Twinning has long intrigued the intellect of mankind,
and early attempts to explain the
phenomenon frequently attributed it to mystical or magical forces. In the African Yoruba tribe, for
example, twins were thought to be good spirits who entered the womb of a pregnant woman to be
born as humans (Scheinfeld, 1967). They were thus considered good-luck omens and celebrations
accompanied the birth of twins. Not all cultures viewed twins in such a benevolent light, however;
many Native American tribes believed twins were harbingers of evil brought about by sinister
powers and routinely smothered them shortly after birth (Scheinfeld, 1967). While biological
advances have since illuminated the true origins of twins, they are still frequently greeted with awe
and fascination by many.
Monozygotic twins, frequently called identical twins due to the striking similarity these
pairs usually exhibit, inspire a particular interest because of the genetic implications of two
individuals who are genetically identical. Many potential reasons for monozygotic twinning events
have been proposed, although it is not definitively known which events are primarily responsible.
Later monozygotic twinning events seem to favor female twins, thus placing an interesting twist on
this question. or the cause of . Medical cases in which female monozygotic twins display
discordant phenotypes for X-linked diseases has pointed a finger at X chromosome inactivation as
a possible player in the cell splitting that results in such
twins.
The principal difference between monozygotic twins
and dizygotic twins and singletons is
that monozygotic twins result from the splitting of early embryonic cells from a single fertilization
event, forming two separate embryos with the same genotype. In dizygotic twinning and singleton
births, by contrast, each embryo results from the union of a separate egg and sperm. The location
of monozygotic twins within the uterus is indicative of the approximate time that initial cell splitting
occurred. Separation of embryonic blastomeres prior to the fifth day of gestation, when the
trophoblast is formed, results in twins with entirely separate chorions and accounts for
approximately one third of monozygotic twins surviving to birth. If splitting occurs after the
separation of inner cell mass and trophoblast on the fifth day of gestation, but prior to the
formation of the amnion on the ninth day, the resulting twins will share a common chorion, but
have separate amnions. Such monochorionic, diamniotic twins constitute the largest proportion of
monozygotic twin births, making up approximately two thirds of the total. A very small fraction of
monozygotic twins are monochorionic and monoamniotic, resulting from a splitting of the inner
cell mass after the formation of the amnion on the ninth day. Conjoined twinning results from
incomplete separation of the inner cell mass, and most likely results from the latest attempt at
separation. (Machin, 1996)
Unlike dizygotic twinning, monozygotic twinning is rarely influenced by hereditary factors
and not induced by fertility drugs (Hall, 1996). Rather, each incidence of twinning is the result of
one of many possible sporadic events that could lead to the splitting of blastomeres from one
another. Among possible events that could result in such cell splitting are abnormalities in the zona
pellucida which surrounds the blastocyst as it travels down the oviduct to the uterus, abnormalities
in the developmental clocks of different cells causing them to be at different stages at the same
time, and discordance between adjacent cells which might cause them to repulse one another (Hall,
1996). It has been speculated that splitting events occurring at later stages, such as those that lead
to monochorionic as opposed to dichorionic twins, may be the result of different developmental
events, a speculation supported by the observation that sex ratio is increasingly skewed toward
females when splitting occurs later (Figure 1, Machin et al., 1996). This observation has lead
researchers to look for events that might cause twinning in females but not males. X-chromosome
inactivation is one such possible event.
Figure 1: Sex ratio (female : male) in twins by zygosity, chorion, and amnion status. A steadily increasing female excess
in monozygotic twins arising from later twinning events is evident. (After
Machin et al., 1996)
In humans, as in the majority of animals, the feature
that differentiates females from males is
that they possess two X chromosomes, whereas males posses one X and one Y chromosome. The
number of X-chromosomal RNA transcripts are kept approximately equal in both sexes, however,
due to a phenomenon called X-chromosome inactivation by which one of the X chromosomes in
every female cell is inactivated (Gilbert, 1994). The inactivated X-chromosome, usually referred to as
a Barr body, is condensed and remains inactivated in all progeny of the initial cell (Griffiths et al.,
1996). In addition to being more condensed, inactivated X-chromosomes also differ from their
active counterparts in a number of other ways, including methylation and acetylation differences,
making it possible to asses which chromosome has been inactivated (Brown and Willard, 1994). The
precise time in development that inactivation occurs is not known, although it is speculated to occur
before the 64 cell stage (Migeon et al., 1996; Gilbert, 1994). X inactivation is usually a random
process, thus females are actually mosaics of cells containing different X genotypes (Griffiths et al.,
1996).
