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\begin{document}

\title{Cardiopulmonary phenotypic variability and discordance in Duchenne
muscular dystrophy}



\author[1]{David Birnkrant}%
\author[1]{John  Carter}%
\affil[1]{MetroHealth Medical Center}%


\vspace{-1em}



  
  \date{May 25, 2024}


\begingroup
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\endgroup





\selectlanguage{english}
\begin{abstract}
Neuromuscular respiratory medicine has traditionally focused on
mechanically assisted lung ventilation and mucus clearance. These
therapies have prolonged survival for patients with Duchenne muscular
dystrophy (DMD). However, the field is rapidly evolving in a new
direction: it is being revolutionized by molecular and genetic
therapies. A good correlation between a patient's dystrophin mutation
and his cardiopulmonary phenotype would allow accurate prediction of
patient prognosis and would facilitate the design of studies that assess
new DMD therapies. Instead, patient prognosis and the design of valid
therapeutic studies are complicated by cardiopulmonary phenotypic
discordance and variability, by which a notable proportion of DMD
patients have unexpectedly good or poor cardiopulmonary function. The
likely cause of phenotypic variability and discordance is genetic
modifiers. Once the modifiers that affect cardiopulmonary function are
better understood, it should be possible to create a personalized
genetic profile that accurately predicts the prognosis of each
individual DMD patient. This would allow investigators to assess the
effect of new therapies in the context of each patient's particular
cardiopulmonary natural history. Amplification of beneficial
cardiopulmonary genetic modifiers and blocking of detrimental modifiers
is a promising strategy for creating new DMD therapies. When patients
with chronic respiratory failure are treated with assisted ventilation,
cardiac function determines their survival. Therefore, prioritizing new
cardiac therapies is most likely to prolong patient survival. By
focusing on these topics we aim to move neuromuscular respiratory
medicine beyond assisted ventilation and coughing and into the age of
translational medicine.%
\end{abstract}%



\sloppy


\textbf{Cardiopulmonary phenotypic variability and discordance in
Duchenne muscular dystrophy: implications for new therapies}

David J. Birnkrant, MD\textsuperscript{1,*}; John C. Carter,
MD\textsuperscript{2}

\textsuperscript{1}Division of Pediatric Pulmonology and Sleep Medicine,
Department of Pediatrics, MetroHealth Medical Center and Case Western
Reserve University School of Medicine

\textsuperscript{2}Division of Pulmonary, Critical Care, and Sleep
Medicine, Department of Medicine, MetroHealth Medical Center and Case
Western Reserve University School of Medicine

\textsuperscript{*}Corresponding author

Correspondence to: David J. Birnkrant, MD

Department of Pediatrics

MetroHealth Medical Center

2500 MetroHealth Drive

Cleveland, Ohio 44109

dbirnkrant@metrohealth.org

Financial Disclosures: Dr. Birnkrant is a Clinical Advisor to Advanced
Bio Machines PTE LTD (ABM Respiratory Care) and has a financial interest
in patents licensed to ABM Respiratory Care

Dr. Cater has no financial disclosures.

Keywords: Duchenne muscular dystrophy, pulmonary function, cardiac
function, modifier genes, phenotype, genotype, survival

Abbreviated title: Cardiopulmonary phenotypic variability in Duchenne
muscular dystrophy

