Mammalian Toxicology, Session 8

Gamete production in mammals; Gestation in mammals; C 9, 10, 20

Gamete Production in Mammals

While there are many, many excellent resources covering the physiology, endocrinology, and other aspects of gamete production and gestation, I have tried to be selective in providing links that seem particularly helpful.  While Casarett and Doull do an adequate job of covering these topics, I have found gaps in a number of places.  So I will try to address those here.

Germline stem cells actually begin their journey as a small group of cells at the edge of the yolk sac dorsal endoderm in the developing embryo, very similar to the start of hematopoietic stem cells.  The germinal cells migrate during embryogenesis from outside the embryo proper along the vessels of the developing ambilical cord, into the upper abdominal area of the embryo.  They continue on a path that leads them down a track near the dorsal midline near the aorta (not in the bloodstream, but exterior to it) to the developing germinal ridges of the early indifferent gonad of the gonadally non-differentiated embryo.  During this passage the stem cells actively undergo mitotic expansion of the germ cell precursor population.  They can be followed histologically by testing for alkaline phosphatase which is a peculiar product of these cells (several other markers are also being sought to allow further investigations of this developmental pathway).  Once in the germinal ridge, they continue to mitose during early gonadal differentiation.  Further proliferation still continues in the testis which develops from the medullary region of the indifferent gonad, but it ceases once oogonia are enclosed in a layer of undifferentiated granulosa cells from the cortex of the indifferent gonad forming a primordial follicle.  The result of this latter process leaves female mammals a pool of potential gametes numbering about 7 million at the peak number 3 months prior to birth while atretic (see below) losses leave the number at about 2 million at the time of birth in the human (Baker in Austin & Short, Reproduction in Mammals, 1st ed., Cambridge University Press: Cambridge, UK, 1972).

Gamete formation is the one time during a mammal's lifespan when meiosis occurs.  Meiosis is qualitatively different from mitosis as a cell division process in that it absolutely requires a relatively prolonged period of double-strand DNA breakage to allow for sister chromatid exchange.  The mechanisms leading up to crossing-over and their subsequent reversal make the DNA of incipient gametes particularly accessible to the actions of toxicants and other environmental insults such as radiation. Spermatogenesis, on the one hand, involves production of millions of gametes on a daily and continuing basis throughout an adult male's reproductive lifespan.  That amount of cell division is bound to involve mutational and repair mistakes leading to nonviable daughter cells.  Sperm competition for oocytes helps minimize, but not completely eliminate, the potential for producing a zygote containing defective or altered parental DNA.  One might anticipate that in long-lived species where the male spermatogonial precursors or stem cells will have undergone many mitotic cell divisions since puberty, there might be some indication that male age plays a role in increasing the numbers of offspring with male-linked genetic defects.  Interestingly, that case is not clear as the reports are mixed.

In females, on the other hand, the mitotic multiplication of oogonia, the oocyte precursor stem cells, ceases before, at, or just after birth, depending on species.  In all cases, oogonia begin the process of meiosis around the time of birth and progress to the dichtyate stage of meiosis I (the stage of meiosis I at or during which sister chromatid exchange occurs) before suspending further progression until still undefined signals trigger them to proceed.  Beyond this stage, oocyte differentiation seems to assume a stoichastic aspect.  Oocyte resume progression in what appears to be a random pattern.  When they do resume progression they will usually continue to completion of meiosis I, extrusion of the first (2n) polar body from the oocyte (still 2n), and to the prophase of meiosis II while still within the ovary.  Completion of meiosis II only occurs at, or just after, interaction with the fertilizing sperm. In the meantime, however, between the resumption of meiosis I and the completion of the prophase of meiosis II, the oocyte will become interdependent with the cells of the developing ovarian follicle in which it resides.  However, that follicle is dependent for its development not only on the signals from the oocyte it contains but on external circulating levels of LH, FSH, estradiol and a variety of growth factors.  Until puberty LH and FSH never reach levels sufficient to support complete development of the ovarian follicles.  Thus, the maturing oocytes in those follicles never complete full maturation.  Indeed, the granulosa cells of the follicle begin to undergo apoptosis in the absence of adequate gonadotropin or steroid support.  Those signals are conveyed to the oocyte so that it, too, undergoes an apoptotic involution.  Follicular losses by this route are called atresia.  Atresia accounts for the loss of well over 99% of the initial pool of oocytes in all mammals studied to date.  All follicles that begin development prior to puberty undergo atresia as do any of the few remaining oocytes that might begin development after the age of menopause.  Indeed, the triggering of menopause (in those species that demonstrate ovarian senescence) seems to be a depletion of the follicular pool to the point that the remaining follicles are insufficient in number to maintain the homeostatic feedback loops necessary to communicate with the hypothalamus and pituitary so as to modulate and entrain healthy follicular growth and maturation.  A pool of dictyate stage oocytes thus remains in that state for the length of not only the reproductive lifespan, but the entire postpartum lifespan (in contrast to male spermatogonia which remain in a G0 arrest until puberty).  It is not terribly surprising, therefore, that there is good evidence that the incidence of genetic defects among live offspring (e.g., Down's syndrome) as well as the rate of gestational loss rises dramatically with maternal age beyond 35 years.

