Why does ATAC data quality vary substantially across sample types? Which cell or tissue types may naturally show lower median high-quality fragment counts?

Answer:
The median number of high-quality ATAC fragments is strongly influenced by the intrinsic chromatin accessibility landscape of the sample. Some cell and tissue types naturally contain fewer accessible chromatin regions due to their biological and epigenetic characteristics. Therefore, relatively low ATAC fragment counts do not necessarily indicate experimental failure.

Examples of sample types that may naturally exhibit lower ATAC signal include:

• Brain tissues
  Neuronal and non-neuronal cell populations exhibit highly distinct chromatin accessibility landscapes. Certain neuronal subtypes may contain more spatially restricted open chromatin regions, resulting in lower detectable fragment counts. 
• Neurons and glial cells
  These cell types often contain relatively compact nuclear architecture and accessibility patterns that differ substantially from highly proliferative cell populations. 
• Solid tumors
  Tumor microenvironments contain complex mixtures of malignant cells, immune cells, and stromal cells, each with distinct epigenetic states. In addition, tumor subclones may exhibit substantial chromatin heterogeneity. 

• Immune cell subtypes 
  • Exhausted T cells (TEX) undergo extensive epigenetic remodeling driven by transcription factors such as TOX, resulting in large-scale reorganization of chromatin accessibility networks. 
  • Regulatory T cells (Treg) exhibit specialized chromatin accessibility programs distinct from conventional effector T cells. 

• Quiescent stem cells
  Chromatin accessibility in quiescent stem cells is generally lower than in actively proliferating states. 
• Terminally differentiated cells
  Cell types such as cardiomyocytes and keratinocytes often display accessibility patterns restricted to lineage-specific regulatory elements, while much of the genome remains relatively inaccessible. 
• Germ cells and early embryonic cells
  These cells undergo extensive chromatin reprogramming during development, resulting in highly dynamic and transient accessibility states. 

When interpreting ATAC QC metrics, biological context should therefore be considered together with standard sequencing and library quality indicators.