Normal sexual development is the result of
numerous genes. Mutation or chromosomal
rearrangements of any of these genes cause partial
or total failure of sex differentiation. The
classification of genetically determined disorders
of sexual development takes the different
developmental processes into account. Pinpointing
the basic defect is a prerequisite for diagnosis
and treatment.
Sunday, April 12, 2009
Male-determining region SRY on the Y chromosome
Normally, the male-determining Y-specific DNA
sequences (SRY) remain on the Y chromosome
during the homologous pairing and crossingover
during meiosis. However, since the maledetermining
region SRY is located very close to
the pseudoautosomal region (PAR), crossingover
in the PAR border region may result in a
transfer of the SRY region to the X chromosome.
This results in a male individual with an XX
karyotype (XX male). Conversely, if the SRY region
is missing from a Y chromosome, a female
phenotype with XY chromosomes (XY female)
results.
sequences (SRY) remain on the Y chromosome
during the homologous pairing and crossingover
during meiosis. However, since the maledetermining
region SRY is located very close to
the pseudoautosomal region (PAR), crossingover
in the PAR border region may result in a
transfer of the SRY region to the X chromosome.
This results in a male individual with an XX
karyotype (XX male). Conversely, if the SRY region
is missing from a Y chromosome, a female
phenotype with XY chromosomes (XY female)
results.
Point mutations in the SRY gene
The human SRY gene has a single exon and encodes
a 204-amino-acid protein from a 1.1 kb
transcript. The middle section of the SRY protein
consists of 79 highly conserved amino acids
with DNA-bending and DNA-binding capability,
the HMG box (high mobility group protein).
Complete or partial gonadal dysgenesis results
from point mutations and deletions in the SRY
gene, in particular the HMG box. (Figure
adapted fromWolf et al., 1992; for an update of
mutations see McElreavey and Fellous, 1999).
Sex reversal also results from mutations in the
SOX9 gene on chromosome 17 at q24 in campomelic
dysplasia.
a 204-amino-acid protein from a 1.1 kb
transcript. The middle section of the SRY protein
consists of 79 highly conserved amino acids
with DNA-bending and DNA-binding capability,
the HMG box (high mobility group protein).
Complete or partial gonadal dysgenesis results
from point mutations and deletions in the SRY
gene, in particular the HMG box. (Figure
adapted fromWolf et al., 1992; for an update of
mutations see McElreavey and Fellous, 1999).
Sex reversal also results from mutations in the
SOX9 gene on chromosome 17 at q24 in campomelic
dysplasia.
Androgen receptor
The fetal testis produces testosterone, the hormone
that induces male sexual differentiation.
Testosterone is taken up by cells of the target
tissues (wolffian ducts and urogenital sinus)
(1). In the urogenital sinus, testosterone is converted
into dihydrotestosterone (DHT) by the
enzyme 5!-reductase. Both testosterone and
dihydrotestosterone bind to an intracellular receptor
(androgen receptor). The activated hormone–
receptor complex (TR* or DR*) acts as a
transcription factor for genes that regulate the
differentiation of thewolffian ducts and the urogenital
sinus. Thus, normal male fetal development
is dependent on normal biosynthesis of
testosterone and normal receptors. Androgen
receptor mutations lead to disorders of sexual
development (2) with X-chromosomal inherited
complete or incomplete androgen resistance
(testicular feminization, TFM).
that induces male sexual differentiation.
Testosterone is taken up by cells of the target
tissues (wolffian ducts and urogenital sinus)
(1). In the urogenital sinus, testosterone is converted
into dihydrotestosterone (DHT) by the
enzyme 5!-reductase. Both testosterone and
dihydrotestosterone bind to an intracellular receptor
(androgen receptor). The activated hormone–
receptor complex (TR* or DR*) acts as a
transcription factor for genes that regulate the
differentiation of thewolffian ducts and the urogenital
sinus. Thus, normal male fetal development
is dependent on normal biosynthesis of
testosterone and normal receptors. Androgen
receptor mutations lead to disorders of sexual
development (2) with X-chromosomal inherited
complete or incomplete androgen resistance
(testicular feminization, TFM).
