XX male syndrome
|XX male syndrome|
|Other names||De la Chapelle syndrome|
XX male syndrome, also known as De la Chapelle syndrome, is a rare congenital intersex condition where an individual with a female genotype has phenotypically male characteristics that can vary among cases. In 90 percent of these individuals, the syndrome is caused by unequal crossing over between X and Y chromosomes during meiosis in the father. This results in the X chromosome containing the SRY gene, as opposed to the Y chromosome where it is normally found. When the X with the SRY gene combines with a normal X from the mother during fertilization, the result is an XX male. Less common are SRY-negative XX males which can be caused by a mutation in an autosomal or X chromosomal gene. The masculinization of XX males is variable.
This syndrome is diagnosed through various detection methods and occurs in approximately 1:20 000 newborn males, making it much less common than Klinefelter syndrome. Treatment is medically unnecessary, although some individuals choose to undergo treatments to make them appear more male or female.
XX male syndrome is not to be confused with sex determination in some non-human mammal species that lack a Y chromosome at all, such as the Transcaucasian mole vole (Ellobius lutescens).
Signs and symptoms
The appearance of XX males can fall into one of three categories: 1) males that have normal internal and external genitalia, 2) males with external ambiguities, and 3) males that have both internal and external genital ambiguities (true hermaphrodites). External genital ambiguities can include hypospadias, micropenis, and clitoromegaly. Typically, the appearance of XX males differs from that of an XY male in that they are smaller in height and weight. Most XX males have small testes, are sterile, and have an increase in maldescended testicles compared to XY males. Some XX male individuals have decreased amounts of body hair and decreased libido. Individuals with this condition sometimes have feminine characteristics, with varying degrees of gynecomastia but with no intra-abdominal Müllerian tissue. According to research at the University of Oklahoma health science centers, despite XX males exhibiting feminine characteristics, their behaviours are usually representative of masculinity in their culture.
The degree to which individuals with XX male syndrome develop the male phenotype is variable, even among SRY-positive individuals. A completely male phenotype usually develops in the presence of the SRY gene but, in some cases, the presence of the SRY gene can result in internal and/or external genitalia ambiguities. Normal XX females undergo X inactivation during which one copy of the X chromosome is silenced. It is thought that X inactivation in XX males may account for the genital ambiguities and incomplete masculinization seen in SRY-positive XX males. The X chromosome with the SRY gene is preferentially chosen to be the active X chromosome 90% of the time, which explains complete male phenotype being observed often in SRY-positive XX males. In the remaining 10%, X inactivation spreads to include a portion of the SRY gene, resulting in incomplete masculinization.
Masculinization of SRY-negative XX males is dependent upon which genes have mutations and at what point in development these mutations occur.
Males typically have one X chromosome and one Y chromosome in each diploid cell of their bodies. Females typically have two X chromosomes. XX males that are SRY-positive have two X chromosomes, with one of them containing genetic material from the Y chromosome, making them phenotypically male but genetically female.
The SRY gene plays an important role in sex determination by initiating testicular development. In most XX males the SRY gene is present. The tip of the Y chromosome contains the SRY gene and, during recombination, a translocation occurs in which the SRY gene on the Y chromosome is moved to become part of an X chromosome. The presence of the translocated SRY gene leads to an XX embryo developing male characteristics.
In 10 % of cases, an XX male does not have the SRY gene, causing variations in their levels of masculinity. The exact cause of this condition is unknown but it has been proposed that mutations in the SOX9 gene may contribute to this syndrome since SOX9 plays a role in testes differentiation during development. Another proposed cause is mutations to the DAX1 gene which encodes a nuclear hormone receptor. DAX1 represses masculinizing genes, therefore, if there is a loss of function of DAX1 then testes can develop in an XX individual. Mutations in SF1 and WNT4 genes are also being studied in connection with SRY-negative XX male syndrome.
Hypothesis that XX occurs in males because of the interaction of the testis-determining portion of the Y chromosome and part of the X chromosome, called the Xg gene, is generally supported by various data. The frequency of the Xg phenotype in XX males is closer to normal males' frequency than normal females' frequency. There have been at least four cases where XX males have inherited the Xg allele from their father, and at least nine cases where XX males did not inherent the allele from their father.
In cases where the individual is being evaluated for ambiguous genitalia, such as a small phallus, hypospadias, or labioscrotal folds, exploratory surgery may be used to determine if male and/or female internal genitalia is present.
Localization of the SRY gene can by determined using fluorescent in situ hybridization.
Indicators include two testes which have not descended the inguinal canal, although this is seen in a minority of XX males, and the absence of Müllerian tissue. External indicators include decreased body weight and small testes.
XX males are sterile due to no sperm content and there is currently no treatment to address this infertility. Genital ambiguities, while not necessary to treat for medical reasons, can be treated through the use of hormonal therapy, surgery, or both. Since XX male syndrome is variable in its presentation, the specifics of treatment varies widely as well. In some cases gonadal surgery can be performed to remove partial or whole female genitalia. This may be followed by plastic and reconstructive surgery to make the individual appear more externally male. Conversely, the individual may wish to become more feminine and feminizing genitoplasty can be performed to make the ambiguous genitalia appear more female. Hormonal therapy may also aid in making an individual appear more male or female.
- 46,XX testicular disorders of sex development
- X chromosome, for other conditions related to the X chromosome.
- For a condition that causes people who have XY chromosomes to have an ambiguous or feminine phenotype, see Androgen Insensitivity Syndrome (AIS)
- For a second condition that causes people who have XY chromosomes to have a feminine phenotype, see XY gonadal dysgenesis (also known as Swyer syndrome)
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