During follicular differentiation of K-selected mammals such as primates and ungulates, groups of follicles begin the last stages of follicular development in rough synchrony during each reproductive cycle (ovarian cycle, estrus cycle, menstrual cycle).  As these progress, supported by circulating FSH, they produce steroids and growth factors that can act in both autocrine and paracrine fashions.  Moreover, they seem to "compete" for available gonadotropin and other vascular support by developing hormone receptors on granulosa cells and oocytes and differentiating an extensive capillary network over the surface of the developing follicle.  The combined impacts of secretion of growth modulators, competion for circulating resources, and the initial slight asynchronies of follicle development, lead a small number and finally only one or two follicles to reach the final ovulatory stages of maturity during each ovarian cycle.  If an ovulatory surge of LH then becomes available either spontaneously (many species including primates) or via copulatory induction (cats, rabbits, mustelids,...), the oocyte enclosed in a surrounding layer of cumulus granulosa cells are shed into either the peritoneal space or the periovarian bursa, depending on species.  The cumulus complex is picked up by the fimbria of the Fallopian tube (oviduct) and transported via the distal, ampullary region of the oviduct.  There they may encounter sperm that were recently deposited in the female tract, and which have undergone two resultant transformations in that tract: capacitation and the acrosomal reaction.

Capacitation appears to involve a stripping of the proteins that normally coat sperm that have passed through the epididymus and been mixed with secretions from the prostate and seminal vesicles.  The stripping occurs as the pH drops from the alkaline environment in semen to the slightly acid environment of the vagina.  Changes in K+ concentration also probably play a role as this is very high in semen but near isotonic levels in vaginal fluid.  Capacitation enhances progressive motility in sperm and helps send them on their way up the female tract (probably with some help from smooth muscular contractions of the female tract induced partially by the prostaglandin content of semen).  Once in the immediate vicinity of (upon contact with?) the oocyte, which is still contained inside the glycoprotein case called the zona pellucida and is still associated with cumulus granulosa cells and the mucoproteins that they secrete at this time, sperm undergo the acrosomal reaction.  During this process the specialized lysosome at the head of the sperm becomes porous and the overlying cell membrane is shed exposing lytic enzymes embedded in the lysosomal membrane lying immediately over the front half of the sperm head.  The exposed acrosomal enzymes allow the sperm to penetrate through the mucoproteins between granulosa cells and through the zona pellucida.  Continuing tail motion helps propel the sperm into the peri-vitteline space overlying the oocyte membrane.  Once within the space specific proteins near the midline of the sperm head bind to complementary receptors at the oocyte surface.  Sperm binding triggers the events associated with completion of fertilization.

Within the oocyte the sperm binding triggers a wave of intracellular Ca++ that may reverberate across the oocyte several times. The calcium pulse, in turn, causes exocytosis of subcortical granules in the oocyte and resumption of meiosis II in the oocyte nucleus.  Extrusion of the subcortical granules into the perivitteline space alters the nature of the oocyte surface and the proteins of the zona pellucida so they are no longer hospitable for sperm attachment.  This forms the block to polyspermy that would otherwise disrupt normal development.  The oocyte nucleus finishes the reductive nuclear division of meiosis II and the oocyte extrudes the second polar body (1n) while forming the female pronucleus.  Meanwhile, the sperm membrane has fused with the oocyte and the sperm nucleus has begun to decondense.  The mitochondria and sperm tail proteins are either degraded or incorporated into the substance of the incipient zygote (fertilized egg).  Fertilization per se involves the fusion of the male and female pronuclei thereby reforming the 2n zygote.