Genes for several recessive traits are located on the X-chromosome, and their expression is
therefore usually restricted to hemizygous males who have inherited a mutated X from their mothers,
who are non-manifesting carriers. On rare occasions women have been found who express these
traits, usually due to genetic events such as chromosome loss or translocation that cause the woman to
have a homozygous genome. If, however, rather than undergoing the typically random inactivation,
the majority of woman's normal X chromosomes became preferentially inactivated, mutant X-linked
traits carried on the active chromosome would become expressed (Jorgensen et al., 1992). This
phenomenon has been well documented in several studies of female monozygotic twins heterozygous
for X-linked disorders. In these pairs one twin has skewed X-inactivation toward the normal
chromosome, and thus expresses the recessive phenotype, while her twin has skewed inactivation
toward the mutant chromosome and thus exhibits no
symptoms.
Several X-linked conditions exhibiting differential
expression in female monozygotic twins
have been documented, most notably Duchenne Muscular Dystrophy (DMD) and color blindness.
In all studies the karyotypes of the twins were examined to rule out the possibility of expression due to
chromosome duplication, deletion, or recombination. In remaining twin pairs heterozygosity for the
examined trait was confirmed and X-chromosome inactivation patterns determined. Since nearly 70
percent of monozygotic twins share blood circulation while in the womb, studies based on the
analysis of non-hematopoietic such as fibroblast tissues have been favored
of studies (Hall, 1996).
A number of cases have been documented in which the patterns of X-inactivation were found
to be skewed in opposite directions, with the affected girl exhibiting preferential inactivation of the
chromosome containing the wild type copy of the gene, while her unaffected twin exhibited
preferential inactivation of the chromosome containing the mutated copy (Table 1).
No cases have been documented in which monozygotic twins known to be heterozygous for an
X-linked trait both manifest the trait or both display normal phenotype (Jorgensen et al., 1992,
Machin et al., 1996), thus a connection between the twinning and X-chromosome inactivation has
been suggested. One hypothesis proposes that random inactivation occurs in the initial cell mass in
such a way that two clusters of cells are produced, each with opposite X inactivation patterns
(Burn et al., 1986, Richards et al., 1990, Machin et al., 1996). The discrepancy in the inactivation
patterns then causes each group of cells to regard the other as a foreign entity, resulting in mutual
repulsion of the cell masses and their development into two separate embryos (Figure 2, Burn et
al., 1986, Richards et al., 1990, Machin et al., 1996). The observation that two of the famous
monozygotic Dionne quintuplets, who were heterozygous for X-linked color-vision deficiency,
were colorblind supports this theory (Jorgensen et al., 1992). This hypothesis would also help
explain skewed X-inactivation leading to X-linked disease expression in apparently singleton births
- these individuals might be monozygotic twins whose normally skewed pair died early in
development (Machin et al., 1996).
Figure 2: Random inactivation followed by mutual
repulsion of cells with differentially inactivated
X-chromosomes resulting in discordant monozygotic twins. Paternal X-chromosome and cells with
paternal X inactivated are shown in bold. (After Machin et al.,
1996)
Cases have also been documented in which the clinically affected twin displays skewed
X-inactivation while the unaffected twin exhibits random inactivation (Table 1). An alternate theory
connecting twinning and X inactivation might account for this observation. In this second proposal,
rather than X inactivation prompting the initial separation of cells, random inactivation occurs
followed by unequal allocation of the cells to each twin in a splitting event triggered by some other
occurance. One twin receives relatively few cells, a significant portion of which have the same X
inactivated, while the other receives a larger allocation of cells with a random, or near random,
inactivation pattern (Figure 3, Machin et al., 1996). Indeed, some twin pairs that exhibit this
inactivation pattern show definite evidence of unequal allotment of cells to the two embryos (Machin
et al., 1996). This theory does not, however, help account for the increased number of female
monozygotic twinning events.
Figure 3: Random inactivation followed by unequal allocation of cells to each twin resulting in
discordant monozygotic twins. Paternal X-chromosome and cells with paternal X inactivated are
shown in bold. (After Machin et al.,
1996)