Deck 20: Developmental Genetics

Question 1

One foot or another
In humans the HOXD homeotic gene cluster plays a critical role in limb development. In one large family, 16 of 36 members expressed one of two dominantly inherited malformations of the feet known as rocker bottom foot (CVT) or claw foot (CMT). One individual had one foot with CVT and the other with CMT. Genomic analysis identified a single missense mutation in the HOXD10 gene, resulting in a single amino acid substitution in the homeodomain of the encoded transcription factor. This region is crucial for making contact and binding to the target genes controlled by this protein. All family members with the foot malformations were heterozygotes; all unaffected members were homozygous for the normal allele.
Given that affected heterozygotes carry one normal allele of the HOXD10 gene, how might a dominant mutation in a gene encoding a transcription factor lead to a developmental malformation?

Answer 1

Congenital Vertical talus (CVT) commonly known as rocker bottom foot deformity is characterised by the dislocation of talonavicular joint with rigidity in navicular over the neck of the talus. This disease is noticed since birth although do not pain or complication seen in childhood but in later life can result in discomfort. This is an autosomal dominant disorder which follows Mendelian pattern of inheritance and incomplete penetrance. This is associated with other abnormalities like neuromuscular syndrome and myelomeningocele or aneuploidy. There are cases which shows occurrence of diseases in offspring born to clinically normal parents. This shows the involvement of single gene.
The other Charcot-Marie Tooth disease (CMT ) also called as Claw foot, is disease with foot abnormalities that typically develop a high arched claw foot look in later life. This is also has an autosomal dominant mode of inheritance with X-linked features sometimes rare autosomal recessive modes of inheritance can occur.
We all know that Homeotic gene cluster ( HOX gene) play important role in the formation and placement of limbs and other body parts. The expression of HOX genes in sequential manner places the body organs along anterior to posterior axis. Both the diseases i.e. CVT and CMT occur due to mutation in one the HOX gene that has been identified as a missense mutation in the HOXD10 gene. Both dieses are heterogenetic in nature.
HOX genes have a conserved 180bp region that encodes for DNA-binding sequence of 60 amino acids. Genetic studies have revealed a single missense mutation, M319K in HOXD10 gene, this mutation was found to be fully penetrant and responsible for both CVT and CMT in heterozygotes.
HOXD cluster contains, HOXD10 as a member while the other members are HOXD13 , HOXD12 , HOXD11 , and HOXD9. Sequencing of these genes has revealed a mutation of a single codon. A single nucleotide Thymine??? -adenine transversion at position 956 in exon 2 of HOXD10 results in the missense mutation at codon 319 (M319K) that codes for lysine (AAG) instead of methionine (ATG). All individual who shows CVT or CMT are Heterozygous for M319K while this codon is absent in normal members. This is how a single mutation results in two developmental disorders.

Question 2

In this chapter, we have focused on large-scale as well as the inter- and intracellular events that take place during embryogenesis and the formation of adult structures. In particular, we discussed how the adult body plan is laid down by a cascade of gene expression, and the role of cell-cell communication in development. Based on your knowledge of these topics, answer several fundamental questions:
(a) How do we know how many genes control development in an organism like Drosophila ?
(b) What experimental evidence demonstrates that molecular gradients in the egg control development?
(c) How did we discover that selector genes specify which adult structures will be formed by body segments?
(d) How did we learn about the levels of gene regulation involved in vulval development in C. elegans ?

Answer 2

(a)The mechanism of mutations allowed scientists to study the normal function and structure of the genes. Mutant screens have been used to determine the components involved in biological processes, such as development. Saturation mutagenesis can identify almost all possible mutations of a particular gene, which can help us to define the lower limit of genes involves in a particular developmental processes in any organism like Drosophila.(b)The experimental evidence on mutational studies has provided insight on gradient control of development. Mutations affecting molecular gradients disrupt normal egg development, allowing scientists to deduce the role of gradients in developmental control. However, fluorescent or radioactive molecular labeling has provided clear evidence of molecular gradients in cells.
(c)The homeotic selector genes are clusters of genes that influence specification of body part through interactions with other genes. The homeotic genes encode homeodomain proteins that interact with Hox genes along with other genes. For example, replacing homeotic genes of antennae with legs development gene would develop legs in place of antennae on the head. So, identifying homeodomain proteins and Hox genes, one can discover homeotic selector genes.
(d)Mutation analysis of the Notch and anchor cell signaling pathways has provided evidence of cell-cell signaling in developmental control. For example, vulva development in Caenorhabditis elegans involves two developmentally equivalent cells that interact to form two different structures. Mutations in signaling and receptor genes for these cells have demonstrated cell-cell signaling between the cells is required for a particular cellular fate.

Question 3

One foot or another
In humans the HOXD homeotic gene cluster plays a critical role in limb development. In one large family, 16 of 36 members expressed one of two dominantly inherited malformations of the feet known as rocker bottom foot (CVT) or claw foot (CMT). One individual had one foot with CVT and the other with CMT. Genomic analysis identified a single missense mutation in the HOXD10 gene, resulting in a single amino acid substitution in the homeodomain of the encoded transcription factor. This region is crucial for making contact and binding to the target genes controlled by this protein. All family members with the foot malformations were heterozygotes; all unaffected members were homozygous for the normal allele.
How can two clinically different disorders result from the same mutation?

