Page 17 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

xvi Guided Tour
4.7 Sex-Linked and Sexually Dimorphic Traits in Humans 121

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This feature is one of the methods used to inte-
Visualizing X Chromosome Inactivation in Transgenic Mice
grate the Mendelian principles introduced early
Scientists have recently used molecular techniques and trans- chromosome from the father. Clonal patches of cells are ei-
genic technology (similar to that described in the earlier Fast ther green or red depending on which X chromosome was in the content with the molecular content that
Forward Box Transgenic Mice Prove That SRY Is the Maleness turned into a Barr body in the original cell that established the will follow.
Factor) to visualize the pattern of X chromosome inactivation in patch (Fig. B).
mice. The researchers generated XX mice containing two differ- Different XX mice display different green and red patchwork
ent transgenes (in this case, genes from a different species). patterns, providing a clear demonstration of the random nature of
One of these transgenes was a jellyfish gene that specifies X chromosome inactivation. The patchwork patterns reflect the
green fluorescent protein (GFP); the other was a gene from red cellular memory of which X chromosome was inactivated in the
coral that makes red fluorescent protein (RFP) (Fig. A). founder cell for each clonal patch. Geneticists currently use these
In the XX mice, the GFP gene is located on the X chromo - transgenic mice to decipher the genetic details of how cells
some from the mother, and the RFP gene resides on the X “remember” which X to inactivate after each cell division.
Figure A Cells of transgenic mice glow either green Figure B Heart cells of a transgenic mouse reveal a
or red in response to X chromosome inactivation. The clonal patchwork of X inactivation. Patches of red or
mouse carries a green (GFP) transgene inserted in the maternal green cells represent cellular descendants of the founders
M
X chromosome (X ), and a red (RFP) transgene in the paternal that randomly inactivated one of their X chromosomes. 18.3 Targeted Mutagenesis 633