ABSTRACT

Neuromuscular respiratory medicine has traditionally focused on
mechanically assisted lung ventilation and mucus clearance. These
therapies have prolonged survival for patients with Duchenne muscular
dystrophy (DMD). However, the field is rapidly evolving in a new
direction: it is being revolutionized by molecular and genetic
therapies. A good correlation between a patient's dystrophin mutation
and his cardiopulmonary phenotype would allow accurate prediction of
patient prognosis and would facilitate the design of studies that assess
new DMD therapies. Instead, patient prognosis and the design of valid
therapeutic studies are complicated by cardiopulmonary phenotypic
discordance and variability, by which a notable proportion of DMD
patients have unexpectedly good or poor cardiopulmonary function. The
likely cause of phenotypic variability and discordance is genetic
modifiers. Once the modifiers that affect cardiopulmonary function are
better understood, it should be possible to create a personalized
genetic profile that accurately predicts the prognosis of each
individual DMD patient. This would allow investigators to assess the
effect of new therapies in the context of each patient's particular
cardiopulmonary natural history. Amplification of beneficial
cardiopulmonary genetic modifiers and blocking of detrimental modifiers
is a promising strategy for creating new DMD therapies. When patients
with chronic respiratory failure are treated with assisted ventilation,
cardiac function determines their survival. Therefore, prioritizing new
cardiac therapies is most likely to prolong patient survival. By
focusing on these topics we aim to move neuromuscular respiratory
medicine beyond assisted ventilation and coughing and into the age of
translational medicine.

1\textbar{} INTRODUCTION

Neuromuscular respiratory medicine has traditionally focused on the use
of devices for mechanically assisted lung ventilation and mucus
clearance. These devices have prolonged patient survival, as reported
for Duchenne muscular dystrophy (DMD).\textsuperscript{1-6} However, the
overall therapeutic approach of neuromuscular medicine is rapidly
evolving in a very different direction. The field is being
revolutionized by the development of molecular and genetic therapies,
some of which have already been approved for patient use, with others in
development.\textsuperscript{7-12}

Neuromuscular respiratory medicine must now find a way to interface with
these emerging therapies in order to advance into the age of
translational medicine.

The central concept of this article is that it is essential to fully
understand the natural history of cardiopulmonary function in patients
with DMD. Natural history is the trajectory of cardiopulmonary function
over time; this trajectory determines whether a patient has a beneficial
or detrimental phenotype. Based on prior studies and clinical
observations, there appear to be three DMD pulmonary phenotypes:
typical, mild and severe.\textsuperscript{13,14} Patients with the mild
pulmonary phenotype have a forced vital capacity (FVC) that peaks at a
higher level and at a later age compared to patients with the typical
pulmonary phenotype. After peaking, the FVC declines more slowly over
time. As a result, patients with the mild pulmonary phenotype have
better pulmonary function through adulthood (Figure
1).\textsuperscript{14} Similarly, a detrimental or beneficial cardiac
phenotype is determined by the age at onset of left ventricular
dysfunction (LVD). In the detrimental phenotype, LVD begins at an early
age and then steadily declines, leading to an early onset of congestive
heart failure (CHF). Once CHF occurs, the mean survival is just 8
months.\textsuperscript{15} In the beneficial phenotype, the onset of
LVD occurs later. While left ventricular dysfunction is progressive, the
later onset of LVD also delays the onset of CHF, prolonging survival
(Figure 2).\textsuperscript{15}

2\textbar{} PHENOTYPIC VARIABILITY AND DISCORDANCE

A good correlation between dystrophin genotypes and cardiopulmonary
phenotypes would be useful. If the natural history of a patient's
cardiopulmonary function could be predicted from the patient's genotype,
then variations from the patient's predicted clinical course could be
attributed to effect of a particular therapy under investigation.
Correction of the dystrophin mutation would be an effective therapeutic
strategy.

