Molecular and Cellular Endocrinology 179 (2001) 3 – 16
www.elsevier.com/locate/mce
Testis determination in mammals: more questions than answers
Reiner A. Veitia a,*, Laura Salas-Cortés a, Chris Ottolenghi a, Eric Pailhoux b,
Corinne Cotinot b, Marc Fellous a
a
Immunogénétique Humaine, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris, Cedex 15, France
b
Biologie du dé6eloppement et Biotechnologies, INRA, 78350 Jouy en Josas, France
Abstract
In humans, testis development depends on a regulated genetic hierarchy initiated by the Y-linked SRY gene. Failure of
testicular determination results in the condition termed 46,XY gonadal dysgenesis (GD). Several components of the testis
determining pathway have recently been identified though it has been difficult to articulate a cascade with the known elements of
the system. It seems, however, that early gonadal development is the result of a network of interactions instead of the outcome
of a linear cascade. Accumulating evidence shows that testis formation in man is sensitive to gene dosage. Haploinsufficiency of
SF1, WT1 and SOX9 is responsible for 46,XY gonadal dysgenesis. Besides, data on SRY is consistent with possible dosage
anomalies in certain cases of male to female sex reversal. 46,XY GD due to monosomy of distal 9p and 10q might also be
associated with an insufficient gene dosage effect. Duplications of the locus DSS can lead to a failure of testicular development
and a duplication of the region containing SOX9 has been implicated in XX sex reversal. Transgenic studies in mouse have shown,
however, that this mammal is less sensitive to gene dosage than man. Here, we will try to put in place the known pieces of the
jigsaw puzzle that is sex determination in mammals, as far as current knowledge obtained from man and animal models allows.
We are certain that from this attempt more questions than answers will arise. © 2001 Elsevier Science Ireland Ltd. All rights
reserved.
Keywords: 46,XY sex reversal; 46,XY gonadal dysgenesis; Sex determination; Gene dosage; XX sex reversal
1. Introduction a partial form. In 46,XY complete GD, there is an
absence of testis formation, whereas in 46,XY partial
Sexual development in mammals is a complex pro- GD there is some testicular development. Individuals
cess that has been formally divided into two steps: with the former condition usually have a completely
determination of the gonad and then its differentiation, female phenotype, whereas in the latter, ambiguous
leading to the development of an appropriate urogeni- external genitalia are common (Polani, 1981; Berkovitz,
tal tract. Gonadal sex determination implies the deci- 1992). Sex reversal also concerns XX individuals with
sion to make a testis or an ovary from the indifferent male or hermaphrodite phenotypes. Testis differentia-
gonadal primordium. Sex determination in man has tion may take place in the absence of Y specific se-
been equated with testis determination. However, a quences (Fig. 1; Table 1). This type of pathology is also
male phenotype is also dependent on the production of encountered in domestic species, especially in dogs,
several hormones (anti-Müllerian hormone — AMH horses, pigs and goats. Recent studies on these animal
— and sex steroids) early in gestation by the developing models indicate that they constitute a promising way in
testis. If a testis fails to develop properly and there is a order to isolate new genes specifically involved in the
consequent deficit in hormone production, the individ- gonadal differentiation process.
ual may present with ambiguous genitalia or even a
completely female phenotype. Clinically, this condition
is termed 46,XY GD and is divided into a complete and 2. SRY: the pivotal gene of sex determination in humans
* Corresponding author. Tel.: 33-14568-8920: Fax: 33-140613153. The gene that triggers the testis determining cascade,
E-mail address: rveitia@pasteur.fr (R.A. Veitia). SRY (Sex determining region, Y), is located on Yp,
0303-7207/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 3 0 3 - 7 2 0 7 ( 0 1 ) 0 0 4 6 0 - 9
, 4 R.A. Veitia et al. / Molecular and Cellular Endocrinology 179 (2001) 3–16
near the pseudoautosomal region. It consists of a single unable to reproduce, then almost all mutations are de
exon with a central conserved motif, which was first novo. However, several cases of familial mutations
identified in the High Mobility Group proteins (Sinclair associated with 46,XY complete GD have been re-
et al., 1990). This motif is termed HMG-box and has ported (see for instance, Jäger et al., 1992; Vilain et al.,
DNA-binding and bending activities, suggesting that 1992). The variant is carried by both normal fertile
SRY functions as a transcriptional regulator (Pontiggia males and 46,XY females. Incomplete penetrance may
et al., 1994). Mutations in SRY or in its flanking be caused by the genetic background, a threshold effect
regions have been identified in cases of 46,XY complete due to altered biochemical properties of the SRY
and partial GD. More than 30 mutations have been protein or an undetected gonadal mosaicism in the
described in the open reading frame and almost all lie fathers. The existence of a threshold for the appropriate
within the HMG-box motif, indicating the functional function of murine Sry has been proposed since reduced
importance of this region (Veitia et al., 1997). In our levels may produce sex reversal (Capel et al., 1993;
experience the frequency of SRY mutations in 46,XY Laval et al., 1995). Fechner et al. (1994) have analyzed
females is approximately 15–20% and most of them cells from a 46,XX (SRY+) true hermaphrodite and
induce a complete GD (McElreavey, 1996). Microdele- described that the translocated X chromosome was
tions 5% and 3% to the SRY gene have also been iden- observed to be late replicating in approximately 50% of
tified in two cases of 46,XY complete and partial cells. As the late-replicating chromosome is supposed to
gonadal dysgenesis, respectively which might modify be the inactive one, this may play a role in the incom-
the regulation of this gene (McElreavey et al., 1992, plete testis determination in these patients. This could
1996). Individuals with mutations in SRY are generally result in an insufficient expression of the SRY protein
necessary to induce full testis formation or X-inactiva-
tion may result in a disruption of the timing of SRY
expression. A more recent cytogenetic study carried on
lymphocytes from SRY+ XX individuals has sug-
gested again a connection between incomplete mas-
culinization and the preferential translocation (resulting
from selection) of the Yp fragment onto the inactive X
chromosome (Kusz et al., 1999). Accordingly, above-
threshold levels of SRY are expected to be required to
assure a normal testicular determination. That this is a
recurrent theme in human sex determination.
In mice, Sry transcripts appear in the somatic cells of
the genital ridge, which will originate Sertoli cells,
between 10.5 and 12.5 dpc peaking at 11.5dpc (Gubbay
et al., 1990; Hacker et al., 1995). In man, SRY mRNA
expression commences at 41 dpo (days post ovulation),
peaks at 44 dpo and persists at low levels throughout
the embryonic period and beyond (Hanley et al., 2000).
The presence of the SRY protein in the genital ridge
right after testis formation has been demonstrated by
Poulat et al. (1995), using polyclonal antibodies. Using
a monoclonal antibody, we have shown the presence of
SRY in the nuclei of Sertoli and germ cells in the testes
of individuals of different ages (26-week-old fetus, 1-
month-old boy and 32-year-old man). This suggests
that the SRY protein is present from the period of
testicular formation early in fetal life until adulthood
(Salas-Cortés et al., 1999). This expression pattern is
also observed in pigs and ruminants (Payen et al., 1996;
Parma et al., 1999). This contrasts with murine Sry
where transcripts are present during a specific temporal
window.
In patients with 46,XY GD without SRY mutations,
Fig. 1. Simplified classification of abnormal testicular differentiation. there may be some differentiation of the gonad. The