Gametogenesis in diagrams:

http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookmeiosis.html

http://www.tjc.edu/science/images/reproduction/female_anatomy.htm

http://www.nature.com/nrg/journal/v1/n1/slideshow/nrg1000_040a_F2.html

http://www.sunydutchess.edu/ahbs/Faculty/Scala/Bio102/PDF/Spermatogenesis.jpg

http://www.tyler.cc.tx.us/science/images/reproduction/male_anatomy.htm

http://www.cvm.okstate.edu/instruction/mm_curr/histology/MR/himrp4.htm

Note the efficiencies of gamete formation in mammals where there is physical competition among sperm for fertilization versus the "chemical/hormonal/developmental" competition that occurs during the maturational phase of oocytes.  Note also how inefficient the overall process to this point actually is.

The process is complex and highly orchestrated by both genetic and epigenetic factors including hormones.  There are many, many sites for possible interference by toxicants or other environmental insults.  These can either be direct such as mutagens intercalating into oocyte DNA during the prolonged prophase of meiosis I, or interference with hepatic clearance of steroids, or indirect via changes in hypothalamic regulatory cell function.

Gestation in Mammals

Developmental toxicology deals with defects arising from problems of gene transcription and expression, cellular biochemistry, intercellular signalling (epigenetic problems); and, it deals with these during embryology, gestation, early growth, and maturation.

Developmental toxicology is often held synonymous with teratology (the science of abnormal development, including biochemistry, morphogenesis, and behavior).  Classic examples are phocomelia (flippered limbs resulting from maternal exposure to thalidomide) or cyclopia (a highly visible midline developmental defect occasionally encountered in obstetrics).

Note that much of development is governed by the products of the HOX genes which initiate elements of developmental differentiation programs.  Examples include those genes determining bilateral symmetry, body segmentation (in insects, e.g.), and specific organ development (e.g., SRY, SOX3, SOX9, in gonad differentiation).

Intrinisic genetic programs of differentiation also seem to occur that limit the potential for most cells to contribute to production of tissues other than their "home" tissues.  Stem cells are often guided into the differentiation spaces via intercellular signal gradients or patterns ("induction gradients" e.g., as studied in the eggs of developing amphibians like frogs and salamanders -- amphioxus-- where small segments from a morula, blastula, gastrula, or neurula when transplanted from one site on the structure to another site give rise to the normal outcome of the first site, e.g., extra eyes or legs).  Note, however, that we are becoming increasingly aware of what factors actually are involved in constraining developmental flexibility.  Indeed the whole area of stem cell research and cloning (especially ala Dolly from adult, fully differentiated cells) is dedicated to the task of describing these pathways.

The stages of development in the mammal are summarized briefly as:

1. Fertilized egg (found near mid-cycle at the time of conception, prior to sperm and oocycte pronuclei fusing)

2. Zygote (one cell embryo)

3. Morula ("ball of grapes," up to about 32 cells; stage at or just before entry of the early embryo into the uterus)

4. Blastula ("hollow sphere," up to about 200 cells; stage at which blastomeres begin to specialize, about 90% go to form the trophoblastic cells that presage the pregnancy membranes and placenta, the supportive tissues; these are the earliest source of hCG beginning about day 7-10 in the human; about 10% of the cells are now embryoblasts that form the inner cell mass and will make up the embryo proper as well as portions of the amnion and yolk sac). About day 3-5 post ovulation the blastula enters the uterine cavity and soon breaks free of the zona pellucida that covers all stages prior to this one during their journey from the ovary through the Fallopian tube. About day 5-7 the blastula begins the process of nidation or implantation by interacting with the decidualized portions of the endometrial lining of the uterus.

5. Gastrula (This stage follows during the period of implantation.  It is the period during which distinct germinal tissue layers differentiate to produce distinct endodermal, mesodermal, and ectodermal cell layers within the embryo.)

6. Neurula (Immediately following gastrulation the neurula forms as the early stages of the notochord and neural development take place along with muscle and somite development.)