Classification of genetically determined disorders of sexual development
1. Defects of sex determination due to mutation
or structural aberration of the SRY region
on the Y chromosome (e.g., XY gonadal
dysgenesis, XX males, and others)
2. Defects of androgen biosynthesis (e.g.,
adrenogenital syndrome due to 21-hydroxylase
deficiency, see p. 392)
3. Defects of androgen receptors (testicular
feminization)
4. Defects of the müllerian inhibition substance
(so-called hernia uteri syndrome)
5. XO/XY gonadal dysgenesis
or structural aberration of the SRY region
on the Y chromosome (e.g., XY gonadal
dysgenesis, XX males, and others)
2. Defects of androgen biosynthesis (e.g.,
adrenogenital syndrome due to 21-hydroxylase
deficiency, see p. 392)
3. Defects of androgen receptors (testicular
feminization)
4. Defects of the müllerian inhibition substance
(so-called hernia uteri syndrome)
5. XO/XY gonadal dysgenesis
Congenital Adrenal Hyperplasia
This disorder, also called adrenogenital syndrome
(AGS, McKusick 201910), is caused by a
genetically determined deficiency of cortisol, a
steroid hormone produced in the fetal adrenal
cortex. A compensatory increase in adrenocortical
hormone (ACTH) excretion leads to secondary
enlargement (hyperplasia) of the
adrenal cortex (congenital adrenal hyperplasia),
increased production of prenatal
steroids and their metabolites with androgenic
effects, and incomplete female sex differentiation.
(AGS, McKusick 201910), is caused by a
genetically determined deficiency of cortisol, a
steroid hormone produced in the fetal adrenal
cortex. A compensatory increase in adrenocortical
hormone (ACTH) excretion leads to secondary
enlargement (hyperplasia) of the
adrenal cortex (congenital adrenal hyperplasia),
increased production of prenatal
steroids and their metabolites with androgenic
effects, and incomplete female sex differentiation.
Clinical phenotype and genetics
Girls are born with ambiguous or virilized genitalia
(1). The adrenal cortex is enlarged (2). Increased
production of androgenic metabolites
causes masculinization. The cortisol deficiency
(3) leads to life-threatening crises due to loss of
sodium chloride (salt-wasting) that require
prompt treatment. AGS is an autosomal recessive
heritable disorder (4). Untreated girls
develop amale physical appearance (5). In boys,
the early signs are limited to salt-wasting. Initially,
skeletalmaturation is accelerated and the
children are tall for their age; however, they
stop growing prematurely and eventually are
too short. Besides the classic form of the disorder
with a frequency of 1:5000, there are
other forms with less pronounced masculinization
due to different mutations.
(1). The adrenal cortex is enlarged (2). Increased
production of androgenic metabolites
causes masculinization. The cortisol deficiency
(3) leads to life-threatening crises due to loss of
sodium chloride (salt-wasting) that require
prompt treatment. AGS is an autosomal recessive
heritable disorder (4). Untreated girls
develop amale physical appearance (5). In boys,
the early signs are limited to salt-wasting. Initially,
skeletalmaturation is accelerated and the
children are tall for their age; however, they
stop growing prematurely and eventually are
too short. Besides the classic form of the disorder
with a frequency of 1:5000, there are
other forms with less pronounced masculinization
due to different mutations.
Biochemical defect
The enzymatic conversion of progesterone to
deoxycortisol (DOC) by hydroxylation at position
21 (steroid 21-hydroxylase) is decreased.
As a result, the concentration of 17-hydroxyprogesterone
is increased.
deoxycortisol (DOC) by hydroxylation at position
21 (steroid 21-hydroxylase) is decreased.
As a result, the concentration of 17-hydroxyprogesterone
is increased.
Gene locus and gene structure
21-Hydroxylase is encoded by the CYP21 gene
(formerly called CYP21B and 21-OHB), a member
of the cytochrome P450 oxidase gene
family. This gene is located within the class III
genes of the major histocompatibility complex
on the short arm of human chromosome 6. It is
part of a tandem paired arrangement of three
other genes: active C4A and C4B genes and a
96–98% homologous inactive CYP21P gene, a
pseudogene due to intragenic deletions resulting
in stop codons (formerly called CYP21A or
21-OHA). These genes originated from a duplication
event in evolution. The CYP21 (21-OHB)
gene consists of 10 exons spanning almost 6 kb
of genomic DNA (the actual distance of 30 kb to
the C4 A and C4 B genes is not shown to scale).
