Making Germ Cells from Human Embryonic Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 57

Ara et al. 2003 ; Besmer et al. 1993 ; Koshimizu et al. 1992 ; Molyneaux et al. 2003 ;
Zou et al. 1998 ). Further, knockout mice with null alleles of the RNA-binding pro-
teins, Tial1 and Nanos3 , have normal numbers of specified PGCs, but no germ cells
that survive the migration and colonize the gonads. Thus, these knockout animals
are both male and female infertile (Tsuda et al. 2003 ; Beck et al. 1998 ; Suzuki et al.
2008 ). Another gene involved in the process of PGC migration is Oct4 , the classic
marker of pluripotency that is critical for the establishment of the embryonic inner
cell mass and the generation of embryonic stem cell lines (Thomson et al. 1998 ).
A conditional mouse knockout lacking Oct4 exclusively in the germ cell lineage
was created using the Cre Recombinase strategy to circumvent the importance of
Oct4 in the blastocyst. These germ cell-specific Oct4 null animals have a reduced
number of PGCs that reach the gonads and diminished fertility or complete infertil-
ity (Kehler et al. 2004 ). Several of the above mentioned genes function as transla-
tional repressors ( Tial1 and Nanos3 ) and transcription factors ( Oct4 ), but exactly
how they regulate PGC survival, migration, reprogramming, and germ cell identity
has yet to be elucidated.
Although we have gained important information about the candidate genes and
signaling pathways of germ cell development from the mouse model, it has become
clear that there are many unique aspects to human germ cell development, espe-
cially pertaining to the genetic requirements. Several of the genes expressed and
functional in human germ cells are distinct from that of model organisms, including
mice and other mammals. Many genes located on the human X and Y sex chromo-
somes have homologs that are expressed in different doses or, in some cases, are
altogether absent in mice (see Sect. 3.3) (Reijo et al. 1995, 1996 ; Skaletsky
et al. 2003 ; Vogt et al. 1996 ; Zinn et al. 1993 ). Women require two X chromosomes
for oocyte development, while female mice are fertile with a single X chromosome
(Zinn et al. 1993 ; Davison et al. 1999 ). Further, meiotic chromosome missegrega-
tion occurs much more frequently in human germ cells (5-20%) than in model
organisms (Hunt 1998, 2006 ; Hunt and Hassold 2008 ). The human genome also
contains rapidly evolving autosomal genes that are uniquely expressed in human
germ cells, but differ greatly in sequence and timing of expression between even
closely related species (Clark et al. 2004b ; Saitou et al. 2002 ; Hendry et al. 2000 ;
Swanson and Vacquier 2002 ). These observations demonstrate the need for further
studies on human germ cell development, and the establishment of an in vitro
human genome-based system to study this process.

3.3 The DAZ Gene Family

3.3.1 The Y Chromosome DAZ Genes

The most well-characterized genes to date definitively involved in human germ cell
development and linked to infertility are the DAZ genes. The DAZ gene cluster,
comprised of four nearly identical duplicated genes in tandem, were discovered in
a screen for genes on the Y chromosome that cause azoospermia or the lack of