7. Embryogenesis (The embryo proper arises during this period of organogenesis.

8. Fetal Development (When the embryo has formed the tissues and organs most of the job of development is growth and maturation.  This is taking place during the fetal period of gestation.)

9. Neonatal Development (Following parturition or birth, rapid growth and maturation of structures continues immediately following birth and up to about the third year in humans.)

10. Childhood Development (Slower growth and maturation of most structures except the brain, which continues to grow and develop rapidly, marks this phase of organismal development.)

11. Puberty (Hormonally triggered changes result in sexual and metabolic maturation of many systems.)

12. Adulthood (Also known as the reproductive period. Growth slows.)

13. Aging/Senescence (Gradual decline in functionality, growth becomes negative.)

Many volumes exist covering each and every one of these stages.  They all involve intricate restructuring of cells and tissues as well as changes in transcriptional programs.  They all involve cell divisions (primarily by mitosis) and associated repair/ checkpoint processes.  Each step has unique features that may be sensitive to a somewhat different spectrum of toxicants and will handle absorption, metabolism, clearance and excretion in differing ways.

To track progress through these various stages a number of useful biomarkers have been identified.  Ultrasound can allow monitoring of the growth and rupture of ovarian Graafian follicles, implying the shedding of a mature egg.  It can also be used to monitor the uterine endometrium for the development of an implantation site (at about 3-4 weeks post ovulation in humans; it provides an alternative to palpation for clinical diagnosis of a pregnancy).  HCG is unique to pregnancy and neoplasias and can be monitored in blood or urine beginning about 7-10 days postovulation; present in apes, it is absent in many other species largely due to the variations in uterine-embryo associations during gestation and the dependency of the pregnancy on the function of the corpus luteum in early pregnancy.  Blood or urine metabolites of the major estrogen and progestins arising from the ovary (and later the placenta) are more variable than hCG and must be tracked longitudinally, but they will remain elevated beyond their normal 10-12 days post ovulation if a pregnancy becomes established (in a nonfertile cycle they decline rapidly near the end of the cycle).  This would also be the timing of the first missed menstrual period in humans.

Under normal conditions, most of the fertilized oocytes and early embryoes are lost, easily >50%.  Yet toxicants can elevate even this level or can cause defects in particular organ systems by interfering with the normal coordination and regulation of the developmental program.

How would one go about testing whether a compound was a developmental (versus a genetic) toxin?

One suggestion offered last year was to begin by checking the components, such as enzymes, in fetal circulation.  This, however, raises the question: "How do you sample fetal blood?"

To consider this it is helpful to look at the anatomy of several stages of fetal development.

Once in this site, go to Biology 30, Unit 2, Human Development: http://www.telusplanet.net/public/firstaid/biology30/develop.htm

The Visible Embryo, informational site at NIH: http://www.visembryo.com/

University of Vermont teaching modules for human embryology: http://cats.med.uvm.edu/cats_teachingmod/embryology/cats_embryo_home.html

Ultrasound image galleries for obstetrics and gynecology: http://www.obgyn.net/us/img_gal.htm

Ultrasound image gallery for human development: http://www.obgyn.net/us/gallery/gallery.htm

Gray's Anatomy OnLine: http://www.bartleby.com/107/%20illus502.html

Fetal circulation by cardiac catheterization: http://user.gru.net/clawrence/vccl/chpt1/fetcirc.HTM

Gray's Anatomy, Embryology, Placental Development OnLine: http://www.bartleby.com/107/12.html

Note that sampling might potentially occur from the umbilical veins (flowing from the placenta to the embryo) or arteries (flowing from the embryo to the placenta) as early as 4-6 weeks post ovulation.  Practically, since the embryo is only 20-30 mm long at 4-6 weeks, fetal blood could probably only be sampled about 8-10 weeks onward once fetal growth is at least 75-100 mm.  And that would need to be done with the guidance of ultrasonic imaging.  More realistically, testing might be accomplished using sampling of chorionic or amniotic fluids or, at very early stages, by sampling of placental villi.

