code is as follows:

DEGs_up_1d = df[(df['fc_1d'] > 1.5) & (df['qval_1d'] < 0.05)]
DEGs_up_1d = DEGs_up_1d.filter(items=['log2FC_6h','log2FC_1d', 'log2FC_3d', 'log2FC_7d'])

To plot the clustergram, the codes are as follows:

from mpl_toolkits.axes_grid1.axes_divider import make_axes_locatable
from mpl_toolkits.axes_grid1.colorbar import colorbar
g = sns.clustermap(DEGs_up_1d, cmap='vlag', method='average', vmin=-2, vmax=2, yticklabels=False)

There are a few Seaborn settings that are displayed in the code, and the documentation can be found here. The colour map to be ‘vlag’ was chosen as it would allow us to have a heatmap where increased expression is red and reduced expression is blue. Note that I have also assigned maximum and minimum value of 2 and -2 respectively, as I wanted to ensure that log2FC = 0 is white (to signify no change). yticklabels = False was chosen because it is near impossible to see all 826 gene names in the clustergram. The output is as shown:

As expected, the day 1 signatures are the most distinct compared to other time-points. The 6 hour and day 7 signatures are clustered together, showing that these time-points have little or no changes in gene expression profile. Interestingly, some of the DEGs have prolonged expression up to day 3, while others resolve very quickly. Do we see the same trends for the downregulated DEGs? Let’s test it out with the following command:

DEGs_down_1d = df[(df['fc_1d'] < -1.5) & (df['qval_1d'] < 0.05)]
DEGs_down_1d = DEGs_down_1d.filter(items=['log2FC_6h','log2FC_1d', 'log2FC_3d', 'log2FC_7d'])
g = sns.clustermap(DEGs_down_1d, cmap='vlag', method='average', vmin=-2, vmax=2, yticklabels=False)

Similar patterns can be seen. However, unlike upregulated DEGs where some DEGs persisted to day 3, most of the downregulated DEGs returned to baseline levels at day 3.

It’s not so hard isnt it? 🙂

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