Instead, even patients who share a common dystrophin genotype may have
disparate pulmonary and cardiac phenotypes, making cardiopulmonary
natural history highly unpredictable. For example, one DMD patient's
forced vital capacity (FVC) may peak at a high absolute value and remain
favorable over time, while his brother's FVC peaks at a markedly lower
level, and subsequently declines at a rapid rate (Figure
3).\textsuperscript{16} Furthermore, unrelated patients with the same or
similar dystrophin genotype can have highly divergent pulmonary
function.\textsuperscript{17} With regard to cardiac natural history, a
DMD patient may experience favorable heart function into adulthood,
while his brother experiences early-onset
cardiomyopathy.\textsuperscript{16} Moreover, patients with identical
dystrophin genotypes have significant variability both in their age at
onset of LVD and in the subsequent rate of deterioration of their
cardiac function over time (Figure 4, Table
1).\textsuperscript{15,18}These are examples of cardiopulmonary
phenotypic variability -- defined as unexpected differences in
cardiopulmonary function among patients with identical dystrophin
genotypes. A related concept is cardiopulmonary phenotypic discordance,
in which an individual DMD patient has a cardiac phenotype that is the
diametric opposite of his pulmonary phenotype; specifically,
surprisingly good heart function in a patient with very poor pulmonary
function, or surprisingly poor heart function in a patient with good
pulmonary function.\textsuperscript{17}With phenotypic discordance, the
skeletal muscles controlling pulmonary function and the cardiac muscles
controlling heart function express phenotypes that are unexpectedly
divergent.

Cardiopulmonary phenotypic variability and discordance have important
implications for determining patient prognosis and for assessing the
effectiveness of DMD therapies. The genetic basis of phenotypic
variability and discordance also provides a promising strategy for
designing new DMD treatments.

3\textbar{} PHENOTYPIC DISCORDANCE AND PATIENT PROGNOSIS

With regard to patient prognosis, it seems self-evident that patients
with better cardiopulmonary function should experience better survival.
For example, in one study of DMD patients \emph{who were not treated
with assisted ventilation} , 5-year survival was just 8\% when the
patients' FVC fell below 1 liter.\textsuperscript{19} Instead, our
studies of phenotypic discordance suggest a more complex reality. When
we studied our DMD patients who could be classified into two
groups--prolonged survivors (alive, at mean age 34.3 years) and those
who experienced early death (at mean age 21.7
years),\textsuperscript{20} -- we discovered that the patients who
experienced early death had unexpectedly poor cardiac function (mean
ejection fraction (EF) 29.2\%) despite comparatively good pulmonary
function (mean FVC 804 mL); i.e., the patients had cardiopulmonary
phenotypic discordance. The patients who experienced prolonged survival
also had phenotypic discordance, but their discordance was the opposite
of patients experiencing early death: the prolonged survivors had
unexpectedly good cardiac function (mean EF 42.2\%) despite their highly
impaired pulmonary function, requiring 24 hour/day assisted ventilation
due to an FVC of 0 mL (Figure 5).\textsuperscript{20} We concluded that
when DMD patients with poor pulmonary function are treated with assisted
ventilation, \emph{cardiac function determines their survival} . Viewed
another way, in patients treated with contemporary respiratory
management, poor cardiac function is the main risk factor for early
death, while favorable cardiac function makes them eligible for
prolonged survival. These results suggest that it is crucial to assess
the effect of new DMD therapies on heart function and that cardiac
outcome measures should be prioritized. Instead, most DMD studies report
on skeletal muscle strength, such as timed function tests, the Northstar
Ambulatory Assessment, upper extremity function and the 6-minute walk
test,\textsuperscript{7,21-23} and data on the cardiac effects of new
DMD therapies are sparse.

These observations related to phenotypic discordance and patient
survival also have important implications with regard to study design.
It is incorrect for studies of emerging therapies to state or
infer\textsuperscript{7,8,24,25} that if a therapy preserves pulmonary
function it will definitely prolong patient survival. If a patient has a
detrimental cardiac phenotype, his survival is likely to be shortened,
even if that patient has good pulmonary function. Conversely, a patient
with a beneficial cardiac phenotype is more likely to experience
prolonged survival, even if he has poor pulmonary function, if that
patient is treated with assisted ventilation.

4\textbar{} IMPLICATIONS FOR THERAPEUTIC TRIALS

When designing therapeutic trials, it is also important to consider that
phenotypic variability and discordance create a population of
``outlier'' patients with surprisingly good or poor pulmonary and
cardiac function. It is common for studies of new DMD therapies to use
aggregate data, derived from a limited number of
subjects.\textsuperscript{7,8,25} This study design is based on the
assumption that phenotypic outliers are rare. If, instead,
cardiopulmonary phenotypic variability and discordance are common, then
``outlier'' patients with unexpectedly good cardiac or pulmonary
function may comprise a significant proportion of the study population,
confounding the results of studies based on aggregate data.

