Rare Disease Detectives

Definition orphan disease

J K Aronson, Chairman of the Editorial Board, British Journal of Clinical Pharmacology

The word orphan comes from the Greek word orphanos, a child who has lost one parent or both, or an adult who has lost a child. It goes back to the putative Indo-European root ORBH, bereft, as in the Latin word orbus. The obsolete English words orbation and orbity meant orphanhood or childlessness. One who is bereft of freedom is a slave, made to work hard – consider the words for work in some modern European languages, such as the German Arbeit and the Czech robota. In his 1920 play R.U.R. (Rossum's Universal Robots) Karel Čapek introduced the word robot (female robotka) for an imagined race of mechanical men and women. And the etymology reminds us of the link between orphans and the workhouse.

In modern English the word orphan is most commonly used in its original Greek sense, but metaphorical meanings have also emerged. An orphan vehicle, for example, is a discontinued model, and an orphan is a line of type that begins a new paragraph at the bottom of a column or page.

Medical orphans

An orphan virus, such as hepatitis G , is not linked to a recognized disease. The term was introduced as long ago as 1954 by Melnick, who described ‘… new viruses, provisionally called “orphan viruses” (as we know so little to what diseases they belong), from patients suspected of having nonparalytic poliomyelitis’. The term is not entirely felicitous – a virus that is initially labelled an orphan may eventually find its missing disease. The same could be said of orphan genes and enzymes. For example, several enzymes have catalytic sites capable of being occupied by millimolar concentrations of ethanol ; their physiological roles are not known, at least not yet.

Orphan receptors are receptors that have been identified from gene sequences but have no known endogenous ligand or physiological function. One such, a member of the family of opioid receptors, is called ORL1 or OP4, although it is gradually losing its orphan status. An endogenous ligand, variously called nociceptin and orphanin FQ , has been identified, but even non-selective ligands with high affinities for OP1, OP2, and OP3 receptors have very low affinity for ORL1 [, ] and its physiological role is not known. On the other hand, there are high-affinity ligands that define subtypes of the ORL1 receptor .

The term orphan disease implies two separate but related concepts. It has been used to describe diseases that are neglected by doctors, and has been applied, for example, to Fabry's disease, alveolar echinococcosis, variant renal cancer, high myopia, and even some common conditions, such as endometrial cancer and tobacco addiction. However, more specifically the term orphan disease is used to designate diseases that affect only small numbers of individuals (so-called health orphans).

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