Background Information

A Healthy Baby Nerve Cell

Tay-Sachs Disease

Protein Structure

Introduction

Background Information

Question

Methods of Research

Data

Conclusions

More Questions

Introduction

There are millions of diseases in the world today that take the lives of countless numbers of people. One of these fatal diseases is Tay-Sachs (TSD), which kills many children. Although rare, it is ruthless. Currently, despite the enormous efforts being made by scientists, there remains no cure. Children who inherit this disease usually die by the age of five. Symptoms do not begin to show until the child is several months old. From then on, the nervous

Gene Therapy

system rapidly deteriorates, development slows, and soon the child can no longer crawl or even sit. By the time they are three or four, the nervous system is so badly affected that it can no longer support life. We decided to research this disease more thoroughly due to its mysterious nature. We were inspired to comprehend what made this disease so merciless, hoping to eventually arrive at our own conclusions about how it can be controlled.

Background Information

What is TSD? Symptoms Other Forms of TSD Populations at Risk Are There Any Cures?

What is TSD? Tay Sachs disease was first described by Warren Tay in 1881 and Bernard Sachs in 1887, hence receiving its name "Tay-Sachs". Tay-Sachs disease is a heritable, autosomal recessive, metabolic disorder. For example, if both parents are carriers of this disease, then there is a 25% chance that their child will be born with TSD. The enzyme hexosaminidase A (HEXA) is important in processing the fatty substance ganglioside (GM2) found in nerve cells. Patients with Tay-Sachs have a lethal genetic mutation on their HEXA gene, which is located on chromosome 15. Therefore, since the enzyme HEXA produced in the nerve cells' lysosomes is no longer functional, the nerve cells are now unable to process the GM2 they store. Consequently, fat accumulates, causing progressive damage as the cells expand, pressing against each other. The most harm occurs in the brain, where there is a large number of neurons. Usually, patients diagnosed with TSD die within several years.
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Fat Cells in Adipose Tissue

Human Brain

Symptoms Children with the infantile onset of Tay-Sachs, the classic form, develop normally until around the age of six months. Then, as the first signs of the disease appear, mental and physical abilities decline. Symptoms may include blindness, deafness, listlessness, and exaggerated startle responses. By the age of two years, the problems are significantly aggravated. The infants appear to have regressed in their growth. They lose basic skills and eventually become even unable to swallow. They may also experience seizures, mental retardation, loss of muscle tone, and paralysis. Ultimately, the central nervous system is so destroyed that life ceases to exist.
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Other Forms of TSD Other forms of Tay-Sachs disease also exist. They include late infantile, juvenile, and adult onset. Basically, patients with these types of TSD have a lower than normal amount of HEXA in their blood. The nearer to the normal level of HEXA they are, the better chance they have to survive. Usually, people with higher levels tend to develop the disease much later than with the classic onset. However, when the symptoms begin, the symptoms are similar.
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Populations at Risk For the general population in the U.S., the incidence of carrying the Tay-Sachs gene is 1:250. Certain populations are especially at risk for Tay-Sachs disorder. One of these is the Ashkenazi (eastern European) Jewish group. Almost 1:27 Jews are carriers of TSD in the United States. For the Cajun community of Louisiana as well as the non-Jewish French Canadians living near the St. Lawrence River, the carrier rate is higher than that of the general U.S. population.
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Are There Any Cures? For the moment, there are no cures for TSD. Presently, over 100 mutations of Tay-Sachs have been discovered, making it very difficult to research. For right now, the best way to control this neurological disorder and prevent the tragedy before it occurs is to review the genetic history of the family and take genetic tests. During the 1970s, a huge government screening program reduced the number of babies dying of TSD from 50-100 to less than 15. However, bioinformatics presents a hopeful future.

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Questions

By charting where the mutations occur, can we determine which areas of the gene are vital to its normal functioning? Are mutations concentrated in certain areas of the gene or scattered throughout? Which amino acid changes are more prevalent in the mutant forms of HEXA? Do different cultural groups have different mutations?

Methods of Research

During this group project, we used various sites on the Internet to obtain information about our topic. These included the National Center for Biotechnology Information (NCBI), the Biology Workbench, the Cold Spring Harbor database, and the HEXA database. The information we were able to gather consisted of background information about the illness, which enabled us to have a better understanding of the mutations that cause the malfunction of the enzyme. By using the Cold Spring Harbor database and the Biology Workbench, we found the sequence of the normal HEXA gene. Using McGill University's HEXA database, we found the locations of the mutations as well as which cultural groups the mutations affected.

