to DNA by its homeodomain. Although it is possible 
that these mutations could permit a partially func- 
tional protein to be made, it is more likely that they 
result in a complete loss of function. These results 
prove that mutations in PAX6 account for aniridia, 
and that aniridia is the human counterpart to the 
Small eye mutation in mice since independently, it 
has been shown that the mouse Pax- 6 gene is mu- 
tated in Small eye mice. Surprisingly, Dr. Maas and 
his colleagues have not detected mutations in six 
other patients; experiments are under v^ay to deter- 
mine if these patients have mutations in the PAX6 
gene that were undetected by SSCP. Experiments are 
also under way to determine whether the PAX6 
gene product functions in inductive processes in 
the developing eye and, if so, how it functions. 
Role of Hox Genes in Nephrogenesis 
Previous experiments indicate that the ureteric 
bud must form properly in order for nephrogenesis 
to occur, since mutants such as the limb deformity 
mouse, in which its outgrowth is retarded, manifest 
a renal agenesis phenotype. Dr. Maas's laboratory 
has characterized a novel Hox gene that is expressed 
in the developing mouse kidney and has been provi- 
sionally identified as Hox- 1.8. Analysis of cDNA 
clones for this gene has revealed an unexpectedly 
complex number of different transcripts that are 
related to each other by alternative splicing. This 
gene is expressed in the periureteral mesenchyme 
during mouse kidney development. Studies are un- 
der way to determine the function of Hox- 1.8 in 
nephrogenesis. 
Dr. Maas is also Assistant Professor of Medicine 
in the Division of Genetics at Brigham and 
Women's Hospital and Harvard Medical School, 
and Associate Physician at Brigham and Women 's 
Hospital, Boston. 
CELL CYCLE-REGULATED PROTEIN KINASES 
James L. Maller, Ph.D., Investigator 
Two events mark the reproductive life of a cell: 
replication of genomic DNA in S phase and distribu- 
tion of that replicated DNA to daughter cells at mito- 
sis or M phase. Dr. Maller is interested in the molecu- 
lar mechanisms involved in the decision to initiate 
mitosis, as well as the control of discrete events in M 
phase itself. The initiation of mitosis is signaled by 
activation of the protein kinase activity of the cdc2 
gene product. Prior to this activation the cell must 
pass a checkpoint in G2 phase whose main function 
is to ensure that cdc2 kinase is not activated until 
DNA replication is complete or damaged DNA is re- 
paired. In the past year substantial progress has been 
made in unraveling the biochemistry of this G2 
checkpoint. In addition, new insight into the mecha- 
nism of metaphase arrest at meiosis II has been 
gained. 
Regulation of the Xenopus cdc25 Phosphatase 
Previous work in this and other laboratories has 
shown that activation of cdc2 kinase occurs via de- 
phosphorylation of tyrosine 1 5 , an event mediated 
in Schizosaccharomyces pombe by the cdc25 
phosphatase. Overexpression of cdc25 in 5. pombe 
leads to entry into mitosis, even with incompletely 
replicated DNA, implying that cdc25 is involved in 
G2 checkpoint control. To study cdc25 in a higher 
eukaryote. Dr. Tetsuro Izumi in this laboratory 
cloned and sequenced the Xenopus cdc25 phos- 
phatase and studied its regulation in egg extracts 
that carry out both DNA synthesis and mitosis in vi- 
tro. Initial experiments showed that the amount of 
cdc25 protein does not change during the cell cy- 
cle, but there is a marked retardation in electropho- 
retic mobility on gels in both meiotic and mitotic M 
phases. Subsequent work showed the shift is due to 
periodic phosphorylation and, moreover, the phos- 
phorylated form is considerably more active in de- 
phosphorylating cdc2 kinase and elevating its pro- 
tein kinase activity. This important work identified 
biochemically two additional elements in the G2 re- 
striction point: the protein kinase and protein phos- 
phatase that control the activity of the cdc25 phos- 
phatase. In vitro studies showed that either protein 
phosphatase 1 (PPl) or 2A could dephosphorylate 
and deactivate cdc25. Analysis of PPl activity dur- 
ing the cell cycle revealed periodic oscillations: ac- 
tivity is high in M phase and S phase but low just 
prior to cdc2 activation and during exit from mito- 
sis. The existence of two peaks of PPl activity dur- 
ing the cell cycle is consistent with previous genetic 
work in yeast, in which altered expression of PPl 
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