Sex Diff Miscellany & Hypothesis

Teresa Binstock binstoct at essex.hsc.colorado.edu
Mon Aug 7 17:57:10 EST 1995


                    A SEX DIFFERENTIATION MISCELLANY
                  showing that SRY is not the only gene
                  involved with sexual differentiation
                      and that some autosomal genes
                can induce cross-sexual differentiation.


Prepared by Teresa C. Binstock
            Researcher
            Developmental and Behavioral Neuroanatomy
            8.7.95

KeyWords: 
     sex reversal, SRY, autosomal sex reversal, homeobox genes,
     tissue-specific genes, regionalization of the embryo.


1. Peters, R., King, C.Y., Ukiyama, E., Falsafi, S., Donahoe, P.K. and
Weiss, M.A. 
     An SRY mutation causing human sex reversal resolves a general
     mechanism of structure-specific DNA recognition: application
     to the four-way DNA junction. 
## Biochemistry 34:4569-4576, 1995. 
##  - SRY, a genetic "master switch" for male development in mammals,
exhibits two biochemical activities: sequence-specific recognition of
duplex DNA and sequence-independent binding to the sharp angles of
four-way DNA junctions. Here, we distinguish between these activities by
analysis of a mutant SRY associated with human sex reversal (46, XY
female with pure gonadal dysgenesis). The substitution (168T in human
SRY) alters a nonpolar side chain in the minor-groove DNA recognition
alpha- helix of the HMG box [Haqq, C.M., King, C.-Y., Ukiyama, E., Haqq,
T.N., Falsalfi, S., Donahoe, P.K., & Weiss, M.A. (1994) Science 266,
1494-1500]. The native (but not mutant) side chain inserts between
specific base pairs in duplex DNA, interrupting base stacking at a site
of induced DNA bending. Isotope-aided 1H-NMR spectroscopy demonstrates
that analogous side-chain insertion occurs on binding of SRY to a
four-way junction, establishing a shared mechanism of sequence- and
structure-specific DNA binding. Although the mutant DNA-binding domain
exhibits > 50-fold reduction in sequence-specific DNA recognition, near
wild-type affinity for four-way junctions is retained. Our results (i)
identify a shared SRY-DNA contact at a site of either induced or
intrinsic DNA bending, (ii) demonstrate that this contact is not
required to bind an intrinsically bent DNA target, and (iii) rationalize
patterns of sequence conservation or diversity among HMG boxes. Clinical
association of the I68T mutation with human sex reversal supports the
hypothesis that specific DNA recognition by SRY is required for male sex
determination.
     Teresa Comment: Note that several following references provide
     exceptions to the prior abstract's final statement.
 