Our recent studies suggest that cardiopulmonary phenotypic discordance
is indeed common. In one study, discordance occurred in one-third of our
DMD patients who were alive and 18 years of age or
older.\textsuperscript{17} In a discordant subset of patients with good
heart function and bad lung function, mean FVC was just 0.18 liters, but
mean ejection fraction was 50\% (Table 2).\textsuperscript{17}Phenotypic
discordance was common among patients in sub-groups with identical or
similar dystrophin mutations and cardiopulmonary function was
unpredictable, having no correlation with the dystrophin mutation (Table
3, Table 4, Figure 6). For DMD patients in clinical trials that are
based on aggregate data, prolonged survival could be attributed to the
effect of a new therapy, when instead a sizable proportion of the study
subjects are simply expressing a beneficial cardiac or pulmonary
phenotype. Additionally, grouping patients by their identical or similar
dystrophin genotypes does not assure a homogeneous study population, as
those patients may have dissimilar cardiopulmonary
phenotypes.\textsuperscript{7,8} These observations suggest that new DMD
therapies should be assessed in the context of each patient's particular
cardiopulmonary natural history, rather than relying on aggregate
patient data, even when the patients share a common dystrophin mutation.

In studies that focus on very young DMD patients as their subjects,
phenotypic variability can be masked, as younger patients have not yet
``declared'' their phenotype. Instead, cardiopulmonary natural history
diverges over time. For example, as previously discussed, we found that
DMD patients with the detrimental cardiac phenotype had onset of LVD
before age 18 years, while those with the beneficial cardiac phenotype
manifested LVD at age 18 years or later, and had prolonged
survival.\textsuperscript{15} After LVD onset, cardiac function worsened
over time in both groups, but survival did not diverge until the
patients reached their late teens and early twenties (Figure
2).\textsuperscript{15,17} Very young patients usually had normal
cardiac function, regardless of whether they ultimately expressed a
beneficial or detrimental cardiac phenotype (Figure
7).\textsuperscript{18} Therefore, the effect of new DMD therapies
should be assessed over an extended period of observation and the study
design should include a significant proportion of ``older'' patients who
have established phenotypes. For example, recent studies of eteplirsen
and ataluren generally included younger patients (10-13 years at
baseline) with a follow-up period of only 2 to 4
years,\textsuperscript{8,21} during which pulmonary function remained
fairly well preserved (as expected given the young age of the subjects
and the relatively short duration of observation). Datasets such as
these may demonstrate treatment-related benefits to respiratory function
in the short term, but these benefits do not necessarily translate into
reduction in the morbidity and mortality associated with long-term
declines in respiratory function. Furthermore, drawing conclusions from
datasets that include primarily younger subjects with short follow-up
periods may incorrectly attribute ``preservation'' of pulmonary or
cardiac function to treatment. Instead, beneficial cardiopulmonary
function may be due to the patients' natural history, independent of any
treatment effect.

To summarize, the natural history of cardiopulmonary function in DMD
requires an understanding of phenotypic variability and discordance,
which can be used to optimize the design of therapeutic trials. However,
in stark contrast to the recommendations for valid trial design outlined
above, current DMD studies involve relatively short durations, study
populations consisting of very young subjects, analyses based on
aggregate patient data, and attempts to create homogeneous study
populations by grouping together subjects who have similar or identical
dystrophin mutations.\textsuperscript{7,8,24,25} Results from such
studies are not necessarily translatable to reducing cardiopulmonary
morbidity or mortality in the long-term. Additionally, using pulmonary
function as a surrogate for survival is inaccurate; rather, cardiac
function is the primary determinant of survival in people with DMD who
are treated with ventilatory support.

