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What are the Genetics Behind Dyslexia?
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Wayne State University
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WHAT IS THE GENETICS BEHIND DYSLEXIA?
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Abstract
This paper discusses why the genetic level of dyslexia is important as well as what has been
found through past research. Dyslexia is very complicated when viewing it in a genetic lens. Its
origin is of neurological association and a lot of research has been done to locate the genes that
are responsible for this disorder. As of 2010 six candidate genes have been identified
(KIAA0319, DYX1C1, DCDC2, ROBO1, MRPL19 and C2ORF3). Therefore, a lot of research
has been focused on those genes to see if a deeper connection can be found. There’s also been a
discovery of primary cilium’s association with neuronal migration. There is also discussion of
dyslexia’s proof of genetic inheritance and its association with gender. Sandu, Specht, Beneventi,
Lundervold, and Hugdahl (2008) discovered that females were more severely affected by
dyslexia than males and many studies have been conducted on families to prove dyslexia’s
genetic inheritance (Smith et al., 1983; Svensson et al., 2010). Then there is also comorbidity
that exists in the majority of dyslexics, which can cause issues when trying to focus on just
dyslexia ( Scerri & Schulte-Korne, 2009). The paper concludes with the fact that more research
must be done in order to uncover dyslexia’s genetic origin.
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Many people have heard about dyslexia and most of those people know that it is a
reading disorder, but the complex genetics behind is unknown and is beginning to be uncovered.
Dyslexia is defined as “a specific learning disability that is neurobiological in origin” and it’s
characterized by difficulties with word recognition and some may have poor spelling or decoding
abilities (Lyon, S. Shaywitz, and B. Shaywitz, 2003, p. 2). About every 1 in 10 people are
dyslexic and many of these cases of dyslexia aren’t even known
(Dyslexia International, 2014). Discovering the genetic behavior of dyslexia could possibly lead
to a screening for young children to see their potential risks at getting dyslexia or maybe a drug
that could be subscribed to those with severe cases of dyslexia ( Scerri & Schulte-Korne, 2009).
The unveiling of the genetic aspect of dyslexia has the capability of better aiding the dyslexics in
our world as well as giving us a better understanding of its neurobiological origin, which could
then lead to the genetic unveiling of other learning disabilities.
I will be looking into the genetics behind dyslexia. It all begins with the realization that
genes have a relationship with dyslexia, which was done through research of generations of the
same family. This research was done due to the frequent reports of educators stating that
dyslexics had a strong positive family history (Smith, Kinberling, Pennington, & Lubs, 1983).
After that research moved towards analysis of the genes that were found to be possibly related to
dyslexia. This is currently the most important aspect of research due to dyslexia’s connection
with genetics, but other topics related to dyslexia are also being studied. These include things
like its gender association and comorbidity. Dyslexia has a very complex origin that is slowly
being revealed, although, research in areas outside of just genes themselves needs to be done to
fully understand how the genetics of dyslexia affect a human.
Dyslexia Candidate Genes
WHAT IS THE GENETICS BEHIND DYSLEXIA?
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Currently the main focus on dyslexic research revolves around candidate genes of
dyslexia. Candidate genes of dyslexia are the identified genes, through a fair amount of research,
that may possibly be associated with dyslexia and how it affects a person genetically. As of 2010
there are six candidate genes related to dyslexia: KIAA0319, DYX1C1, DCDC2, ROBO1,
MRPL19 and C2ORF3 (Svensson et al.). Below is just a discussion of a few of the candidate
genes.
KIAA0319
KIAA0319 is a candidate gene that lies on chromosome 6 (Cope et al., 2005). According
to Cope et al. (2005), the research they conducted displayed strong evidence of KIAA0319’s
association with dyslexia. Their data matches up with data from previous studies; therefore, the
evidence is adding up. In addition, Scerri et al. (2011) provided further support of KIAA0319’s
association with reading skills. Then KIAA0319 became a little more interesting, in 2013 a study
was conducted on in Indian population. It determined that KIAA0319 was expressed in different
parts of the brain and also that it has a role in neuronal migration and the “development of the
neocortex” (Venkatesh et al., 2013, p. 534). Its association of neuronal migration, like the
association of primary cilium discussed above, gives strong evidence to its association with
dyslexia because it has to do with how the neurons develop within you, mainly your brain. If
dysfunction within neuronal migration occurs, the brain affected is underdeveloped, like those
with learning disabilities like dyslexia. Therefore, KIAA0319 has a long line of evidence that
justifies its title of a candidate gene for dyslexia.
DCDC2
DCDC2 is one of a few candidate genes for dyslexia. DCDC2 has been researched with
relation to primary cilium, which will explained in further detail below, but other research has
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been done as well to corroborate its association to dyslexia. In a study performed by Wilke et al.
(2009), DCDC2 was looked at in 72 German dyslexics and the results display that DCDC2
played a role within the dyslexics. This also increases the thought of damaged neuronal
migration as a possible cause of dyslexia. Recent research also finds evidence of DCDC2’s
association of mathematics (Marino et al., 2010). There seems to be a pleiotropic effect of
DCDC2 within addition and subtraction of one digit numbers, and multiplication of “nuclear
families of developmental dyslexia” (Marino et al., 2010, p. 67). DCDC2 has evolving evidence
that strongly suggests that it is in some way associated with dyslexia.
