The genetic material we inherit from our parents is organized into structures known as chromosomes. We have 22 chromosomal pairs, plus two X chromosomes (women) or an X and Y chromosome (men). Like beads on a string, each chromosome carries thousands of genes. Genes contain information; they are the blueprints for creating a human being. Each gene contains instructions for creating a single tiny element of a person. They are the fundamental units of heredity that allow specific qualities to be handed down from generation to generation. Some genes cause diseases, for example, if a person gets a gene for a certain type of muscular dystrophy, they will almost likely have that ailment later in life. Other genes are known as risk factor genes; they may not cause a person to get a specific ailment, such as diabetes, high blood pressure, or Alzheimer’s disease, but they can make it more or less likely. 

Familial Alzheimer's Disease (FAD)

One unusual form of Alzheimer’s disease is passed down through generations. This is referred to as Familial Alzheimer’s disease (FAD). Each child has a 50% risk of inheriting a faulty gene that causes FAD if one parent has it. The existence of the gene indicates that the person will get Alzheimer’s disease at some point, probably in their 40s or 50s. This type of Alzheimer’s disease affects a very tiny number of people. 

Three genes have been found that, when altered in specific ways, produce FAD. Presenilin 1 (chromosome 14), Presenilin 2 (chromosome 1), and the amyloid precursor protein gene on chromosome 21 are the names of these genes. 

According to studies by Weill Institute for Nuerosciences, less than 5% of Alzheimer’s disease is caused by a single genetic mutation that is transmitted through families. In these families, people usually show symptoms well before the age of 65 and symptoms sometimes begin as early as the 30s or 40s. This form of AD is called early-onset familial Alzheimer’s disease. 

How Common is Genetic Dementia?

Genetic dementia, also known as family or genetic types of dementia, is uncommon in comparison to sporadic dementia cases. Most dementia cases are not caused by genetic causes. However, the prevalence of hereditary dementia varies depending on the type of dementia under consideration.  

According to a study by the National Library of Medicine, In the case of frontotemporal dementia (FTD), which encompasses several different subtypes, it is estimated that about 10-30% of cases have a genetic component.  

It is crucial to note that these prevalence estimates can differ based on the population investigated and the diagnostic criteria utilized. Advances in genetic testing and research continue to improve our understanding of the hereditary basis of dementia. 

How can we Determine Familial Alzheimer's Disease in an Individual?

Genetic testing is a form of medical examination that examines your DNA for alterations. DNA is an abbreviation for deoxyribonucleic acid. It includes all living organisms’ genetic instructions.  

The decision to have FAD testing has been quite complicated, and the benefits and drawbacks must be carefully evaluated. The test does not provide a relative risk of dementia, but it does indicate whether a person will develop a significant and progressive illness in their middle years. The test can only be done with the person being tester’s informed consent. Nobody should ever be forced to take such a test. 

In a patient who already exhibits symptoms, genetic tests can be done to confirm the presence of an FAD gene. Testing can also indicate who may become ill in the future. Knowing you have the gene could help some people plan. It allows children to contemplate future lifestyle options and express their desires to someone they trust. However, because there is no cure, an individual must decide whether they want to know that they will get dementia at some point in the future. 

Doctors may recommend DNA testing in cases when a young or middle-aged person has symptoms of Alzheimer’s disease and a family history of early onset AD. Typically, doctors will only recommend this if there is a family history of Alzheimer’s disease. That person contributes blood, which is then analyzed for a mutation in the presenilin 1, presenilin 2, or APP genes. If these tests come out negative, the possibility of a genetic aetiology of Alzheimer’s disease in the family remains unaddressed. If they are positive, the doctor will be able to identify the mutation that causes AD in the family. 

Specialized genetic counselling is required to assist people in considering these issues. The doctor can provide more information about this service. When preventive therapies for Alzheimer’s disease become accessible, there may be more reasons to seek testing in the future. 

This type of testing is mostly utilized in research settings to identify study participants who may be at a higher risk of getting Alzheimer’s. This method enables scientists to detect early brain alterations and assess the efficacy of potential treatments for patients with varied APOE levels. 

What are some other Rare Forms of Inherited Dementia?

Other rare kinds of dementia can be inherited as well. These include Huntington’s illness and certain kinds of Fronto Temporal Lobar Degeneration, in which conduct changes before memory changes. All these inherited illnesses are quite rare in the general population. 

Huntington's Disease

It is a rare, genetic condition that involves the progressive breakdown (degeneration) of brain nerve cells. Huntington’s disease affects a person’s functional abilities in a variety of ways, frequently resulting in movement, thinking (cognitive), and psychological issues. Huntington’s disease symptoms can arise at any age, but they are most common in adults in their 30s or 40s. If the disorder appears before the age of 20, it is referred to as juvenile Huntington’s disease. When Huntington’s disease occurs early, the symptoms are somewhat different, and the condition may advance more quickly. There are medications available to aid with the symptoms of Huntington’s disease. However, medications cannot prevent the condition’s physical, mental, and behavioral degeneration.

Frontotemporal Lobar Degeneration (FTLD)

Frontotemporal lobar degeneration (FTLD) is a clinical and pathologically diverse condition characterized by gradual impairment in behavior or language linked with frontal and anterior temporal lobe degeneration. While the first cases were described at the beginning of the twentieth century, FTLD has only recently been recognized as a prominent cause of dementia, particularly in patients under the age of 65. FTLD is a clinically and pathologically diverse condition marked by a progressive decrease in conduct or language linked with frontal and anterior temporal lobe degeneration. 

Creutzfeldt-Jakob Disease

 Creutzfeldt-Jakob illness is a rare brain ailment that causes dementia. It is one of several human and animal diseases known as prion disorders. CJD is a rare and fatal neurological condition caused by prion proteins that are aberrant. Creutzfeldt-Jakob disease symptoms can be comparable to those of Alzheimer’s disease. However, Creutzfeldt-Jakob disease normally progresses faster and results in death. While the majority of CJD cases are sporadic, meaning they occur on their own, roughly 5-10% of cases are inherited. Mutations in the prion protein (PRNP) gene can cause the disease to run in families.   

Niemann-Pick Disease Type C

NPC is a slowly progressing lysosomal condition with age-dependent symptoms. The perinatal and infancy signs are primarily visceral, with hepatosplenomegaly, jaundice, and (in some cases) pulmonary infiltrations. Neurologic symptoms dominate the presentation from late infancy forward. The youngest children may have hypotonia and developmental delay, followed by ataxia, dysarthria, dysphagia, and, in certain cases, epileptic seizures, dystonia, and gelastic cataplexy. Although cognitive decline may be modest at first, affected individuals eventually develop progressive dementia. NPC is a rare hereditary illness characterized by aberrant lipid buildup within cells. It can result in neurological signs such as developing dementia. Mutations in the NPC1 or NPC2 genes cause NPC. 


These are only a few examples of rare inherited dementia. Each illness has its own genetic foundation, symptoms, and progression. Individuals with a family history of dementia may benefit from genetic counselling and testing to better understand the risks and make educated decisions. 

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