Molecular characterization and mapping of ATOH7, a human atonal
homolog with a predicted role in retinal ganglion cell development
Nadean L. Brown,
Susan L. Dagenais,
Department of Pediatrics at Children’s Memorial Institute for Education and Research, Northwestern University Medical School,
Chicago, Illinois 60614-3394, USA
Department of Human Genetics, University of Michigan School of Medicine, Ann Arbor, Michigan 48109-0618, USA
Department of Internal Medicine, 4510 MSRB I, University of Michigan School of Medicine, Ann Arbor, Michigan 48109-0650, USA
Received: 29 June 2001 / Accepted: 18 September 2001
Abstract. The human ATOH7 gene encodes a basic helix-loop-
helix (bHLH) transcription factor that is highly similar to Dro-
sophila Atonal within the conserved bHLH domain. The ATOH7
coding region is contained within a single exon. We mapped
ATOH7 to Chromosome (Chr) 10q21.3-22.1, a region syntenic to
the segment of mouse Chr 10 where Atoh7 (formerly Math5)is
located. The evolutionary relationship between ATOH7 and other
atonal homologs was investigated using parsimony analysis. A
direct comparison of ATH5/7 and ATH1 protein subgroups to
Atonal also revealed a nonrandom distribution of amino acid
changes across the bHLH domain, which may be related to their
separate visual and proprioceptive sensory functions. Among
bHLH genes, ATOH7 is most closely related to Atoh7. This se-
quence conservation extends significantly beyond the coding re-
gion. We define blocks of strong homology in flanking human and
mouse genomic DNA, which are likely to include cis regulatory
elements. Because targeted deletion of Atoh7 causes optic nerve
agenesis in mice, we propose ATOH7 as a candidate for human
optic nerve aplasia and related clinical syndromes.
Early development of the mammalian retina proceeds through a
series of cell fate decisions in which progenitor cells in the optic
cup progressively exit mitosis, migrate to specific laminar posi-
tions, and terminally differentiate into one of seven basic neural or
glial cell types. Defects in retinal cell determination or differen-
tiation lead to specific malformations in humans, including Leber’s
congenital amaurosis (Freund et al. 1998; Sohocki et al. 1998),
cone-rod dystrophy (Freund et al. 1997; Sohocki et al. 1998; Swain
et al. 1997), enhanced S-cone syndrome (Cepko 2000; Haider et al.
2000), and optic nerve aplasia or hypoplasia (Lee et al. 1996; Scott
et al. 1997; Weiter et al. 1977). Although the genetic basis for
retinal histogenesis is poorly understood, recent findings suggest
that key proteins controlling some of these developmental pro-
cesses are nuclear transcription factors (Brown et al. 2001; Freund
et al. 1997; Kobayashi et al. 1999; Swain et al. 1997; Wang et al.
Basic helix-loop-helix (bHLH) transcription factors regulate
multiple aspects of retinal neuron formation in vertebrates and
invertebrates (Jan and Jan 1993; Vetter and Brown 2001; Cepko
1999). Proneural bHLH proteins contain basic DNA-binding and
helix-loop-helix dimerization motifs and act to promote neuron
formation. They are generally expressed by groups of equivalent
neural progenitor cells, with each group ultimately giving rise to
one or more neurons. A key proneural bHLH gene that controls
photoreceptor development in Drosophila is atonal, named for its
chordotonal stretch receptor mutant phenotype (Jarman et al.
1993). While several vertebrate ato
nal homologs (Atoh proteins)
have been identified, most act only during peripheral or central
nervous system development and are not expressed significantly
during eye formation. In contrast, the ATH5/7 subclass (named for
Xenopus Xath5 and mouse Atoh7) is highly expressed by retinal
progenitors during the early stages of eye development in ze-
brafish, chick, frog, and mouse (Brown et al. 1998; Kanekar et al.
1997; Liu et al. 2001; Masai et al. 2000).
Mutations in the fly atonal, zebrafish Zath5, or mouse Atoh7
(Math5) genes cause agenesis of the initial neuron class in the
diverse eye types of these organisms, which consist of R8 photo-
receptors in Drosophila (Jarman et al. 1994, 1995) and retinal
ganglion cells (RGCs) in vertebrates (Brown et al. 2001; Jarman et
al. 1994, 1995; Kay et al. 2001; Wang et al. 2001). Because Dro-
sophila photoreceptors are induced sequentially, by an iterative
process that depends on previously formed photoreceptors, atonal
mutant flies are nearly eyeless, whereas zebrafish Zath5 and mouse
Atoh7 mutants have eyes but lack RGCs and optic nerves. These
phenotypes are consistent with gain-of-function experiments in
which ectopic expression of frog Xath5 or chicken Cath5 was
shown to bias retinal progenitors toward an RGC fate (Kanekar et
al. 1997; Liu et al. 2001).
We report the cloning, chromosome mapping, and phyloge-
netic analysis of human ATOH7. By comparing the flanking ge-
nomic DNA in humans and mice, we have identified multiple
discrete segments of sequence homology. On the basis of these
conserved features, we propose that ATOH7 is the human ortholog
of Atoh7. Given the central role of Atoh7 (Math5) in RGC and
optic nerve formation, ATOH7 mutations may underlie clinically
important congenital malformations or degenerative diseases of
the optic nerve.
Materials and methods
DNA library screening.
A Sau3A-partial human genomic DNA library
constructed in Fix (Stratagene) was screened using a radiolabeled full-
length Atoh7 cDNA probe (Brown et al. 1998) at reduced stringency. Two
positive clones (#3 and #6) were purified for further analysis.
DNA cloning and sequence analysis.
Restriction maps of phage
clones were prepared by using BamHI, EcoRI, SalI, HindIII, and XhoI, and
Southern analysis. Human genomic DNA fragments hybridizing to the
Atoh7 cDNA probe were subcloned in pGEM3Z and used as templates for
automated sequencing. A 5.7-kb segment from the assembled sequence of
phage clone #6 was deposited in GenBank (accession no. AF418922) and
is largely identical to the sequence of an independent BAC clone
(AC016395). We also identified a short 3Ј EST for ATOH7 (H05728) and
sequenced the corresponding 1.7-kb cDNA clone. The genomic sequence
Correspondence to: T. Glaser; E-mail: email@example.com
The nucleotide sequence data reported in this paper have been submitted to
GenBank and assigned the accession numbers AF418922 and AF418923.
Mammalian Genome 13, 95–101 (2002).
© Springer-Verlag New York Inc. 2002