(formerly called CYP21B and 21-OHB), a member
of the cytochrome P450 oxidase gene
family. This gene is located within the class III
genes of the major histocompatibility complex
on the short arm of human chromosome 6. It is
part of a tandem paired arrangement of three
other genes: active C4A and C4B genes and a
96–98% homologous inactive CYP21P gene, a
pseudogene due to intragenic deletions resulting
in stop codons (formerly called CYP21A or
21-OHA). These genes originated from a duplication
event in evolution. The CYP21 (21-OHB)
gene consists of 10 exons spanning almost 6 kb
of genomic DNA (the actual distance of 30 kb to
the C4 A and C4 B genes is not shown to scale).
Molecular genetic analysis
Point mutations, deletions, and duplications
occur in the CYP21 gene. The deletions and duplications
result from misalignment of the homologous
chromatids during meiosis and unequal
crossing-over. Deletions occur in about
20–25% of patientswith classic 21-hydroxylase
deficiency. Duplications have no clinical consequences.
Deletions and duplications can be
easily detected by Southern blot analysis. The
most frequent type of deletion is loss of a 30 kb
region including the 3' part of the CYP21P pseudogene,
the entire C4B gene, and the 5' part of
the CYP21 gene. The resulting fusion gene of
CYP21P and CYP21 carries a TaqI restriction site
in the 5' region of CYP21P that is not present in
the CYP21 gene. Therefore, the fusion gene has a
characteristic 3.2 kb TaqI fragment. This distinguishes
the rearrangement from the normal
CYP21 gene, which has a characteristic 3.7 kb
fragment. In the example shown, the CYP21
gene (21-OHB) is represented by a 3.7 kb DNA
fragment, the pseudogene CYP21P (21-OHA) by
a 3.2 kb fragment after TaqI digestion (1). Thus,
the normal pattern is a 3.7 kb and a 3.2 kb fragment
(2). Homozygous deletion of either of the
genes may be apparent by lack of either of the
two fragments (3, 4). A heterozygous deletion
shows reduced intensity (5) and a duplication
shows increased intensity (6).
occur in the CYP21 gene. The deletions and duplications
result from misalignment of the homologous
chromatids during meiosis and unequal
crossing-over. Deletions occur in about
20–25% of patientswith classic 21-hydroxylase
deficiency. Duplications have no clinical consequences.
Deletions and duplications can be
easily detected by Southern blot analysis. The
most frequent type of deletion is loss of a 30 kb
region including the 3' part of the CYP21P pseudogene,
the entire C4B gene, and the 5' part of
the CYP21 gene. The resulting fusion gene of
CYP21P and CYP21 carries a TaqI restriction site
in the 5' region of CYP21P that is not present in
the CYP21 gene. Therefore, the fusion gene has a
characteristic 3.2 kb TaqI fragment. This distinguishes
the rearrangement from the normal
CYP21 gene, which has a characteristic 3.7 kb
fragment. In the example shown, the CYP21
gene (21-OHB) is represented by a 3.7 kb DNA
fragment, the pseudogene CYP21P (21-OHA) by
a 3.2 kb fragment after TaqI digestion (1). Thus,
the normal pattern is a 3.7 kb and a 3.2 kb fragment
(2). Homozygous deletion of either of the
genes may be apparent by lack of either of the
two fragments (3, 4). A heterozygous deletion
shows reduced intensity (5) and a duplication
shows increased intensity (6).
Atypical Inheritance Pattern
Heritable changes in the number of repeated
groups of three nucleotides each (trinucleotide
or triplet repeat) represent a newclass of mutations
in man for which there is no parallel in
other organisms. They either occur within the
gene and are translated or occur outside the
gene in an untranslated region, and they are unstable
during transmission through the germline.