DYX1C1
DYX1C1 is also considered a candidate gene for dyslexia (Svensson et al., 2010). One of
its earliest appearances was in 1983 in the study done by Smith, Kimberling, Pennington, and
Lubs. This research was done through linkage analysis of families. The authors determined that
there were probably many different etiologies to dyslexia, but due to the fact that many families
being affected by it, they concluded that the primary effect may be of genetic origin (Smith et al.,
1983). So they analyzed the linkage of families and found that a gene that was significantly
involved resided on chromosome 15. Eventually researches found that one particular area of
chromosome 15 seemed to be what was associated with dyslexia, DYX1C1. The research
continued on into present day determining that DYX1C1 was associated with reading and
spelling ability and it also had effects on short term memory (Bates et al., 2009). Once looking
into a study performed in 2010 I found that DYX1C1 was finally considered a candidate gene for
dyslexia.
The research done to even consider a genes association to dyslexia is extensive. It took
DYX1C1 27 years before it was finally named a candidate gene for dyslexia and even this title
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doesn’t necessarily mean that it has a definite role in dyslexia. It just means that there is a high
probability that these candidate genes are associated to dyslexia in some way.
Connection to Primary Cilium
Primary cilium is defined by Satir, Pedersen, and Christensen (2010) as “a solitary
organelle that emanates from the cell surface of most mammalian cell types during growth
arrest” (p. 499). There have been 3 hypotheses about its function that have existed, up until now.
Out of the three, 2 have been proven to be correct functions of primary cilium while one was
proven false by experimentation. The function that we are focusing on here is that the primary
cilium is a sensory structure. The reasoning behind our focus on this function is that the signaling
that occurs within the cilium “coordinates key processes during development”.
Dyslexia is due to the irregular development of the neurological system, so it would make
sense that due to the primary cilium’s key role in development, researchers are looking into more
detail about primary cilium and how it could associate with dyslexia. One study, conducted by
Massinen et al. (2011) examined the role of a candidate gene of dyslexia, DCDC2, and primary
cilium to see if there was an association of the gene, primary cilium, and dyslexia. The results of
the experiment displayed that yes, there was an association between primary cilium and DCDC2,
but this association was very small (Massinen et al., 2011). This relationship didn’t affect the
development of newborn neurons, which would have created a big relationship between the
primary cilium, DCDC2, and dyslexia. This led researchers to believe that there could be another
candidate gene of dyslexia that has a bigger association with primary cilium (Massinen et al.,
2011). Due to the primary cilium’s key role during development it has become an increasing
topic in the genetic world of dyslexia.
Genetic Inheritance
WHAT IS THE GENETICS BEHIND DYSLEXIA?
Dyslexia is “a highly heritable disorder” that is causing neurological dysfunction (Scerri
& Schulte-Korne, 2009). Due to its high level of heritability it is common to see generations of
families with this learning disability. In order to prove that dyslexia is genetically inherited,
many studies have been conducted within families to see where the dyslexia lies within the
family. Within the study conducted by Svensson et al. (2010) a large genomic study of six
generations of one family was conducted (p. 43). The study found that out of the 6 generations
35% of the family was considered dyslexic under their criteria (Svensson et al., 2010, p. 44).
Gender Association
The gender ratio of reading disabilities has recently been uncovered by Hawke et al.
(2009) and their discoveries state that reading difficulties is typically higher in males than
females. But if you’re looking at the severity of the reading disability that we are discussing,
dyslexia, Sandu et al. (2008) uncover that females are more severely impacted when it comes to
dyslexia. The gray and white matter of dyslexic boys and girls were compared to those of the
control or normal boys and girls (Sandu et al., 2008). The biggest significance was shown
between the normal girls and the dyslexic girls. The white matter volume of dyslexic girls was
significantly lower than that of non-dyslexic girls (Sandu et al., 2008). Therefore, dyslexia’s
impact on females is more severe than its impact on males.
Comorbidity
Comorbidity is when there is a presence of more than one disease or condition and
dyslexics are often not just dyslexic. In most cases, dyslexics experience comorbidity with other
learning disabilities like speech sound disorder (SSD) and language impairment (LI) (Raskind,
Peter, Richards, Eckert, & Berninger, 2012; Pennington & Bishop, 2009). The research done by
Pennington and Bishop (2009) displays evidence of overlapping within areas of analyzed
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etiology, cognition, and diagnostic, but due to the complexity of each disability and the little
knowledge we contain of each disability, it is difficult to pin point their relationship with one
another. Pennington and Bishop (2009) state that “new data are needed to better define their
relations” (p. 283). The co-occurrence of dyslexia and other learning disabilities is often what a
person experiences.
Conclusion
Dyslexia’s genetics has been looked at since the late 1900s and is slowly making
progress. The continuation of research on candidate genes is getting us closer to a better
understanding of dyslexia. The early discovery of its genetic inheritance through families has led
us to discovering candidate genes of dyslexia through examining the genetics of these families
(Smith et al., 1983). Discovering an association between DCDC2 and primary cilium has led us
to believe of the association of neuronal migration to dyslexia (Massinen et al., 2011). Although,
the areas that lack some research are that of comorbidity and gender association. A lot is being
done to look at what is specific genes are causing dyslexia, but more research needs to revolve
around how it is affecting a person. In most cases a person is not just dyslexic, there lie other
factors that need to be considered while uncovering the genetics of dyslexia. Why are females
more severely affect by dyslexia than males? How can we distinguish between dyslexia and the
other learning disabilities that often reside in one person? There is much more research that must
be done in order to determine dyslexia’s genetic origin.
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