2. Boucekkine, C., Toublanc, J.E., Abbas, N., Chaabouni, S., Ouahid, S.,
Semrouni, M., Jaubert, F., Toublanc, M., McElreavey, K., Vilain, E. and
et al,  
     Clinical and anatomical spectrum in XX sex reversed patients.
     Relationship to the presence of Y specific DNA-sequences. 
## Clinical.Endocrinology 40:733-742, 1994. 
##  - OBJECTIVE: Testicular differentiation can occur in the absence of
the Y chromosome giving XX sex-reversed males. Although Y chromosomal
sequences can be detected in the majority of male subjects with a 46,XX
karyotype, several studies have shown that approximately 10% of patients
lack Y material including the SRY gene. The aim of this study was to see
if the classification of XX sex-reversed individuals into three groups,
Y-DNA-positive phenotypically normal XX males, Y-DNA-negative XX males
with genital ambiguities and Y-DNA-negative true hermaphrodites can be
applied to our cases. DESIGN: Endocrinological and genetic studies were
conducted in 20 XX sex- reversed patients. PATIENTS: Twenty patients
with various phenotypes were studied. They were between 20 days and 35
years old. Ten presented ambiguous external genitalia (Prader's stages
II to IV). After laparotomy or gonadal biopsy, the diagnosis was 46,XX
true hermaphroditism in five, and XX male in 15. MEASUREMENTS: Blood
samples were obtained from all patients for hormonal and molecular
studies. Basal levels of testosterone, oestradiol and pituitary
gonadotrophins were measured by RIA. In addition, two stimulation tests
were performed: gonadotrophin stimulation with GnRH and testicular
stimulation with hCG. Several Y-specific DNA sequences of the short arm
of the Y chromosome were analysed by Southern blot and polymerase chain
reaction methods. RESULTS: In this study, three categories of XX
sex-reversed individuals were observed: phenotypically normal males with
or without gynaecomastia, males with genital ambiguities, and true
hermaphrodites. Endocrinological data were similar in XX males and in
true hermaphrodites. Testosterone levels exhibited normal (n = 9) or
decreased (n = 11) values. The hCG response was low. FSH and LH were
elevated in 13 patients. Molecular analysis in ten patients showed
varying amounts of Y material including the Y boundary and SRY. Ten
patients with various phenotypes lacked Y chromosomal DNA. There was no
relation between Leydig cell function (as indicated by testosterone
levels before or after hCG stimulation) and the presence of Y chromosome
material. CONCLUSION: Although the presence of Y-specific DNA generally
results in a more masculinized phenotype, exceptions do occur. In the
Y-DNA- negative group, complete or incomplete masculinization in the
absence of SRY suggests a mutation of one or more downstream non- Y,
testis-determining genes.  
 
3. Capel, B., Rasberry, C., Dyson, J., Bishop, C.E., Simpson, E.,
Vivian, N., Lovell-Badge, R., Rastan, S. and Cattanach, B.M. 
     Deletion of Y chromosome sequences located outside the testis
     determining region can cause XY female sex reversal. 
## Nature Genetics 5:301-307, 1993. 
##  - An approach designed to map and generate mutations in the region
of the short arm of the mouse Y chromosome, known to be involved in sex
determination and spermatogenesis, is described. This relies on
homologous Yp-Sxra pairing and asymmetrical exchange which can occur at
meiosis in XY males carrying Sxra on their X chromosome. Such exchange
potentially generates deficiencies and duplications of Yp or Sxra. Three
fertile XY females were found out of about 450 XY offspring from XSxra/Y
x XX crosses. In all three, despite evidence for deletion of Y
chromosomal material, the Sry locus was intact. Each deletion involved a
repeat sequence, Sx1, located at a distance from Sry. Since expression
of Sry was affected these results suggest that long range position
effects have disrupted Sry action. 
     ### TERESA Comment: Note that a repeat sequence was deleted
     and induced sex reversal, even though Sry was present. Might
     we have here a shift within the nuclear matrix? Perhaps the
     difference between X and Y alphoid repeats is extremely
     important.  
 
4. Josso, N., Lamarre, I., Picard, J.Y., Berta, P., Davies, N.,
Morichon, N., Peschanski, M. and Jeny, R. 
     Anti-mullerian hormone in early human development. 
## Early Human.Development 33:91-99, 1993. 
##  - Anti-mullerian hormone (AMH) is a glycoprotein produced by
immature Sertoli cells and responsible for the regression of mullerian
ducts in male fetuses. The ontogeny of the hormone in early human
development was investigated. While no detectable AMH could be found in
female fetal serum, in males, the mean +/- S.E.M. AMH serum
concentration was 40.5 +/- 3.9 ng/ml from 19 to 30 weeks (n = 13), and
28.4 +/- 6.1 ng/ml from 30 weeks to term (n = 9). The latter value is
significantly different from the mean AMH concentration in serum from
boys aged 2 months to 2 years (43.1 +/- 3.7), suggesting that AMH
production is sluggish during the perinatal period. The serum AMH
concentration of a 46, XX male fetus was in the normal range for males.
Using in situ hybridization, AMH transcripts were detected in the
testicular tissue of all fetuses from 8 weeks onwards, but not in fetal
ovaries nor in the yet undifferentiated gonadal tissue of a 7- week-old
fetus bearing male-determining DNA sequences. Together, these data
indicate that AMH is a reliable marker for the presence of functional
testicular tissue and, as such, may be helpful for the diagnosis of
fetal sex, particularly in the presence of sex chromosome abnormalities.