These tests would, however sophisticated, be checks only on the first generation involved.  They might include direct evaluation of enzymes and tissues sampled from placental villi, cells from amniocentesis, both of which yield information on potential genetic toxins as well, from: karyotype; molecular tests like RFLP -- DNA, treated with restriction enzymes like ECORI, gel electrophoresis, and/or Southern blots and probes, or sequencing of bands --, SNP tests using PCR, LCR, or sequencing); PCR; metabolic information from protein and enzyme evaluations (Western blots, 2D gels, enzyme activities, cell growth in vitro); tissue growth information from imaging, e.g., ultrasound or NMR; physiology from levels of steroids, proteins, hormones; (maternal) immunoglobulins in fetal compartment(s).  They probably would not indicate if development overall was normal or resulted in an offspring capable of optimal function including reproduction.

Indeed, these tests don't really have the capacity to evaluate full development, coordination of tissue growth, or rate of organismal growth until we look at whole animal tests.  An acute dose to the mother may imply an impact or loss of the embryo or fetus and/or a decrease in the capacity to fertilize and implant zygotes.  A chronic dose to the mother may not lead to as precipitate a response.  Either insult might allow testing of early stages as above.  Lower, chronic doses may also allow examination of developmental impacts on the F1 generation either as assessed directly by the kinds of tests listed above or more overall impacts on growth and development as assessed up to and after birth and through reproduction of the F1 to produce F2 progeny.

Developmental toxins may not necessarily change the genetics of the F1 generation.  They are distinguished by post-genetic or epi-genetic impacts as often seen in changes in cell structure and/or physiology.  Sometimes these are readily apparent, sometimes they are so subtle they are not apparent until the F1 is functioning outside the uterus or even attempting to produce the F2 generation.

Multigeneration tests imply the need to use small animals to allow handling of enough individuals to statistically see the increment of alterations caused by the toxicant above those occurring normally (often at rather high rates, e.g., in cases of loss of early embryos).  The numbers need to allow estimations above a high level of natural background "noise."  These tests also require the use of animals with a short generation time so enough tests can be run in a short period of time to allow many compounds to be tested in a reasonable period.  Mice are obvious candidates that fit these needs.

But notice that use of such short lived animals may run risks of missing important problems that arise only in longer lived species that require other or more coordination of developmental events over a more prolonged lifespan, e.g., maintenance of stem cell lineages.  This is also complicated by differences in the levels and types of protection/repair/detoxification processes occurring in small versus larger longer lived species.

Special sensitivities also arise in the developing embryo or fetus:

1. Organs may function differently in the fetus versus the adult, e.g., the liver is hematopoietic in the fetus, the adrenal cortex is hyperplastic and very active in a modification of steroidogenesis peculiar to the fetus, blood flow in the heart is different.

2. Key molecules in the adult differ from those in the fetus, e.g., fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin implying it also has higher affinities for CO, CN, NO, and reactive oxygen species and therefore potentially higher susceptibility to such gases or products of endogenous pathways in mother or fetus producing such compounds.

3. The placenta itself is a site of high P450 enzyme activity in relation to steroidogenesis and detoxification/toxin activation.

4. Rapidly growing fetal tissues are sites of high expression of receptors for growth factors and of paracrine growth factors themselves.  They rapidly take up molecules involved in macromolecular synthesis and overall metabolism.  Therefore, they have increased susceptibility to structural mimics of such building blocks and/or inhibitors of growth processes, e.g., lead vs calcium, phorbol esters vs diacyl glycerides, D- vs L- amino acids, L- vs D- glucose.

5. Unique activities are occurring in the embryo versus the adult, e.g., active growth of nervous and eye lens tissues, migration of germ line cells, migration of various cell lines (Rathke's pouch> anterior pituitary, ultimobrachial anlage> thyroid and parathyroid tissues).  Some signaling paths may be unique to the embryo or fetus, e.g., chemotactic substances, certain transcription factors.

Germline mutations

Note also there are important differences in somatic versus germ cell mutations and how they are evaluated.


1. nondeleterious?: recessive, silent, and conservative amino acid substitutions

2. deleterious: chromosomal aberrations, point mutations, deletions, insertions, homozygosity?

