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The distinction between diseases and phenotypes

The community uses the word phenotype with multiple meanings. The HPO defines a disease as an entity that has all four of the following features:

  • an etiology (whether identified or as yet unknown)
  • a time course
  • a set of phenotypic features
  • if treatments exist, there is a characteristic response to them

A phenotype (better: phenotypic feature) is a component of a disease. HPO terms can be used to describe the set of phenotypic features that characterize a disease. For instance, if the disease is the common cold, then the phenotypes would be runny nose, fever, cough, fatigue, etc. Therefore, in the following description:

a sepsis-like condition with intestinal pseudoobstruction, transient hypoglycemia,
cholestatic hepatitis, and transient renal failure (maximum plasma creatinine
132 μmol/L and urea 11 mmol/L at day 5 of life, which normalized on day 10).

We would conceptualize the "sepsis-like condition" as a disease, and would use HPO terms to describe

  • Intestinal pseudo-obstruction
  • Hypoglycemia with the modifier Transient.
  • Cholestatic liver disease.
  • Elevated serum creatinine.
  • Increased blood urea nitrogen.

Thus, the HPO considers a disease to be an entity that has a known or unknown cause, is characterized by one or more phenotypic features which can affect all or only a subset of individuals with the disease, a time course over which the phenotypic features may have onset and evolve, and in some cases one or more indicated treatments and a response to treatment. For instance, if the disease entity is the common cold, then the cause is a virus, the phenotypic features include fever, cough, runny nose, and fatigue, the time course usually is a relatively acute onset with manifestations dragging on for days to about a week, and the treatment may include bed rest, aspirin, or nasal sprays. In contrast, a phenotypic feature such as fever is a manifestation of many diseases. There is a grey zone between diseases and phenotypic features. For instance, diabetes mellitus can be conceptualized as a disease, but it is also a feature of other diseases such as Bardet Biedl syndrome. The HPO takes a practical stance and provides terms for such entities.