Human testis development is initiated when SOX9, a dosage-sensitive transcription factor gene at 17q24.3, is upregulated above a critical threshold in the supporting-cell lineage of the embryonic gonad, normally in response to SRY. In 2011, Cox and colleagues described a family in which a roughly 178 kb duplication of non-coding DNA about 0.5 Mb upstream of SOX9 caused complete testicular development in 46,XX (SRY-negative) individuals, while the 46,XY carrier father was normal and fertile — establishing copy-number gain of a SOX9 regulatory region as a cause of 46,XX testicular disorders of sex development (DSD) while leaving the mechanism unexplained. Later work identified the duplicated interval (the RevSex region) and gonadal enhancers within and adjacent to it (eSR-A, eSR-B, eALDI) that are activated by SF1 and SOX9, repressed by FOXL2, and active in fetal Sertoli cells, and documented that the duplication is recurrent but incompletely penetrant in 46,XX carriers. What remains open is the quantitative mechanism: how enhancer copy-number gain raises gonadal SOX9 above the male-determination threshold in the absence of SRY, why no gonadal effect has been reported in any 46,XY carrier in the published families while many 46,XX carriers are asymptomatic, and direct functional demonstration of duplication-driven SOX9 upregulation in human gonadal-lineage cells.

Background

In mammals, the embryonic gonad arises as a bipotential primordium whose supporting-cell precursors can become either Sertoli cells — the organizing cell type of testis cords — or granulosa cells, the ovarian supporting lineage. The Y-chromosomal gene SRY (sex-determining region Y), encoding a DNA-binding transcription factor, is expressed in a narrow window in male embryos and acts on a single major target, SOX9 (SRY-box 9, an HMG-box transcription factor gene on chromosome 17q24.3). If SOX9 activity rises above a critical threshold in a supporting-cell precursor, SOX9 autoactivates and locks in Sertoli fate together with the nuclear receptor SF1 (also called NR5A1); if SOX9 stays below threshold, ovarian factors such as FOXL2, WNT4 and RSPO1 suppress it and granulosa fate follows. Disorders (or differences) of sex development (DSD) are congenital conditions in which gonadal or genital development deviates from the chromosomal sex. In '46,XX testicular DSD', individuals with two X chromosomes develop testicular tissue; in the large majority SRY has been translocated onto an X chromosome or an autosome, and only a minority are SRY-negative. SOX9 is dosage-sensitive in two tissues: heterozygous loss-of-function mutations cause campomelic dysplasia, a severe skeletal dysplasia frequently accompanied by XY sex reversal, and balanced chromosomal rearrangements of the 17q24.3 region that leave the SOX9 coding sequence intact cause either isolated DSD or isolated skeletal/craniofacial phenotypes such as Pierre Robin sequence. These observations imply that tissue-specific enhancers — short non-coding DNA segments that bind transcription factors and activate their target promoter over long genomic distances, usually constrained within topologically associated domains (TADs), the chromatin folding units of the genome — are dispersed across the ~2 Mb gene desert upstream of SOX9, and that copy-number variants (CNVs) duplicating or deleting these segments alter enhancer dosage for one tissue without affecting others. In the mouse, the gonadal Sox9 enhancers TESCO (testis-specific enhancer of Sox9 core) and Enh13 (enhancer 13) have been defined by targeted deletion; deleting Enh13 alone reduces Sox9 expression enough to cause complete XY male-to-female sex reversal. Against this background, Cox, Willatt, Homfray and Woods (NEJM 2011) reported the first familial duplication case: a ~178 kb duplication in the gene desert ending roughly 500 kb upstream of SOX9, in which SRY-negative 46,XX carriers developed testes and presented as infertile males, while 46,XY carriers, including the father, were completely normal and fertile. The letter closed by flagging as unresolved: the molecular mechanism by which the duplication enhances SOX9 expression specifically in the gonad (enhancer content or tissue-specific chromatin effects); why SOX9-dependent skeletal development is unaffected; whether the duplication is penetrant only in the absence of SRY or requires additional genetic or epigenetic modifiers (the unaffected 46,XY father); whether the mechanism generalizes beyond one family; the incomplete breakpoint-level characterization of the duplicated segment; and the absence of direct functional assays of SOX9 expression in relevant cell types. Later work resolved several of these strands. Successive overlapping CNVs narrowed the dosage-sensitive interval to a 78 kb 'sex determining region' (Benko et al. 2011), then to a ~41 kb and a 24 kb minimal region nicknamed RevSex (Hyon et al. 2015; Croft et al. 2018), and Kim et al. (2015) showed that 46,XY and 46,XX isolated DSD map to two separable upstream regulatory regions (XYSR and XXSR). Croft et al. (2018) then identified three human gonadal SOX9 enhancers — eSR-A, eSR-B (within RevSex) and eALDI — using luciferase reporter assays (in which a candidate enhancer sequence is cloned upstream of a light-emitting reporter gene to quantify its ability to drive transcription), enhancer-lacZ transgenic mice (expression in fetal Sertoli and granulosa cells) and CRISPR deletion in mouse; the enhancers are activated by SF1 plus SOX9, act synergistically, and are strongly repressed by the ovarian factor FOXL2. The accepted working model is thus that duplication adds copies of gonadal enhancers that can be activated without SRY, tipping the bipotential gonad toward testis. What that model still does not deliver is the quantitative dosage-to-threshold mechanism in the human gonadal context, an explanation of the puzzling penetrance pattern (no gonadal effect reported in 46,XY carriers in any published family; a substantial fraction of 46,XX carriers asymptomatic, with no duplication-size/phenotype correlation), and any direct measurement of SOX9 expression from the duplicated allele in human gonadal supporting-lineage cells.

Problem Statement

Treat the causal link between the duplication and gonadal SOX9 upregulation as the accepted working model to be assumed, not something the answer must re-derive.

(1) Mechanism. Explain the quantitative mechanism by which duplication of non-coding regulatory DNA upstream of SOX9 (the gonadal enhancer region at 17q24.3, including the eSR-A and eSR-B enhancers located some 63 kb apart, eSR-B lying within the ~24 kb RevSex interval) raises SOX9 expression in the embryonic gonad sufficiently to switch supporting-cell fate from granulosa to Sertoli and drive testicular development in SRY-negative 46,XX individuals. Quantitative here means the account must relate enhancer copy number to the level of SOX9 activity relative to the male-determination threshold, not merely describe the enhancers qualitatively.

(2) Penetrance. Account for the duplication's penetrance pattern: the absence of any reported gonadal or sex-development effect in 46,XY carriers of these duplications in all published families and cohorts to date (Cox et al. 2011; Vetro et al. 2011, 2015; Pinti et al. 2019; Unal et al. 2025), together with incomplete penetrance and variable expressivity among SRY-negative 46,XX carriers (for example the unaffected carrier mother of the 3.7 kb duplication in Sajan et al. 2023, and 5 of 12 asymptomatic carriers in Unal et al. 2025), with no correlation between duplication size and phenotype.

(3) Demonstration experiment. Specify the experiment that would directly demonstrate upregulation of SOX9 from the duplicated allele in human gonadal supporting-lineage cells.

The already-settled components of the original 2011 question — recurrence in multiple families, the enhancer identity and tissue specificity of the duplicated interval, and breakpoint-level characterization in new cases — are part of the answer's foundation, not its remaining target.

References

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