Data

We aimed our project at discovering different areas of the HEXA protein that were vital to the folding of the enzyme. After identifying different mutations in the protein that cause Tay-Sachs disease, we decided to focus on missense or point mutations because they are most prevalent. Since Tay-Sachs is a result of a structural change in HEXA, a mutation that causes Tay-Sachs must affect the folding of the protein. By creating a bar graph of the mutations, we were able to determine regions that seemed to be crucial because there was a greater number of mutations in that specific region. As shown on Graph 1, there are areas dense with mutations. Also there were two regions that were void of mutations. (See Graph 1 for more detail.)

Then, we attempted to determine why the structure of the protein was altered when the amino acid changed by comparing the chemical structure, solubility, and surface area of the original amino acid with the mutated amino acid. (See Chart 1 for data)

We noticed that argenine seemed to change more often than any of the other amino acids. So, we decided to determine the percentage of change per amino acid in order to compare them. (See Graph 2 and Graph 3 for our results). Analyzing the data showed that argenine and methionine had the highest percentages of missense mutations. There are only 7 methionines in normal HEXA compared to 25 argenines. There are 2 mutations of methionine compared to 7 mutations of argenine. Both resulted in similar percentages - approximately 28%.

In order to compare mutations of different cultural groups, we examined Chart 1 to see if there were relationships between mutations and cultural groups. We concentrated on the location of the mutation and its type.

Chart 1: Structural Changes of Missense Mutations in HEXA Enzyme

Amino Acid #	Mutation	Cultural Group Affected	Onset	Chemical Structure Change	Original Solubility	Mutated Solubility	Solubility Difference (g/100g @ 25C)	Original Surface Area	Mutatant Surface Area	Surface Area Difference
1	Met-Leu	Ukrainian	--	none	3.381	2.426	0.955	185	170	15
1	Met-Val	Black American	Infantile	none	3.381	8.85	-5.469	185	155	30
1	Met-Thr	English	Infantile	Hydrophobic - Hydrophilic	3.381	very	3.381-very	185	140	45
25	Pro-Ser	English	Late Infantile	Hydrophobic - Hydrophilic	162.3	5.023	157.277	145	115	30
39	Leu-Arg	Polish	Infantile	Hydrophobic - Basic	2.426	15	-12.574	170	225	-55
127	Leu-Phe	Hispanic-German- Jewish / Spanish- Arab	Infantile	none	2.426	2.965	-0.539	170	210	-40
127	Leu-Arg	Italian	Infantile	Hydrophobic - Basic	2.426	15	-12.574	170	225	-55
166	Arg-Gly	Syrian	Late Infantile	Basic - Hydrophobic	15	24.99	-9.99	225	75	150
170	Arg-Trp	French Canadian / Italian	Infantile	Basic - Hydrophobic	15	1.136	13.864	225	255	-30
170	Arg-Gln	Japanese / Moroccan-Jewish / Scottish	Infantile	Basic - Acid	15	0.864	14.136	225	180	45
178	Arg-Cys	Czechoslovakian	Infantile	Basic - Hydrophilic	15	very	15-very	225	135	90
178	Arg-His	Wide Distribution	Late Infantile / Juvenile	none	15	4.19	10.81	225	195	30
178	Arg-Leu	English	Infantile	Basic - Hydrophobic	15	2.426	12.574	225	170	55
180	Tyr-His	Ashkenazi Jewish	Adult	Hydrophilic - Basic	0.0453	4.19	-4.1447	230	195	35
192	Val-Leu	German / Romanian / English / Irish	Infantile	none	8.85	2.426	6.424	155	170	-15
196	Asn-Ser	French Canadian	?	Acidic - Hydrophilic	0.778	5.023	-4.245	160	115	45
197	Lys-Thr	Dutch	Adult	Basic - Hydrophilic	very	very	very-very	200	140	60
200	Val-Met	German / Romanian / English / Irish	--	none	8.85	3.381	5.469	155	185	-30
204	His-Arg	German	Infantile	none	4.19	15	-10.81	195	225	-30
210	Ser-Phe	North African	Infantile	Hydrophilic - Hydrophobic	5.023	2.965	2.058	115	210	-95
211	Phe-Ser	Italian	Infantile	Hydrophobic - Hydrophilic	2.965	5.023	-2.058	210	115	95
226	Ser-Phe	Azorean	?	Hydrophilic - Hydrophobic	5.023	2.965	2.058	115	210	-95
247	Arg-Trp	Wide Distribution	--	Basic - Hydrophobic	15	1.136	13.864	225	255	-30
249	Arg-Trp	French Canadian	--	Basic - Hydrophobic	15	1.136	13.864	225	255	-30
250	Gly-Ser	French Canadian	--	Hydrophobic - Hydrophilic	24.99	5.023	19.967	75	115	-40
250	Gly-Asp	Lebonese Maronite	Juvenile	Hydrophobic - Hydrophilic	24.99	3.53	21.46	75	150	-75
252	Asp-His	Portuguese	Adult	Hydrophilic - Basic	3.53	4.19	-0.66	150	195	30
258	Asp-His	Scottish / Irish	Infantile	Hydrophilic - Basic	3.53	4.19	-0.66	150	195	-45
269	Gly-Ser	French Canadian	Infantile	Hydrophobic - Hydrophilic	24.99	5.023	19.967	75	115	-40
269	Gly-Asp	---	Infantile	Hydrophobic - Hydrophilic	24.99	3.53	21.46	75	150	-75
279	Ser-Pro	Druze	Late Infantile	Hydrophilic - Hydrophobic	5.023	162.3	-157.277	115	145	-30
301	Met-Arg	Yugoslav	Infantile	Hydrophobic - Basic	3.381	15	-11.619	185	225	60
314	Asp-Val	Ashkenazi Jewish / Irish-English	?	Hydrophilic - Hydrophobic	3.53	8.85	-5.32	150	155	-5
335	Ile-Phe	American	?	none	4.117	2.965	1.152	175	210	-40
388	Ile-Met	French Canadian	?	none	4.117	3.381	0.736	175	185	-10
391	Val-Met	Greek	Adult	none	8.85	3.381	5.469	155	185	-30
399	Asn-Asp	Black American	--	Acidic - Hydrophilic	0.778	3.53	-2.752	160	150	10
420	Trp-Cys	Irish / German	Infantile	Hydrophobic - Hydrophilic	1.136	very	1.136-very	255	135	120
436	Val-Ile	Black American	--	none	8.85	4.117	4.733	155	175	-20
454	Gly-Ser	Italian	Infantile	Hydrophobic - Hydrophilic	24.99	5.023	19.967	75	115	-40
455	Gly-Arg	Portugal	Late Infantile	Hydrophobic - Basic	24.99	15	9.99	75	225	-150
458	Cys-Tyr	Japanese	Infantile	none	very	0.0453	very-.0453	135	230	-95
474	Trp-Cys	German-Dutch	Juvenile	Hydrophobic - Hydrophilic	1.136	very	1.136-very	255	135	120
482	Glu-Lys	Italian / Chinese / Moroccan-Jewish	Infantile	Acidic - Basic	2.5	very	2.5-very	190	200	-10
484	Leu-Pro	Japanese	Infantile	none	2.426	162.3	-166.016	170	145	25
485	Trp-Arg	Chinese	Infantile	Hydrophobic - Basic	1.136	15	-13.864	255	225	30
499	Arg-Cys	Slavic / Irish / English / Polish	Infantile	Basic - Hydrophilic	15	very	15-very	225	135	90
499	Arg-His	Jewish / Scottish-Irish	Juvenile	none	15	4.19	10.81	225	195	30
504	Arg-Cys	German / French / Algerian	Infantile	Basic - Hydrophilic	15	very	15-very	225	135	90
504	Arg-His	Wide Distribution	Juvenile	none	15	4.19	10.81	225	195	30