Note that most of the conditions we think about are actually the rare cases that are compatible with survival through gestation. They actually represent a minority of the aberrations that actually occur.  Losses of zygotes up to the stage of implantation (about 5-7 days postfertilization in humans) is unclear but success in fertilizing mature eggs in vitro is at best 30-50%, with 15-20% being more typical (i.e., losses range from 50-85%).  Losses of zygotes and embryos up to the stage of clear biochemical and clinical recognition (about 2-3 weeks post-fertilization) appears to be between 30% and 70% under the best of conditions.  Another 15-20% of concepti are lost by miscarriage during the remainder of pregnancy.  Thus, somewhere between 24% and 60% of human fertilizations result in a live birth.  So 40%-76% are lost due to genetic, metabolic, morphogenetic, or maternal causes.  Prior research suggests that genetic causes top the list of reasons for these losses.  Thus, evaluating the effects among the survivors may miss the most important elements of genetic deviation induced by exposures to toxicants.

Assays for gene mutations.

-- Ames assay

-- Enhanced Ames assay

-- Mammalian cell tests

-- In vivo tests in Drosophila and mice

-- SNP screens

-- microarray evaluations

Assays for chromosome aberrations.

-- Karyotypes of cultured or lymphoid cells

-- Flow cytometry

-- FISH

Other

-- Unscheduled DNA repair synthesis

-- Yeast recombination

-- Sister chromatid exchange

-- Transgenic animals

-- PCR

In considering toxicity models for evaluating genetic impacts of toxicants it is worth considering the reproductive strategies being employed by the species being studied.  Rodents are a common model yet their reproduction is set up in an R-selected mode. They have a relatively brief lifetime and would not be expected to display a full array of repair, detoxication, or barrier processes that would prevent intoxication of either the adult or developing young.  Nor might we expect them to display robust protective mechanisms for gamete production.  Many larger mammals including humans, on the other hand, are K-selected and would be expected to display a full array of protections for gametes, developing offspring, and adults.  Is there any good evidence for or against these statements?

R-selected: life history favors early reproduction, high rates of reproduction, usually small offspring requiring minimal parental investment, quantity vs quality, favored in a variable environment

vs

K-selected: life history favors late reproduction, low rates of reproduction, large offspring requiring much parental investment, quality vs quantity, favored in stable environments or in species with large ranges

Other possible model systems for evaluating germline genetic insults in mammals

And what might make good markers to track gamete insults in males and females?  Why might the DAZ1 gene make a good biomarker for testing genetic toxicity in male gamete production?  What is the "secret" of the Plains Viscachia?  Why might it be a good model for looking at toxic effects on female gametes?

Many genes that are required for fertility have been identified in model organisms.  Mutations in these genes cause infertility due to defects in development of the germ cell lineage, but the organism is otherwise healthy.  Although human reproduction is undoubtedly as complex as that of other organisms, very few fertility loci have been mapped.  This is in spite of the prevalence of human infertility, the lack of effective treatments to remedy germ cell defects, and the cost to couples and society of assisted reproductive techniques.  Fifteen percent of couples are infertile and half of all cases can be traced to the male partner.  Aside from defects in sperm production, most infertile men are otherwise healthy.  This review is divided into two distinct parts to discuss work that: (i) led to the identification of several genes on the Y chromosome that likely function in sperm production; and (ii) implicates DNA repair in male infertility via increased frequency of mutations in DNA from men with meiotic arrest. (Mol Cell Endocrinol 2001 Nov 26;184(1-2):41-9).

Source: http://www.ncbi.nlm.nih.gov/

The AZFc region of the Y chromosome features massive palindromes and uniform recurrent deletions in infertile men. (Kuroda-Kawaguchi T, Skaletsky H, Brown LG, Minx PJ, Cordum HS, Waterston RH, Wilson RK, Silber S, Oates R, Rozen S, Page DC. Howard Hughes Medical Institute, Whitehead Institute, and Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.)

Deletions of the AZFc (azoospermia factor c) region of the Y chromosome are the most common known cause of spermatogenic failure. We determined the complete nucleotide sequence of AZFc by identifying and distinguishing between near-identical amplicons (massive repeat units) using an iterative mapping-sequencing process.  A complex of three palindromes, the largest spanning 3 Mb with 99.97% identity between its arms, encompasses the AZFc region.  The palindromes are constructed from six distinct families of amplicons, with unit lengths of 115-678 kb, and may have resulted from tandem duplication and inversion during primate evolution.  The palindromic complex contains 11 families of transcription units, all expressed in testis.  Deletions of AZFc that cause infertility are remarkably uniform, spanning a 3.5-Mb segment and bounded by 229-kb direct repeats that probably served as substrates for homologous recombination. (Nat Genet 2001 Nov;29(3):279-86).

