The genetics of auricular development and malformation: new findings in model systems driving future directions for microtia research

Abstract

Microtia is a term used to describe a wide array of phenotypic presentations of the outer ear. Although the majority of the cases are isolated in nature, much of our understanding of the causes of microtia has been driven by the identification of genes underlying syndromic forms where the anomaly co-presents with various other craniofacial and extra-craniofacial structural defects. In this review we discuss recent findings in mice deficient in Hoxa2, a key regulator of branchial arch patterning, which has necessitated a revision to the canonical model of pinna morphogenesis. The revised model will likely impact current classification schemes for microtia and, as we argue in this review, the interpretation of the developmental basis for various auricular malformations. In addition, we highlight recent studies in other mammalian species that are providing the first clues as to possible causes of at least some isolated anomalies and thus should now accelerate the search for the more elusive genetic contributions to the many isolated and non-syndromic cases of microtia. These findings, together with the application of new genome-level sequencing technologies and more thorough quantitative assessment of available mutant mouse resources, promise an exciting future for genetic studies in microtia.

Keywords: Anotia; Auricular development; Craniofacial microsomia; Goldenhar syndrome; Microtia; Mouse models; OAVS.

Figure 1. Development of auricular form in humans and mice

(a) updated model of auricular morphogenesis adapted from Porter & Tan (2005), the original sketches from Streeter (1922), and modified to accommodate the data in mice from Minoux et al (2013). (I) the branchial arches contributing to auricular development - the mandibular process of branchial arch 1 (dark pink) and arch 2 (orange) – are evident during week five of gestation. (II) in week six, distinct tubercles or hillocks (the “hillocks of His”) appear, (III) the hillocks coalesce, (IV) free ear flap appears caudal to the arch 2 hillock region, (V) during the ninth week, the arch 1 and 2 tissue completely merge to obliterate the branchial cleft; the EAM forms via invagination within branchial arch 1, (VI) the final ear form becomes apparent after the 13^th week. The first branchial cleft is marked in II and III with a white asterisk. The EAM is marked in IV, V and VI by a white arrowhead. (b) Early mouse embryos (embryonic day 10-11 [E10-E11]) showing the early hillocks and their initial growth and merging. (c) Representative photographs of the auricle from human embryos at approximately 57 days, 94 days and 118 days of gestation (top row) are compared to roughly equivalently staged auricles from mouse embryos at E13.5, E16, and E18.5. Note that in the mouse the main body of the auricle, which derives from branchial arch 2, folds over to cover the EAM until after birth. Human conceptal specimens courtesy of the Birth Defects Research Laboratory, University of Washington.

Figure 2. Mirror-image auricular duplications: a role for ectopic expression of the HOXA2 genetic program?

The Hoxa2-regulated genetic program is normally restricted to branchial arch 2-derived mesenchyme where it is required for normal pinna morphogenesis (a) E18.5 wildtype embryo. (b) Ectopic expression of Hoxa2 in all neural crest-derived mesenchyme, including that contributing to branchial arch 1 structures, produces embryos with mirror-image duplications of auricular structures (black arrow) (c) higher magnification image of a mutant embryo showing duplicated auricular structures as well as small ectopic structures reminiscent of pre-auricular tags (white arrowhead). (d) Partial mirror-image duplication of auricular structures in a patient. Images in a, b and c (albeit flipped in orientation from their original presentation) are reproduced from Minoux et al (2013) with permission from the journal Development. doi: 10.1242/dev.098046.

Figure 3. Schematic representation of adult (a) human and (b) mouse auricles

The labeling of structures of the mouse pinna was adapted from Theiler & Sweet, 1986. The obscured opening to the mouse EAM is enlarged in the red rectangle and the tragus represented transparently to better show the EAM location.

Figure 4. Auricular dysmorphism in syndromic microtia patients with confirmed genetic diagnoses

a. Comparison of ear presentations from patients with different types of syndromic microtia. In most cases the external ear morphology is very specific, and the images representative, for each syndrome. b. In some disorders, such as Treacher-Collins syndrome, the auricular malformation can vary greatly. In some cases the ears in different patients are strikingly similar (top row) while others bear little similarity (bottom row). Photographs of patient ears for each of the following syndromes: CHARGE, Auriculocondylar, Miller, BOF, and Treacher-Collins syndrome, were generously provided by Prof Michael Cunningham (Seattle Children's Craniofacial Center). The image of auricular phenotype in LAMM syndrome was reproduced from GeneReviews (www.ncbi.nlm.nih.gov/books/NBK1116/; copyrighted to University of Washington, Seattle).