Unaffected persons may carry a premutation,
whichmay be converted to a full mutation
when passed through the germline to the next
generation. Therefore, the effects of the mutation
differ in severity in affected members
within the same family. Occasionally, there is
regression and a generation is skipped.
groups of three nucleotides each (trinucleotide
or triplet repeat) represent a newclass of mutations
in man for which there is no parallel in
other organisms. They either occur within the
gene and are translated or occur outside the
gene in an untranslated region, and they are unstable
during transmission through the germline.
Unaffected persons may carry a premutation,
whichmay be converted to a full mutation
when passed through the germline to the next
generation. Therefore, the effects of the mutation
differ in severity in affected members
within the same family. Occasionally, there is
regression and a generation is skipped.
Genetic diseases
Genetic diseases with increased
numbers of trinucleotides
Some important genetically determined diseases
are based on a greater than normal number
of trinucleotides: Huntington disease,
fragile X syndrome, myotonic dystrophy,
spinobulbar muscular atrophy type Kennedy,
and spinocerebellar ataxia type 1.
numbers of trinucleotides
Some important genetically determined diseases
are based on a greater than normal number
of trinucleotides: Huntington disease,
fragile X syndrome, myotonic dystrophy,
spinobulbar muscular atrophy type Kennedy,
and spinocerebellar ataxia type 1.
Huntington disease
Huntington disease is a progressive disease of
the brain. Within 5–10 years, it leads to
complete loss of motor control and intellectual
abilities (1). It usually begins around age 40–50
with uncoordinated movements (chorea, St.
Vitus’ dance), excitation, hallucinations, and
psychological changes. The disease is transmitted
by autosomal dominant inheritance and
shows complete penetrance. It presents an affected
family with two difficult problems: (i)
due to its late onset, carriers of the mutation
have usually completed their family planning
before the disease is manifest, and (ii) children
of affected persons first learn as young adults
that they are at a 50% risk of developing the disease
later in life. Thus, the introduction of a
direct predictive DNA diagnostic procedure is
very important. However, before such a genetic
test is carried out, it must be established
through genetic counseling that the persons at
risk have decided for themselves whether they
want to have the test performed. The gene is located
on the distal short arm of chromosome 4
(2). It spans 210 kb and codes for a protein
(called huntingtin) of important function. The
5! end of the gene contains numerous copies of
a trinucleotide sequence consisting of cytosine,
adenine, and guanine (CAG), a codon for the
amino acid glutamine. Normally the gene has
10–34 CAG repeats; in patients there are
42–100. The diagnostic test (3) demonstrates
that affected individuals (here, individuals 1, 2,
and 4) have enlarged DNA fragments due to expanded
CAG repeats. (Findings of the Institut
für Humangenetik of the Universität Göttingen
with kind permission by Prof.W. Engel; Zühlke
et al., Hum. Mol. Genet. 2:1467–1469, 1993).
the brain. Within 5–10 years, it leads to
complete loss of motor control and intellectual
abilities (1). It usually begins around age 40–50
with uncoordinated movements (chorea, St.
Vitus’ dance), excitation, hallucinations, and
psychological changes. The disease is transmitted
by autosomal dominant inheritance and
shows complete penetrance. It presents an affected
family with two difficult problems: (i)
due to its late onset, carriers of the mutation
have usually completed their family planning
before the disease is manifest, and (ii) children
of affected persons first learn as young adults
that they are at a 50% risk of developing the disease
later in life. Thus, the introduction of a
direct predictive DNA diagnostic procedure is
very important. However, before such a genetic
test is carried out, it must be established
through genetic counseling that the persons at
risk have decided for themselves whether they
want to have the test performed. The gene is located
on the distal short arm of chromosome 4
(2). It spans 210 kb and codes for a protein
(called huntingtin) of important function. The
5! end of the gene contains numerous copies of
a trinucleotide sequence consisting of cytosine,
adenine, and guanine (CAG), a codon for the
amino acid glutamine. Normally the gene has
10–34 CAG repeats; in patients there are
42–100. The diagnostic test (3) demonstrates
that affected individuals (here, individuals 1, 2,
and 4) have enlarged DNA fragments due to expanded
CAG repeats. (Findings of the Institut
für Humangenetik of the Universität Göttingen
with kind permission by Prof.W. Engel; Zühlke
et al., Hum. Mol. Genet. 2:1467–1469, 1993).