In humans DAZ1 (Deleted in Azoospermia), resides on the distal portion of the euchromatic portion (the part that is transcriptionally active and has been sequenced and mapped; for the Y chromosome the map is a physical one because only the distal part of the short arm, the psuedoautosomal region, can undergo crossing over during meiosis I when it pairs with the homologous region of the long arm of the X chromosome) of the long arm (q, not for quinacrine, an antimalarial and a green fluorescent stain for AT rich DNA, but for the next letter after p which stands for, petite, in reference to the short arm of a chromosome; quinacrine is a common stain for generating band patterns that assist in karyotyping) of the Y chromosome, just proximal (relative to the centromere) of the heterochromatic region (the part that is not transcriptionally active and is highly repetitive) of the long arm of Y.  It is a gene identified by David Page et al. at the Whitehead Institute at MIT as part of a cluster of duplicated genes that is found to be lost in a high percentage (2-20%) of men clinically demonstrated to be azoospermic and therefore infertile.  About 15% of couples are infertile in western societies.  About 50% of infertile couples demonstrate male-associated infertility including ~35% among them demonstrating male azoospermia.  Thus, about (0.15 x 0.1 x 0.35 = 0.00525 = 0.525% or 1/190) 1 in every 200 males lack part or all of the DAZ1 gene!

Because DAZ1 and its clustered homologs are repeats and contain repetitive sequences within each gene, it is not very surprising to find high rates of deletion occurring during the meiotic process involved in spermatogenesis.  Since the deletion is apparently nonlethal for the initial stages of development, it does not compromise the vitality of the males that harbor the gene - it simply makes them unable to reproduce.  Clearly each DAZ1 deletion that is found represents a de novo mutation occurring during spermatogenesis in the father of the affected offspring.  (The gene product of DAZ1 appears to be an RNA binding protein that affects progression of spermatogenesis beyond the spermatocyte stage.  It may block meiosis II.)

The high endogenous rate of deletion for DAZ means the site is labile.  This makes it a potentially useful marker for monitoring genetic toxicant insults to male gamete production, particularly if the homologs in other species besides man and the great apes also lie in the region in which they are found in man.

Background on genetics of gametogenesis and DAZ, in particular, are found at the following sites:

Lecture on chromosomal biology: http://www.bi.umist.ac.uk/users/mjfasjw/2MMB/chromosomes/lecture1.asp

DAZ1 technical gene description: http://gdb.tokyo.jst.go.jp/HOWDY/HOWDY.pl

DAZAP Article: http://www.biomedcentral.com/content/pdf/1471-2164-2-6.pdf

DAZ circa 1998: http://wwwvet.murdoch.edu.au/spermatology/iss982.html

Frequency of azoospermia/AZF deletions: http://www.ich.ucl.ac.uk/cmgs/ydels99.htm

European Guidelines on Diagnosis of Y Chromosomal Deletions: http://www.emqn.org/guidelines/azf.htm

Male infertility, genetic analysis of the DAZ genes on the human Y chromosome and genetic analysis of DNA repair.  (Fox MS, Reijo Pera RA.  Departments of Obstetrics, Gynecology, and Reproductive Sciences, Physiology and Urology, and Programs in Human Genetics and Cancer Genetics, University of California, 94143-0546, San Francisco, CA, USA). Source: http://www.nature.com/

I leave you to figure out why the Plains Viscachia might make a good model in which to explore insults to oogenesis.  ;)

Discussion Questions

 

Genetic Toxicology & Evolution (QS4Q3)

 

26.    If pyrimidine dimers and chemical adducts to DNA are preferentially removed in transcribed and active DNA sequences relative to untranscribed and inactive DNA sequences, what does that suggest about the hot spots for genetic drift upon which evolution is based?  If you were looking for gene sequences to use to differentiate among related species, what kind of genes would you tend to look at as indicators?

 

Genetic Toxicity: Possible Models? (QS4Q4)

 

27.    Why might the Plains Viscacha of South America be a good model for toxicity associated with oogenesis?  Could the DAZ1 gene found on the long arm of the Y chromosome be used as a screen for germ cell toxicity in the primate male?  How might the latter be evaluated?

 


2005 Kenneth L. Campbell