Familial risk of autism spectrum disorder

Familial risk of autism spectrum disorder

What is the issue?

Familial risk (or ‘heritability’) refers to the extent to which a trait or condition is genetically
determined, as opposed to being triggered by environmental factors. Given that no
single ‘cause’ for autism spectrum disorder (ASD) has yet been pinpointed, the role of
genetics in the development of these conditions remains a matter of some debate.

This issue may be of concern to adults who have a diagnosis ASD and who want to
understand the likelihood of their children ‘inheriting’ the condition. It may also be
significant for parents who already have a child on the autism spectrum and who are
unsure whether any subsequent children they have may likewise be born with ASD.

What does the research say?

One of the most reliable methods for assessing the heritability of a condition is the twin
study. To conduct a twin study, researchers must gather a sample group of identical
twins and another of non-identical (fraternal) twins. They will then record whether a ASD
is present in one, both or neither of the twins in each sibling pair.

Because identical twins have exactly the same genetic profile, any genetically
determined condition that affects one twin is almost certain to affect the other as well – in
other words, it will have a close to 100 per cent concordance rate. On the other hand,
non-identical twins have only around half of their genetic material in common. There is
therefore a much smaller chance that both non-identical twins in a pair will inherit a
genetically determined condition.

Twin studies enable researchers to make judgements about the role of environmental
factors in determining developmental outcomes. Since most pairs of twins grow up in the
same household they are likely to share very similar experiences relating to the home,
family life and upbringing. For example, they will probably be exposed to the same
parenting styles, play opportunities, family activities, and schooling. If ASD could be
traced solely to external factors such as these (and genetics played no part at all) then
we would expect to see similar concordance rates for the condition between both
identical and non-identical twins.

Researchers have found that if one of a pair of identical twins meets the diagnostic
criteria for ASD there is approximately a 90 per cent chance that the second twin will
also have the disorder. In contrast, the concordance rate for ASD between fraternal twin
siblings seems to around 10 per cent (Bailey et al., 1995).

The idea that ASD is, at least in part, genetically determined has been further supported
by research indicating that ASD is often present in multiple cases within extended
families (Silverman et al., 2002; Lauritsen et al., 2005). Additionally, it has been found

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that individual ASD traits, such as obsessive behaviour, communication deficits and
impaired social interaction, are highly heritable (Ronald et al., 2005; Zhao et al., 2007).

Research into families who have members with ASD have shown that:

• There are some families where only one member has a diagnosis of ASD. These
'one-off' incidences are referred to as 'simplex' ASD. Genetics research suggests
that some of these cases may be due to isolated, spontaneous changes to DNA
sequences that occur at the cell formation stage. It is believed that these rare
mutations account for around one in ten of all people diagnosed with ASD (Sebat
et al., 2007).

• On the other hand, there are many families in which more than one individual has
an ASD diagnosis, and/or in which several members have strong ASD traits.
These are referred to as 'multiplex' families. It is thought that there are specific
genetic variations, passed down through generations, which might underlie the
increased familial risk for ASD in these cases. However, there is no evidence as
yet to prove that the condition is traceable to a single, consistent genetic defect.

Identifying individuals who have genetic ASD markers may in the future provide options
for early diagnosis and interventions. Researchers have recently developed a blood test
that looks promising for identifying differences in brain cell development in children as
young as one year of age (Courchesne, 2013). As the multi-genetic basis of ASD is
discovered new diagnostic and treatment options should follow.

In summary

The results of twin and family studies to date indicate that there is almost certainly a
genetic component to ASD. Whilst there appears to be a growing body of evidence to
suggest that particular genetic variations, passed through families, may predispose
some individuals to a greater likelihood of developing ASD, at this point in time no
individual gene or set of genes has been categorically identified.

References

Bailey A, Le Couteur A, Gottesman I, Bolton P, Simonoff E, Yuzda E, Rutter M (1995)
‘Autism as a strongly genetic disorder: Evidence from a British twin study’, Psychological
Medicine, Vol. 25, No. 1, pp. 63–77.

Courchesne, E. (2013) ‘Genetic Pathways Underlying Neural Defects and Implications
for Early Diagnostic Screening’. Proceedings of the Asia Pacific Autism Conference’.
Adelaide.

Autism Spectrum Australia (Aspect) ABN 12 000 637 267
Building 1, Level 2, 14 Aquatic Drive Frenchs Forest NSW 2086
PO Box 361 Forestville NSW 2087 T 02 8977 8300 www.autismspectrum.org.au

Copyright Autism Spectrum Australia (Aspect) 2013

Lauritsen M, Pedersen C, Mortensen P (2005) ‘Effects of familial risk factors and place
of birth on the risk of autism: a nationwide register-based study’, Journal of Child
Psychology and Psychiatry, and Allied Disciplines, Vol. 46, No. 9, pp. 963–971.
Ronald A, Happé F, Plomin R (2005) ‘The genetic relationship between individual
differences in social and nonsocial behaviours characteristic of autism’, Developmental
Science, Vol. 8, No. 5, pp. 444-458.
Sebat J, Lakshmi B, Malhotra D, Troge J, Lese-Martin C, Walsh T, Yamrom B, Yoon S,
Krasnitz A, Kendall J, Leotta A, Pai D, Zhang R, Lee Y-H, Hicks J et al., (2007) ‘Strong
association of de novo copy mutations with autism’, Science, Vol. 316, pp. 445–449.
Silverman J, Smith C, Schmeidler J, Hollander E, Lawlor B, Fitzgerald M, Buxbaum J,
Delaney K, Galvin P, (2002) ‘Symptom domains in autism and related conditions:
Evidence for familiality’, American Journal of Medical Genetics, Vol. 114, pp. 64– 73.
Zhao X, Leotta A, Kustanovich V, Lajonchere C, Geschwind D, Law K, Law P, Qiu S,
Lord C, Sebat J, Ye K, Wigler M (2007) ‘A unified genetic theory for sporadic and
inherited autism’, Proceedings of the National Academy of Sciences of the United States
of America, Vol. 104, No. 31, pp. 12831–12836.

Resources

What support is available? Australian Government resources

Autism Spectrum Australia (Aspect) ABN 12 000 637 267
Building 1, Level 2, 14 Aquatic Drive Frenchs Forest NSW 2086
PO Box 361 Forestville NSW 2087 T 02 8977 8300 www.autismspectrum.org.au

Copyright Autism Spectrum Australia (Aspect) 2013