     ### TERESA Comment: If AMH is in serum, it may well be in
     brain and in other tissues. If so, which seems likely, is it
     bioactive? Ought we presume a priori that it is not bioactive
     in vivo? Is AMH a further aspect of sex differentation?  
 
5. Kuhnle, U., Schwarz, H.P., Lohrs, U., Stengel-Ruthkowski, S., Cleve,
H. and Braun, A. 
     Familial true hermaphroditism: paternal and maternal
     transmission of true hermaphroditism (46,XX) and XX maleness
     in the absence of Y-chromosomal sequences. 
## Human.Genetics 92:571-576, 1993. 
##  - We report on 46,XX true hermaphroditism and 46,XX maleness
coexisting in the same pedigree, with maternal as well as paternal
transmission of the disorder. Molecular genetic analysis showed that
both hermaphrodites as well as the 46,XX male were negative for
Y-chromosomal sequences. Thus, this pedigree is highly informative and
allows the following conclusions: first, the maternal as well as
paternal transmission of the disorder allows the possibility of an
autosomal dominant as well as an X- chromosomal dominant mode of
inheritance; second, testicular determination in the absence of
Y-specific sequences in familial 46,XX true hermaphrodites as well as in
46,XX males seems to be due to the varying expression of the same
genetic defect; and third, there is incomplete penetrance of the defect. 

 
6. Zeng, Y.T., Ren, Z.R., Zhang, M.L., Huang, Y., Zeng, F.Y. and Huang,
S.Z. 
     A new de novo mutation (A113T) in HMG box of the SRY gene
     leads to XY gonadal dysgenesis. 
## Journal.of.Medical.Genetics 30:655-657, 1993. 
##  - We describe a new point mutation in the SRY gene of a Chinese XY
female with gonadal dysgenesis (Swyer syndrome). Using the double
stranded DNA cycle sequencing method, a single nucleotide substitution
of G-->A was identified at codon 113 of the patient's SRY gene,
resulting in a conservative amino acid change from alanine (A) to
threonine (T) at a residue that lies within the putative DNA binding
motif. With this mutation, one MnlI recognition site is abolished and a
new BsmAI site is present in the DNA sequence of the SRY gene;
therefore, it is easily detected by analysis of the digestion of the
amplified SRY DNA fragment on an electrophoretic agarose gel. In situ
hybridisation to the XY female's chromosomes showed that her mutant SRY
gene was indeed located on the short arm of her Y chromosome. The SRY
mutation in the XY female reported here occurred de novo, as sequence
analysis showed that it was not present in her father or other family
members.  
 
