MOLECULAR GENETICS OF THE HUMAN SEX CHROMOSOMES 
Larry J. Shapiro, M.D., Investigator 
Research in Dr. Shapiro's laboratory is directed 
primarily toward elucidating the biology of the 
human sex chromosomes. The organization of 
genes on the X and Y chromosomes and the evolu- 
tionary history of these two structures is under 
study. The role of the sex chromosomes in primary 
sex determination and the functional regulation of 
gene expression of X-encoded genes through X in- 
activation are also major areas of interest. Investiga- 
tion is proceeding into the basis of X-Y recombi- 
nation in the pseudoautosomal region of the sex 
chromosomes and the generation of populational 
polymorphism in the sex chromosomes. In addi- 
tion, one gene that encodes the enzyme steroid sul- 
fatase (STS) is receiving particular scrutiny. The 
biosynthesis, post-translational processing, and tar- 
geting of this protein to subcellular organelles is 
the subject of a number of experiments. The evolu- 
tionary history of this gene, the mechanism by 
which it escapes regulation by X chromosome inac- 
tivation, and the significance of frequent deletions 
of this locus in human populations is receiving de- 
tailed consideration. 
I. Organization of the X and Y Chromosomes. 
It is commonly believed that the very dissimilar 
X and Y chromosomes evolved from a pair of ho- 
mologous ancestral chromosomes. The finding that 
some modern day vertebrates lack sex chromo- 
somes or a genetic mechanism of primary sex deter- 
mination supports this view. One example is pro- 
vided by certain fish that can change gonadal and 
phenotypic sex in response to environmental cues. 
Studies of the expression of putative regulatory 
genes in this dynamic system are under way. Pairing 
and segregation of the human X and Y chromo- 
somes is facilitated by a chromosomal domain of se- 
quence identity, the pseuduoautosomal region. In 
turn, this sequence homogeneity is maintained by 
frequent meiotic recombination between the X and 
Y chromosomes in this region. Absence of the 
pseudoautosomal region results in a complete 
failure of pairing of the X and Y during spermato- 
genesis. Studies of the pseudoautosomal, sex chro- 
mosome-unique boundaries in primates have 
provided evidence that gross chromosomal rear- 
rangements, which include a pericentric inversion, 
have been responsible for creating this sharp junc- 
tion. It is suspected that the pseudoautosomal re- 
gion gradually expands through nonhomologous 
recombination and contracts through processes of 
chromosomal rearrangement over evolutionary 
time. Several consequences of this are the presence 
of sequence similarities between the X chromo- 
some short arm and the Y chromosome long arm, 
which could be the basis for X/Y translocations en- 
countered clinically At least two genes (as well as a 
number of anonymous sequences) that map to 
the short arm of the X have pseudogenes on the 
Y chromosome that also permit observation of the 
processes by which genes, not under selective pres- 
sure, degenerate. More recent studies of a highly 
polymorphic family of sequences on the X chromo- 
some and a related group of nonpolymorphic se- 
quences on the Y chromosome should permit in- 
vestigation of the role of genetic recombination in 
generating certain kinds of polymorphism. 
II. Deletions of the Short Arm of the Human X 
Chromosome. 
Deficiency of STS enzymatic activity produces a 
visible cutaneous abnormality called ichthyosis. 
This common inborn error of metabolism in man 
has an X-linked pattern of inheritance and affects as 
many as 1 in 2,000 males. Most often the clinical 
abnormalities are limited to ichthyosis, but occa- 
sionally more complex phenotypes are observed. At 
least 90% of STS subjects have substantial deletions 
of the X chromosome short arm that may involve 
contiguous genes to produce the more complicated 
clinical situations. Frequent deletions also seem to 
occur at another X short arm site, the dystrophin 
locus, resulting in Duchenne muscular dystrophy. 
Dr. Shapiro and his colleagues have constructed a 
detailed, rare cutting enzyme restriction map of the 
distal X short arm, using pulsed-field gel electro- 
phoresis and cloning segments of this region as 
yeast artificial chromosomes. This information has 
been used to localize many of the breakpoints in 
these deletions. The deletions range from 40 kb to 
well over 1 Mb in length. Although the breakpoints 
are heterogeneous, they appear to cluster in a few 
regions. One deletion junction has been cloned 
and sequenced; no repetitive elements or second- 
ary structures are present at the breakpoint, but an 
8 bp direct repeat was introduced at the junction. 
Additional deletion breakpoints are being analyzed 
to gain further insight into the mechanisms that 
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