XX male syndrome
XX male syndrome, also known as De la Chapelle syndrome, is a rare congenital intersex condition where an individual with a 46 XX karyotype has phenotypically male characteristics that can vary among cases. Synonyms include 46,XX testicular difference of sex development, 46,XX sex reversal, nonsyndromic 46,XX testicular DSD, and XX sex reversal.
In 90 percent of these individuals, the syndrome is caused by the Y chromosome's SRY gene, which triggers male reproductive development, being atypically included in the crossing over of genetic information that takes place between the pseudoautosomal regions of the X and Y chromosomes during meiosis in the father. 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.
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. 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.They generally have small testes and may also have abnormalities such as undescended testes or the urethra opening on the underside of the penis. A small number of affected people have external genitalia that do not look clearly male or clearly female. Affected children are typically raised as males and are likely to have a male gender identity.
Masculinization
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.
Genetics
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.In about 80 percent of individuals with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes. This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The translocation causes the SRY gene to be misplaced, almost always onto an X chromosome. If a fetus is conceived from a sperm cell with an X chromosome bearing the SRY gene, it will develop as a male despite not having a Y chromosome. This form of the condition is called SRY-positive 46,XX testicular disorder of sex development.
About 20 percent of those with 46 XX testicular disorder of sex development do not have the SRY gene. This form of the condition is called SRY-negative 46,XX testicular disorder of sex development. The cause of the disorder in these individuals is often unknown, although changes affecting other genes have been identified. Individuals with SRY-negative 46,XX testicular disorder of sex development are more likely to have ambiguous genitalia than are people with the SRY-positive form.
SRY-positive
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.SRY-negative
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.Xg gene
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 inherit the allele from their father.Diagnosis
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.A standard karyotype can be completed to cytogenetically determine that an individual with a partial or complete male phenotype has a XX genotype.
FISH analysis determines the presence or absence of the SRY gene.
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.