P
P
X chromosome (X ). Cells in which X is inactivated (top) glow © Hao Wu and Jeremy Nathans, Molecular Biology and Genetics,
M
green; cells glow red (bottom) when X is inactivated. Neuroscience, and HHMI, Johns Hopkins Medical School.
GFP X M
RFP X P TOOLS OF GENETICS Tools of
Barr body
How Bacteria Vaccinate Themselves Against Viral Genetics Essays
Barr body Infections with CRISPR/Cas9 Current readings explain
GFP X M
Researchers discovered clustered sequence repeats (CRISPRs) At the CRISPR locus of bacterial genomes, short direct re-
RFP X P in bacterial genomes as early as 1987. When in 2005 some of peats are interrupted at regular intervals by unique spacer se- various techniques and
these sequences were found to originate from bacteriophage quences (Fig. A). The spacer sequences are fragments of tools used by geneticists,
genomes, several astute scientists speculated that CRISPRs bacteriophage genomes captured by the host cell and inte-
might mediate a viral immunity system in bacteria. These ideas grated into the host genome by the action of two bacterially including examples of
were largely ignored for several more years until the mecha- encoded Cas proteins (Cas1 and Cas2). The repeats within the
nism of resistance became clarified. And finally, in 2012–2013, CRISPR arrays are added by these endonucleolytic enzymes applications in biology
the so-called CRISPR craze reached its full bloom when re- during the capture and integration process. and medicine.
searchers including Feng Zhang, Jennifer Doudna, and Viral immunity results from steps that begin with transcrip-
Emmanuelle Charpentier developed methods to adapt this viral tion of the CRISPR array into long RNA molecules called
immunity system to engineer genomes in bacterial cells and in pre-crRNAs that are processed into short (24–48 nt) so-
have a different pattern of X chromosome inactivation. In Recall that the two tips of the X chromosome, the NAs). In the bacterial species
called CRISPR RNAs (crR
eukaryotic organisms.
Fig. 4.25b, females heterozygous for the X-linked recessive pseudoautosomal regions (PARs), contain genes also pres-
ent at the tips of the Y chromosome (Fig. 4.8). In order to
trait anhidrotic epidermal dysplasia have patches of skin that The CRISPR/Cas9 locus vaccinates bacteria against viruses.
Figure A
lack sweat glands interspersed with patches of normal skin; equalize the dosage of these genes in XX and XY cells, the
Repeats
Spacers
Chapter 14 Bacterial Genetics
494
CRISPR
the phenotype of a patch depends upon which X chromo- PAR genes on the Barr body X chromosome escape inacti-
some is inactivated. Each patch is a clone of skin cells de- tracrRNA cas9
vation. This feature of dosage compensation may explain at
rived from a single embryonic cell that made the decision to least in part why XXY males (Klinefelter syndrome) and
cas9 mRNA
inactivate one of the X chromosomes. In a second example, XO females (Turner syndrome) have abnormal morpholog- 3' Pre-crRNA
5'
5'
women heterozygous for an X-linked recessive hemophilia 3' ical features. Although one of the two X chromosomes in Crowd: © Image Source/Getty Images RF
GENETICS AND SOCIETY
allele are called carriers of the disease allele, even though XXY males becomes a Barr body, Klinefelter males have
they may have some symptoms of hemophilia. The severity three doses (rather than the normal two) of the genes in the
Cas9
of the condition depends on the particular random pattern of PAR regions. The single X chromosome in XO cells does
The Human Microbiome Project
RNase III
cells that inactivated the disease allele and cells that inacti- not become a Barr body, yet these cells have only one dose
Established in 2008 and funded by the U.S. National Institutes
vated the normal allele. In Chapter 3, we discussed how of the PAR genes (rather than two in XX females).
of Health, The Human Microbiome Project (HMP, Fig. A) is one
chance events work through genes to affect phenotype; X X chromosome inactivation is common to mammals,
of several international consortia aiming to understand the com-
inactivation is a perfect example of such an event. and we will present the molecular details of this process in Pre-crRNA processing
3'
3' 3' plex relationship between our bodies and the trillions of micro-
3'
5'
5'
5' organisms that inhabit them. 5'
5' The HMP has already achieved its first goal of describing
3'
the diversity of the organisms that make up the human microbi-
ome. Investigators analyzed the microbial metagenomes lo-
cated at several different sites in the bodies of more than
crRNAs
250 people from around the globe. These studies focused on
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har00909_ch04_089-132.indd 121 the sequence of the gene encoding the 16S rRNA of the ribo-
somal small subunit of these bacteria, because these sequences
diverge substantially in different bacterial species and thus
serve as markers for those species. The results showed that a
tracrRNA
single person can harbor up to 1000 different bacterial species, Figure A
crRNA
but people vary widely in the types of bacteria that make up © Anna Smirnova/Alamy RF
their microbiome. Thus, it appears that worldwide more than
10,000 different bacterial species colonize human bodies. The
researchers of the HMP have already sequenced the complete an effect? One method is to investigate in detail how the bio-
genomes of many of these kinds of bacteria. logical properties of the microbiome and the host might be
The second phase of the HMP began in 2014, and is changed by the interactions of bacteria and the humans they
aimed ultimately at determining whether changes in the mi- colonize. Thus, scientists will characterize whether and how the
crobiome are the causes or effects of diseases or other im - transcriptomes and proteomes of the bacteria and human cells
portant traits in humans. Diseases potentially linked to the are changed by bacterial colonization of human organs. These
microbiome include cancer, acne, psoriasis, diabetes, obesity,
Viral DNA cleavage studies will further delve into metabolomics (characterizing me-
and inflammatory bowel disease; some investigators have tabolites in the human bloodstream).
suggested that the composition of microbiomes could also A second and even more powerful method for establish-
influence the mental health of their hosts. The first step in ing the cause and effect of microbiome changes is the use of
these studies will be to establish whether statistical correla- germ-free mice raised in sterile environments. Surprisingly,
tions exist between specific kinds of microbial communities germ-free mice can survive although they are not normal: they
and disease states. As one example, one HMP phase II proj - have altered immune systems, poor skin, and they need to eat
ect currently underway is an analysis of vaginal host cells and more calories than do normal mice to maintain a normal body
Genetics and Society Essays microbes during pregnancy. Approximately 2000 pregnant weight. Researchers can populate germ-free mice with a sin-
har00909_ch18_618-645.indd 633 women will be studied and their birth outcomes recorded. gle bacterial species or a complex microbial community, and
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Dramatic essays explore the social and ethical The goal of this project is to determine if changes in the mi - thus determine how microbiomes influence physiological
states. Problem 8 at the end of this chapter will allow you to
crobiome correlate with premature birth or other complica-
issues created by the multiple applications of tions of pregnancy. explore this approach by discussing an experiment recently
Of course, the existence of any correlations found between
performed with germ-free mice that asks if the microbiome
modern genetic research. microbiomes and disease does not prove cause or effect. But plays a causal role in obesity.
even if bacteria correlated with a disease state do not cause the If microbial communities indeed contribute to disease
disease, the existence of the correlation could be useful as a states in humans, then future treatments might aim to alter
way to diagnose certain conditions. Nevertheless, the most ex- resident microbiomes. Thus, the flip side of the HMP is to in -
citing potential outcomes of the HMP will be results that point to vestigate how human interventions might change bacterial
bacteria within microbiomes as agents that contribute to com- communities. How effective are dietary changes or dietary
plex diseases. Such bacteria would become obvious targets for additives such as probiotics in effecting long-lasting altera-
therapeutics such as drugs that target proteins specifically tions in microbiomes? If acute infections are treated with anti-
made by these microorganisms. biotics, how will bacterial communities change over time?
How can researchers establish whether a statistical corre- Several HMP projects are already exploring these important
lation between microbiomes and diseases reflects a cause or questions.
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