How is "Ear Protrusion" encoded by the genome?
The external part of the ear, the human pinna is made up of a piece of cartilage covered with skin and attached to the skull by ligaments, muscles, and fibrous tissue. Every ear shape is slightly different, but the term ear protrusion refers to a common condition that simply means an ear sticks out by more than 2cm—or in scientists speak, their pinna is 2cm away from the head.
While this isn’t typically causing concern, some disorders can affect how a human pinna is developed. For example, the study of rare, familial microtia cases (a disorder characterized by a small, abnormally shaped pinna) has established that mutations in the HOXA2 gene can severely impact pinna development. There is also great non-pathological variation between humans in pinna shape and size. This variation has been reported to be influenced by age, sex, and ethnicity. However, no genetic variants influencing normal pinna morphology have yet been reported.
To identify genetic variants associated with normal pinna morphology, researchers performed a genome-wide association study (GWAS) in 5,062 Latin Americans—using facial photographs, a qualitative assessment was performed for 10 pinna traits, including helix rolling, lobe attachment, lobe size, and ear protrusion. Each trait was scored as an ordered categorical variable, with 0 being the lowest level of expression of the trait and 2 the highest. Results showed that rs3827760 near the Ectodysplasin A receptor (EDAR) gene was significantly associated with ear protrusion. The EDAR gene has been known to be a key regulator of embryonic skin appendage development. Additionally, it showed that the expression of EDAR in the developing mice impacted their ears; EDAR-deficient mice had an abnormally shaped ear. The variant has also been associated with characteristic tooth morphologies, hair type, and sweat gland density in East Asians, where this allele is present at high frequency while being nearly absent in European or African populations. The variants also showed significant associations with helix rolling, lobe attachment, and lobe size in this study.
This study helped determine how genetics influence the shape of ears. It will be important to further evaluate these regions' role in ear development and disorders. The development of such applications may benefit from a refined knowledge of the genetic determinants of pinna morphology.
https://pubmed.ncbi.nlm.nih.gov/26105758/
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