7. Mittwoch, U. 
     Sex determination and sex reversal: genotype, phenotype, dogma
     and semantics. [Review]. 
## Human.Genetics 89:467-479, 1992. 
##  - The genetic terminology of sex determination and sex
differentiation is examined in relation to its underlying biological
basis. On the assumption that the function of the testis is to produce
hormones and spermatozoa, the hypothesis of a single Y-chromosomal
testis-determining gene with a dominant effect is shown to run counter
to the following observed facts: a lowering in testosterone levels and
an increase in the incidence of undescended testes, in addition to
sterility, in males with multiple X chromosomes; abnormalities of the
testes in autosomal trisomies; phenotypic abnormalities of XX males
apparently increasing with decreasing amounts of Y-chromosomal material;
the occurrence of patients with gonadal dysgenesis and XY males with
ambiguous genitalia in the same sibship; the occurrence of identical SRY
mutations in patients with gonadal dysgenesis and fertile males in the
same pedigree; and the development of XY female and hermaphrodite mice
having the same genetic constitution. The role of X inactivation in the
production of males, females and hermaphrodites in T(X;16)16H mice has
previously been suggested but not unequivocally demonstrated; moreover,
X inactivation cannot account for the observed bilateral asymmetry of
gonadal differentiation in XY hermaphrodites in humans and mice. There
is evidence for a delay in development of the supporting cells in XY
mice with ovarian formation. Once testicular differentiation and male
hormone secretion have begun, other Y-chromosomal genes are required to
maintain spermatogenesis and to complete spermiogenesis, but these genes
do not function effectively in the presence of more than one X
chromosome. The impairment of spermatogenesis by many other chromosome
abnormalities seems to be more severe than that of oogenesis. It is
concluded that the notion of a single testis- determining gene being
responsible for male sex differentiation lacks biological validity, and
that the genotype of a functional, i.e. fertile, male differs from that
of a functional female by the presence of multiple Y-chromosomal genes
in association with but a single X chromosome. Male sex differentiation
in XY individuals can be further impaired by a euploid, but
inappropriate, genetic background. The genes involved in testis
development may function as growth regulators in the tissues in which
they are active.  
 
8. de la Chapelle, A., Hastbacka, J., Korhonen, T. and Maenpaa, J. 
     The etiology of XX sex reversal. [Review]. 
## Reproduction,.Nutrition,.Development Suppl 1:39s-49s, 1990. 
##  - The primary testis-determining function is exerted by a gene in
the sex-determining region of the human Y chromosome. This gene is
termed the sex-determining factor or TDF. A zinc finger gene, ZFY,
residing in this region has been cloned and characterized. It is a
candidate for TDF. A challenge to future molecular research is to
clarify the function of a zinc finger gene on the X chromosome, ZFX,
that shows high structural similarity to ZFY. Furthermore, the existence
of other genes involved in sex determination is likely but so far
unproven. Sex reversal leading to testes in apparently XX individuals
(XX males) is most often due to the presence of TDF on the paternally
derived X chromosome. The abnormality arises during meiosis in the
father when an abnormal exchange leads to the transfer onto the X of the
entire pseudoautosomal region plus a portion of the Y
chromosome-specific region including TDF from the Y. An XX male
resulting from such an exchange is described. 10-20% of XX males do not
have Y DNA. Two major mechanisms to explain such Y(-) XX males are
discussed. First, several published pedigrees show clear-cut dominant
autosomal or X chromosomal inheritance of XX maleness. These patients
are always Y(-) and usually have sexual ambiguity. This indicates the
existence of other genes, obviously 'downstream' from TDF, that when
mutated can trigger testis determination. Nothing concrete is presently
known about these putative genes, but their phenotypic effect is
slightly different from that of TDF. Second, mosaicism with a prevalent
XX lineage and a hidden or scarce lineage containing a Y chromosome can
explain some apparently Y(-) XX males. Two XX/XXY mosaic patients are
described in detail. In one, only a combination of DNA hybridization and
cytogenetic studies led to the discovery of the XXY cell line. In
conclusion, XX sex reversal in man is caused by at least 3 mechanisms,
viz. abnormal Y-X interchange, genes other than TDF, and mosaicism.  
 
*****

Teresa Commentary: Now, let's go one more step and consider (i) homeobox
genes that are very involved with embryonic segmentation and (ii)
tissue-specific genes. If tissue specific genes were dysregulated in
ways affecting the aforementioned SRY-related or autosomal sex-reversal
genes within specific homeobox-determined sub-locales of the embryonic
human brain, the result might be non-hetero sexual orientation and/or
non-hetero-typical gender orientation -- even as most of the embryo
proceeded to develop in perfect accord with its XX or XY chromosomal
constitution.

*****

                        ***  Copyright 1995  ***
   (regarding five lines of title, keywords list, and last paragraph)




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