1. Prader-Willi Syndrome
Stage 1: Muscular Hypotonia
Roselaure Anstral
December 5, 2013
BIL 374: Human Disease: Genetic and Ultrastructural Interactions

2. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Abstract
This paper explores the genetic disorder known as Prader-Willi Syndrome (PWS) and
ultrastructural alterations that may contribute to stage one of PWS: hypotonia. Prader–Willi
syndrome affects multiple body systems, whose most constant major indicator includes
hypotonia with poor suck and poor weight gain in infancy. PWS can occur by three main
mechanisms, which lead to absence of expression of paternally inherited genes in the 15q11.2–
q13 region: paternal microdeletion, maternal uniparental disomy, and imprinting defect.
Hypotonia is the most common characteristic of PWS. Thus, the earliest research studies
examine this characteristic along with the changes that accompany it. This paper aims to
examine these early findings.

3. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Muscular Hypotonia in Prader Willi Syndrome
In 1956, Swiss physicians A. Prader, H. Willi and A. Labhart, identified Prader-Willi
Syndrome as “a constellation of symptoms” (Cassidy, et al. 2009). Prader-Willi Syndrome
(PWS) is a genetic disorder that affects multiple organ systems within the body and is a disorder
that crosses cultural and racial boundaries. It is the most common genetic disorder that leads to
obesity. It affects between 350,000 and 400,000 individuals worldwide. Within the United
States, the rate of prevalence was approximately 1 in 16,000 in 1990 (Klish et al., 2013), this
number is much larger today. Outside of the US, reported prevalence rates for PWS range from 1
per 8000 in rural Sweden to 1 per 16,000 in Western Japan, and a birth incidence of 1 per 27,000
in Flanders (Klish et al., 2013).
The Genetics of PWS
The Prader-Willi Syndrome disorder is caused by “failure of expression of paternally
inherited genes in the PWS region of chromosome 15,” (Cassidy, et al. 2009). More than one
gene is involved in PWS, and these genes are near each other in a small area of what is called the
“long arm” of chromosome 15—in a region labeled 15q11-q13 (Cassidy, et al. 2009). Prader-
Willi syndrome (PWS) is caused by deficiency for one or more paternally expressed imprinted
transcripts within chromosome 15q11-q13 (see Appendix A, Image 1 for detailed explanation of
genetics), including SNURF-SNRPN and multiple small nucleolar RNAs (snoRNAs) (Cassidy et
al., 2011). Ducker et al (2010) stated that the “balanced chromosomal translocations that
preserve expression of SNURF-SNRPN and centromeric genes but separate the snoRNA HBII-85
cluster from its promoter cause PWS. A microdeletion of the HBII-85 snoRNAs in a child with
PWS provides, in combination with previous data, effectively conclusive evidence that

4. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
deficiency of HBII-85 snoRNAs causes the key characteristics of the PWS phenotype.” It
appears likely that the characteristic features of PWS is caused by the deletion or dysfunction of
small nucleolar RNAs (snoRNAs) which are genes that provide instructions for making
molecules. These molecules have a variety of functions, including helping to regulate other types
of RNA molecules (Cassidy et al. 2011).
There are at least three different chromosome errors that can keep these key genes from
working normally, and all result in the child having Prader-Willi syndrome (Butler et al, 2010).
About 70% of all cases of PWS are due to paternal deletion. It is the most common form of
PWS. Part of the chromosome 15 inherited from the child’s father is deleted. About 25% of
cases are due to maternal uniparental disomy (UPD). In this less common form of PWS, the baby
inherits both copies of chromosome 15 from the mother. In these cases, the developing baby
usually starts out with three copies of chromosome 15 (a condition called trisomy 15) because
there was an extra chromosome 15 in the mother’s egg. Later, one of the three is lost—the
chromosome 15 that came from the father. The result has the same effect as a deletion. Less than
55 percent of cases is an imprinting defect. The PWS genes on the father’s chromosome are
present but do not work because the imprinting process is “faulty.” (Cassidy et al. 1998)
Hypotonic Stage: Ultrastructure Alteration
There are three stages of Prader- Willi Syndrome: hypotonic stage (prenatal-infancy),
hyperphagic stage (childhood), and adolescence & adulthood. My research focused on the first
stage.

5. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
In the early studies of Prader Willi Syndrome, there was an innate focus on the hypotonia
stage of PWS. Hypotonia is the common factor among babies with PWS. The weak muscle tone
(hypotonia) of infants born with PWS is a nearly universal finding. It is said to improve over
time, but adults remain mildly hypotonic with decreased muscle bulk and tone.
A. K. Afifi et al. (1956) was among the first to examined the muscles of infants by light
and electron microscopy. They found ultrastructural changes including subsacrolemmal
mitochondrial aggregates, abnormal Z-line and myofilamentous disarray and loss – comparing a
“normal” and PWS muscle fiber (see Appendix B, Image 1 and 2). When they compared muscle
fibers under the same microscope the differences became even more apparent (see Appendix B,
Image 4). They concluded that although there are abnormalities, they are nonspecific and too
mild to explain the muscle symptoms in PWS.
In a later study, S. Sone (1994) reexamined the previous conclusions of A. K. Afifi
(1956) that these alterations were not enough to explain the hypotonia. S. Sone (1994) conducted
a study of 259 ‘floppy’ infants using muscle histochemistry. The results were that there were
histochemical abnormalities of PWS including fiber size variations, fiber atrophy, and the
increase and decrease in numbers of certain fibers. S. Sone (1994) found that primary muscle
pathology, including muscle fiber immaturity and abnormal muscle fiber type distribution, play a
role in muscle hypotonia and weakness.
Conclusion/Discussion
Both of these studies are significant to the understanding of PWS. A.K. Afifi et al.
(1994) sample size was very small (7 cases) which can account for their conclusion, whereas S.

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Genetic Disorder: Prader-Willi Syndrome
Sone (1994) findings were based on a much larger sample size (259) and also included findings
on the pathology of the muscles. Which they feel contributed to their findings greatly instead of
just focusing on the ultrastructure of the muscle. In present day, PWS has gained some
popularity in the mainstream via media coverage. It is still a disorder that has not been cured;
however, there is great promise with growth hormone treatment to combat some of the
symptoms of PWS including hypotonia (Cassidy et al. 2011). Research supporting growth
hormone treatment provides hope to families dealing with this disorder, including hope for my
family as well.

7. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
References:
Afifi AK, Zellweger H. Pathology of muscular hypotonia in the Pradwer-Willi syndrome. J
Neuro Sci 1969; 4: 46-61.
Butler, M.G. and Thompson, T. (2000) Prader-Willi Syndrome: Clinical and Genetic Findings.
The Endocrinologist 10 (4) Suppl 1:3S-16S
Carrel, A. L., Myers, S. E., Whitman, B. Y., Eickhoff, J., & Allen, D. B. (2010). Long-term growth hormone
therapy changes the natural history of body composition and motor function in children with
Prader-Willi syndrome. Journal of Clinical Endocrinology & Metabolism, 95(3), 1131-1136.
Cassidy, S.B. and Schwartz, S. (1998) Prader-Willi and Angelman Syndromes: Disorders of
Genomic Imprinting. Medicine 77: 140-151.
Cassidy, S.B., Driscoll, D.J. (2009). Prader–Willi syndrome. European Journal of Human
Genetics 17, 3–13; doi:10.1038/ejhg.2008.165; published online 10 September 2008
Cassidy, S. B., Schwartz, S., Miller, J. L., & Driscoll, D. J. (2011). Prader-Willi
syndrome. Genetics in Medicine, 14(1), 10-26.
Duker, A. L., Ballif, B. C., Bawle, E. V., Person, R. E., Mahadevan, S., Alliman, S., ... & Sahoo,
T. (2010). Paternally inherited microdeletion at 15q11. 2 confirms a significant role for
the SNORD116 C/D box snoRNA cluster in Prader–Willi syndrome. European Journal
of Human Genetics, 18(11), 1196-1201.
Klish, W. J., Kirkland, J. L., & Hoppin, A. G. Epidemiology and genetics of Prader-Willi
syndrome.
Sone, S. (1994). Muscle histochemistry in the Prader-Willi syndrome. Brain and
Development, 16(3), 183-188.

8. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Appendix A
Genetics of Prader Willi Syndrome
Image 1: genetic and expression map of chromosomal region 15q11.2-q13 (Butler et al., 2000),
shows that the position of genes and genetic markers (circles) are shown. In the PWS region
(shown in blue), there are six paternal-only (PWS region) expressed unique copy genes
(MKRN3, MAGEL2, NECDIN, C15ORF2 and SNURF- SNRPN and a family of five paternal-
only expressed snoRNA genes). The first type of chromosome error may extend from the
deletion of BP1 to BP3 or that of BP2 to BP3.

9. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Appendix B
Comparison of Muscle Fibers
Image 1: Normal Muscle Fiber (A.K. Afifi, et al. 1969). A sample of muscle fiber of a ‘normal’
infant; it shows an electron micrographic image of a normal muscle fiber, showing the nucleus
(N), Z line (Z) intact, and the A and I bands (A, I) and mitochondria (M) clearly recognizable
and organized.

10. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Image 2: Muscle Fiber of child with PWS (A.K. Afifi, et al. 1969). A sample of muscle fiber of a
child with PWS; it shows the Z line tortuosity and irregularity as shown by the arrow. There are
also myofilamentous disarray and disruption of the sacromeral structure as represented by (**).
The myofibril remained intact (Mf) along with the Z line (Z).

11. Roselaure Anstral
December 5, 2013
Genetic Disorder: Prader-Willi Syndrome
Image 3: Two muscle fibers; lower fiber shows intact ultrastructure (normal muscle fiber)
whereas upper fiber contains abnormal distribution of ultrastructure (PWS muscle fiber). (A.K.
Afifi, et al. 1969)