Identification of SOX3 as an XX male sex reversal gene in mice and humans

Abstract

Sex in mammals is genetically determined and is defined at the cellular level by sex chromosome complement (XY males and XX females). The Y chromosome-linked gene sex-determining region Y (SRY) is believed to be the master initiator of male sex determination in almost all eutherian and metatherian mammals, functioning to upregulate expression of its direct target gene Sry-related HMG box-containing gene 9 (SOX9). Data suggest that SRY evolved from SOX3, although there is no direct functional evidence to support this hypothesis. Indeed, loss-of-function mutations in SOX3 do not affect sex determination in mice or humans. To further investigate Sox3 function in vivo, we generated transgenic mice overexpressing Sox3. Here, we report that in one of these transgenic lines, Sox3 was ectopically expressed in the bipotential gonad and that this led to frequent complete XX male sex reversal. Further analysis indicated that Sox3 induced testis differentiation in this particular line of mice by upregulating expression of Sox9 via a similar mechanism to Sry. Importantly, we also identified genomic rearrangements within the SOX3 regulatory region in three patients with XX male sex reversal. Together, these data suggest that SOX3 and SRY are functionally interchangeable in sex determination and support the notion that SRY evolved from SOX3 via a regulatory mutation that led to its de novo expression in the early gonad.

Did sex chromosome turnover promote divergence of the major mammal groups?

“…SRY is thought to have evolved from SOX3 , a highly conserved gene that lies on the X in mammals 44 and is expressed in the central nervous system and germcells, but not the somatic cells of the testis, where the sex determining message must be received 45. Mis‐expression of SOX3 in the Sertoli cells causes sex reversal in humans and mice 46, suggesting that a change in tissue of expression, perhaps by fusion with a gonad‐specific promotor, created the male‐dominant SRY gene that took over sex determination from the ancestral gene 47.…”

Testis development in the absence of SRY: chromosomal rearrangements at SOX9 and SOX3

“…35 Duplications involving SOX3 and deletions of its 5′ region (Figure 1d) have been reported in at least four cases of XX DSD. 16,17 Moreover, a mouse model in which SOX3 is ectopically expressed in the developing gonads shows complete XX sex reversal, suggesting that gain-of-function mutations of SOX3 might act as an SRY surrogate in sex determination, promoting SOX9 gonadal expression. 16 Interestingly, SOX3 duplications have been reported in two unrelated 46,XY individuals with X-linked hypopituitarism, whereas their carrier mothers were unaffected.…”

“…16,17 Moreover, a mouse model in which SOX3 is ectopically expressed in the developing gonads shows complete XX sex reversal, suggesting that gain-of-function mutations of SOX3 might act as an SRY surrogate in sex determination, promoting SOX9 gonadal expression. 16 Interestingly, SOX3 duplications have been reported in two unrelated 46,XY individuals with X-linked hypopituitarism, whereas their carrier mothers were unaffected. 36,37 No hypopituitarism was present in our case 20 or in the patient reported by Moalem et al 17 An X-linked dominant but leaky mutation affecting sex development in a portion of XX subjects might be hypothesized, either as a consequence of the X-inactivation pattern in the developing gonad or of specific genomic modifiers.…”

“…Ectopic expression of Sox3 in the mouse bipotential gonad frequently leads to complete XX sex reversal. 16 Moreover, duplications of SOX3 or its 5′ region have been reported in 46,XX DSD patients. 16,17 Here, we present two new subjects with cryptic duplications upstream of SOX9 that were identified among 19 unrelated novel cases with 46,XX isolated DSDs, either familial (2 cases) or sporadic (17 cases), all SRY-negative.…”

“…16 Moreover, duplications of SOX3 or its 5′ region have been reported in 46,XX DSD patients. 16,17 Here, we present two new subjects with cryptic duplications upstream of SOX9 that were identified among 19 unrelated novel cases with 46,XX isolated DSDs, either familial (2 cases) or sporadic (17 cases), all SRY-negative. In the same cohort, we also identified the second case of reciprocal translocation upstream of SOX9, causing XX DSD.…”