Myotonic dystrophy (MDY1)
Myotonic dystrophy is an autosomal dominant
hereditary disease that predominantly affects
the central nervous and muscular systems (1).
The myotonia causes a masklike facies (2). The
disease is very variable and in many families
shows increasing severity in consecutive
generations (anticipation). An increased number
of CTG repeats, more than 50 copies compared
with 5–35 in normal individuals (3), is
found immediately beyond the 3! end of the
gene in affected persons. This is demonstrated
in a Southern blot as an enlarged DNA fragment
(4). (Schematic representation of a Southern
blot at the gene locus D19S95, probe pBBO.7
after DNA cleavage with EcoRI.
hereditary disease that predominantly affects
the central nervous and muscular systems (1).
The myotonia causes a masklike facies (2). The
disease is very variable and in many families
shows increasing severity in consecutive
generations (anticipation). An increased number
of CTG repeats, more than 50 copies compared
with 5–35 in normal individuals (3), is
found immediately beyond the 3! end of the
gene in affected persons. This is demonstrated
in a Southern blot as an enlarged DNA fragment
(4). (Schematic representation of a Southern
blot at the gene locus D19S95, probe pBBO.7
after DNA cleavage with EcoRI.
Fragile X Syndrome
The fragile X syndrome (McKusick 309550;
other designations: fraX syndrome, X-chromosomal
mental retardation with fragile site on
the X chromosome, Martin–Bell syndrome) is
the most frequent form of hereditarymental retardation
in males, with a frequency of about
1:2000–4000 individuals. The responsible
mutation usually consists of an increased number
of unstable trinucleotide repeats. Unlike in
classic X-chromosomal inheritance, there are
males without manifestations, and a large proportion
of female carriers showpartial manifestations.
other designations: fraX syndrome, X-chromosomal
mental retardation with fragile site on
the X chromosome, Martin–Bell syndrome) is
the most frequent form of hereditarymental retardation
in males, with a frequency of about
1:2000–4000 individuals. The responsible
mutation usually consists of an increased number
of unstable trinucleotide repeats. Unlike in
classic X-chromosomal inheritance, there are
males without manifestations, and a large proportion
of female carriers showpartial manifestations.
trinucleotide
The unstable expansion of a trinucleotide repeat
(CGG) is located in the 5'-untranslated region
of the FMR1 gene. Recent findings indicate
that an increase beyond 200 repeats impedes
the migration of the 40S ribosomal subunit.
This causes translational suppression.
(CGG) is located in the 5'-untranslated region
of the FMR1 gene. Recent findings indicate
that an increase beyond 200 repeats impedes
the migration of the 40S ribosomal subunit.
This causes translational suppression.
Phenotype
The phenotype is very variable. The mental retardation
varies; there is no distinct neurological
dysfunction. In adult males, the testes are
enlarged (macroorchidism). Affected individuals
can usually be integrated well into the
family and learn to function in a familiar environment.
varies; there is no distinct neurological
dysfunction. In adult males, the testes are
enlarged (macroorchidism). Affected individuals
can usually be integrated well into the
family and learn to function in a familiar environment.
Fragile site Xq27.3
The gene locus (FRAXA) for the gene (FMR1) is
located on the distal long arm of the X chromosome
in region 2, band 7.3 (Xq27.3). In this region
the great majority of patients and some of
the female heterozygotes show a constriction
(fragile site) in the affected X chromosome in
about 2–25% of metaphases. The constriction
must be induced by folic acid deficiency in the
culture medium, and it must be differentiated
from other fragile sites in this region.
located on the distal long arm of the X chromosome
in region 2, band 7.3 (Xq27.3). In this region
the great majority of patients and some of
the female heterozygotes show a constriction
(fragile site) in the affected X chromosome in
about 2–25% of metaphases. The constriction
must be induced by folic acid deficiency in the
culture medium, and it must be differentiated
from other fragile sites in this region.