Answer 3

A single mutation in HOXD10 gene results in the occurrence of two clinically different abnormalities related to foot. This gene is responsible for the normal limb formation in humans. Hox genes codes for transcription factors which leads to specific expression during embryogenesis.
HOXD11 , HOXD10 , HOXD13 and HOXA11 genes play important roles for forming limb buds during developmental stage. These genes are expressed in collinear manner that means most anterior genes such as HOXD11and HOXD10 will be expressed first followed by the expression of other posterior genes. Also more anterior genes are expressed at the anterior end whereas posterior genes like HOXD13 will be expressed only towards posterior end. Some reports clarify that genes at the posterior end are more expressive than the anterior ones and in a way they tend to control them.
Reports suggest that all individual carrying these disorders are heterozygous for the mutated gene where normal ones were recessive for normal allele. This shows that if the mutated gene has only one allele for diseases at any of the gene then it will be expressed. It may be possible that both the alleles for mutated gene are heterozygous of mutation in that case both abnormalities occur simultaneously. The only condition of being normal is to be homozygous recessive for the mutated allele. However the occurrence of CVT in one foot and CMT in another in same individual indicates that there could be same factors important during foot development that are random that related with the genes exclusively.

Question 4

Review the chapter concepts list on page 403. Most of these ate concerned with the cascade of gene transcription that convers a zygote into an adult organism. Write a short essay outlining the differences and similarities in the gene families used by plants and animals to establish the body axis and to regulate gene expression of these gene sets.

Question 5

One foot or another
In humans the HOXD homeotic gene cluster plays a critical role in limb development. In one large family, 16 of 36 members expressed one of two dominantly inherited malformations of the feet known as rocker bottom foot (CVT) or claw foot (CMT). One individual had one foot with CVT and the other with CMT. Genomic analysis identified a single missense mutation in the HOXD10 gene, resulting in a single amino acid substitution in the homeodomain of the encoded transcription factor. This region is crucial for making contact and binding to the target genes controlled by this protein. All family members with the foot malformations were heterozygotes; all unaffected members were homozygous for the normal allele.
What might we learn about the control of developmental processes from an understanding of how this mutation works?

Question 6

Nuclei from almost any source may be injected into Xenopus oocytes. Studies have shown that these nuclei remain active in transcription and translation. How can such an experimental system be useful in developmental genetic studies?

Question 7

Distinguish between the syncytial blastoderm stage and the cellular blastoderm stage in Drosophila embryogenesis.

Question 8

(a) What are maternal-effect genes? (b) When are gene products from these genes made, and where are they located? (c) What aspects of development do maternal-effect genes control? (d) What is the phenotype of maternal-effect mutations?

Question 9

(a) What are zygotic genes, and when are their gene products made? (b) What is the phenotype associated with zygotic gene mutations? (c) Does the maternal genotype contain zygotic genes?

Question 10

List the main classes of zygotic genes. What is the function of each class of these genes?

Question 11

Experiments have shown that any nuclei placed in the polar cytoplasm at the posterior pole of the Drosophila egg will differentiate into germ cells. If polar cytoplasm is transplanted into the anterior end of the egg just after fertilization, what will happen to nuclei that migrate into this cytoplasm at the anterior pole?

Question 12

How can you determine whether a particular gene is being transcribed in different cell types?

Question 13

You observe that a particular gene is being transcribed during development. How can you tell whether the expression of this gene is under transcriptional or translational control?

Question 14

What are Hox genes? What properties do they have in common? Are all homeotic genes Hox genes?

Question 15

The homeotic mutation Antennapedia causes mutant Drosophila to have legs in place of antennae and is a dominant gain-of-function mutation. What are the properties of such mutations? How does the Antennapedia gene change antennae into legs?

Question 16

The Drosophila homeotic mutation spineless aristapedia ( ss a ) results in the formation of a miniature tarsal structure (normally part of the leg) on the end of the antenna. What insight is provided by ( ss a ) concerning the role of genes during determination?

Question 17

A number of genes that control expression of Hox genes in Dro­ sophila have been identified. One of these homozygous mutants is extra sex combs, where some of the head and all of the thorax and abdominal segments develop as the last abdominal segment. In other words, all affected segments develop as posterior segments. What does this phenotype tell you about which set of Hox genes is controlled by the extra sex combs gene?

Question 18

In Arabidopsis, flower development is controlled by sets of homeotic genes. How many classes of these genes are there, and what structures are formed by their individual and combined expression?

Question 19

The floral homeotic genes of Arabidopsis belong to the MADS-box gene family, while in Drosophila, homeotic genes belong to the homeobox gene family. In both Arabidopsis and Drosophila, members of the Polycomb gene family control expression of these divergent homeotic genes. How do Polycomb genes control expression of two very different sets of homeotic genes?

Question 20

Dominguez et al. (2004) suggest that by studying genes that determine growth and tissue specification in the eye of Drosophila, much can be learned about human eye development.
(a) What evidence suggests that genetic eye determinants in Drosophila are also found in humans? Include a discussion of orthologous genes in your answer.
(b) What evidence indicates that the eyeless gene is part of a developmental network?
(c) Are genetic networks likely to specify developmental processes in general? Explain fully and provide an example.