Graph 2

Graph 3

Conclusions

By examining Graph 1, it was clear that the mutations of HEXA were more concentrated in certain areas, suggesting that those areas were more vital to the proper functioning of the HEXA protein.
Other amino acids have far lower percentages of missense mutations than argenine or methionine, despite the fact that some of the other amino acids have more occurrences in normal HEXA.
Through examining the data collected on mutations and cultural groups, we were unable to find any trends or patterns among cultural groups.

More Questions

? Why do mutations so often cause changes in methionine and argenine?

? Efforts to control Tay Sachs have been made by injecting the HEXA protein into the blood, which would deliver it to the brain. However, there is a blood-brain barrier, which prevents HEXA from entering the brain. Is there any other possible way that we can get HEXA into the brain?

?It has been found that mice with the mutant HEXA gene are still able to produce HEXA. How does this happen? If it is a gene that allows this, do humans have the gene?

? What makes different cultures more susceptible to this disease?

? Are there evolutionary commonalties between Jews, French Canadians, and Cajuns, who are the leading carriers of this disease?

The Fiji's

Shuyan Huang, Megan Campbell, Courtney Turner, Cindy Hodakoski

References

HEXA database of Cold Springs http://vector.cshl.org/html/sequences/sequences.html

Biology workbench http://workbench.sdsc.edu

NCBI http://www.ncbi.nlm.nih.gov

http://www.cryst.bbk.ac.uk/PPS2/course/section2/SideChains/primary4.html

McGill University's HEXA Database http://data.mch.mcgill.ca/cgi-bin/hexadb/hexadb_mutQ.cgi?field=id_mut&value=

Malacinski, George M. and David Freifelder. Essentials of Molecular Biology, 3rd edition. Jones and Barrlett Publishers; Boston; 1998.

National Tay-Sachs & Allied Diseases Association, Inc. http://www.ntsad.org/ntsad/t-sachs.htm