Expanded CGG repeats in the fragile X syndrome
The heritable unstable sequences explain two
unusual characteristics of the fraX syndrome;
(i) the transition from a premutation (about
60–200 CGG repeats) without clinical manifestation
into a full mutation (more than 200 CGG
repeats) during transmission through the
germline, and (ii) differences in the FRAXA
locus within a given family.
Fragile X syndrome is heritable as an X-linked
dominant trait. The risk of transmission and
clinical manifestation varies according to the
type of mutation (premutation or full mutation),
the gender of the patient and of the parent
carrying an expanded trinucleotide repeat, and
the relationship within the family. Males with
the full mutation are mentally retarded and do
not reproduce. Heterozygous females for the
full mutation have a risk of variable mental retardation
of 50%. They transmit the full mutation
to 50% of their offspring. A premutation
present in amale (“normal male transmitter”) is
transmitted to all daughters and none of the
sons. Female carriers of the premutation or full
mutation have a 50% risk of transmitting the
mutant allele. The actual risk of manifest fragile
X syndrome depends on the number of CGG repeats
and varies between 10% (60–69 repeats)
and 50% (more than 100 repeats) for sons (Gene
Clinics at http: www.geneclinics.org).
The number of CGG repeats is variable within a
family. In the pedigree shown (1), individuals II-
3 and III-1 have more than 200 repeats and have
fragile X syndrome. Individuals I-2, I-3, II-1, and
III-3 are carriers of the premutation with 79–82
repeats. The normal number of CGG repeats is 6
to about 50, the premutation is defined by
about 55 to 100, and the full disease-causing
mutation by more than 200 repeats (2).
The different numbers of CGG repeats can be
demonstrated in Southern blots as DNA fragments
of different sizes (3). The normal gene is
represented by a small DNA fragment (S). A premutation
leads to slightly enlarged fragments.
The full mutation is characterized by large fragments
(L). With this procedure, a reliable diagnosis
of the genotype is possible. (Photograph of
a Southern blot: HindIII digestion and hybridization
with probe Ox1.1; P. Steinbach, Ulm).
unusual characteristics of the fraX syndrome;
(i) the transition from a premutation (about
60–200 CGG repeats) without clinical manifestation
into a full mutation (more than 200 CGG
repeats) during transmission through the
germline, and (ii) differences in the FRAXA
locus within a given family.
Fragile X syndrome is heritable as an X-linked
dominant trait. The risk of transmission and
clinical manifestation varies according to the
type of mutation (premutation or full mutation),
the gender of the patient and of the parent
carrying an expanded trinucleotide repeat, and
the relationship within the family. Males with
the full mutation are mentally retarded and do
not reproduce. Heterozygous females for the
full mutation have a risk of variable mental retardation
of 50%. They transmit the full mutation
to 50% of their offspring. A premutation
present in amale (“normal male transmitter”) is
transmitted to all daughters and none of the
sons. Female carriers of the premutation or full
mutation have a 50% risk of transmitting the
mutant allele. The actual risk of manifest fragile
X syndrome depends on the number of CGG repeats
and varies between 10% (60–69 repeats)
and 50% (more than 100 repeats) for sons (Gene
Clinics at http: www.geneclinics.org).
The number of CGG repeats is variable within a
family. In the pedigree shown (1), individuals II-
3 and III-1 have more than 200 repeats and have
fragile X syndrome. Individuals I-2, I-3, II-1, and
III-3 are carriers of the premutation with 79–82
repeats. The normal number of CGG repeats is 6
to about 50, the premutation is defined by
about 55 to 100, and the full disease-causing
mutation by more than 200 repeats (2).
The different numbers of CGG repeats can be
demonstrated in Southern blots as DNA fragments
of different sizes (3). The normal gene is
represented by a small DNA fragment (S). A premutation
leads to slightly enlarged fragments.
The full mutation is characterized by large fragments
(L). With this procedure, a reliable diagnosis
of the genotype is possible. (Photograph of
a Southern blot: HindIII digestion and hybridization
with probe Ox1.1; P. Steinbach, Ulm).
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