5\textbar{} IMPLICATIONS FOR PATIENT PROGNOSIS AND THE DESIGN OF NEW
THERAPIES

The genetic basis of cardiopulmonary phenotypic variability and
discordance provides a strategy to more accurately assess patient
prognosis and for designing new DMD treatments. The unpredictability of
cardiopulmonary phenotype in patients with identical dystrophin
genotypes is likely due to the presence of genetic modifiers. These
modifiers may include different isoforms of the dystrophin gene product,
or separate genes that change the expression and products of the mutated
dystrophin gene. Thus, when modifiers are present, the phenotype cannot
be predicted from the dystrophin mutation alone; i.e., by the degree to
which a dystrophin mutation is predicted to alter the gene's reading
frame, disrupting the production of functional dystrophin protein.

While some modifiers appear to affect skeletal muscle and cardiac muscle
synchronously, there are examples of tissue-specific modifiers, such as
a modifier that is detrimental to skeletal muscle but is associated with
later onset of cardiomyopathy.\textsuperscript{26} Thus, modifiers have
the potential to explain both phenotypic variability (divergent
phenotypes among groups of patients who share a common dystrophin
mutation) and phenotypic discordance (individual patients who have a
cardiac phenotype that is diametrically opposite to their pulmonary
phenotype). If the most common and most potent modifiers are identified
and characterized, it might be possible to predict an individual
patient's long-term cardiopulmonary natural history early in life, from
his extended genetic profile, consisting of his dystrophin mutation and
his relevant genetic modifiers. That would allow clinicians to predict
patient prognosis sooner and more accurately and would allow
investigators to assess DMD therapies in the context of each patient's
particular cardiopulmonary natural history.

A promising strategy for identifying those modifiers is the use of Whole
Exome Sequencing in a study population consisting of patients with
``extreme'' cardiopulmonary phenotypes. Recent work utilizing such a
strategy in DMD patients with early loss of ambulation identified
variants in the modifier gene \emph{TCTEX1D1} that were associated with
earlier loss of ambulation, and were also noted in patients with earlier
and more severe onset of cardiomyopathy.\textsuperscript{27} These
``extreme phenotypes'' are equivalent to the phenotypic ``outliers'' we
have identified in our studies of phenotypic discordance. As discussed
previously in this article, we showed that DMD patients who experience
early death and those who experience prolonged survival both manifest
unexpected levels of heart function: unexpectedly poor cardiac function
in the patients who experience early death and unexpectedly good cardiac
function in the prolonged survivors. Thus, Whole Exome Sequencing of
patients with phenotypic discordance may be a way to identify the
modifier genes that cause phenotypic outliers and ``extreme
phenotypes,'' allowing creation of extended genotypic profiles that have
a high degree of prognostic accuracy.

There are numerous candidates for modifiers of the dystrophin gene and
the latest candidates have been described in various
publications.\textsuperscript{26,28,29} Most of the studies describe how
modifiers affect skeletal muscle, with a few reports focusing on
modifiers of pulmonary and cardiac function.\textsuperscript{29-31}While
there is an association between pulmonary phenotype and age at loss of
ambulation (for example, patients with the most severe pulmonary
phenotype experience loss of ambulation at the earliest
age),\textsuperscript{14} the relationship between skeletal muscle
deterioration and cardiopulmonary phenotypes is not well-studied.
Further complicating the picture, exogenous factors such as
administration of glucocorticoids may have a differential effect in
patients with certain genetic modifiers. For example, increasing doses
of glucocorticoid were shown to increase expression of the modifier
gene\emph{SPP1} in patients who possessed the G allele of this
genotype,\textsuperscript{32} suggesting a mechanism for paradoxical
increases in muscle inflammation in the presence of glucocorticoids.

With these limitations in mind, amplifying beneficial modifiers and
downregulating deleterious ones could be a promising therapeutic
strategy. For example, Yamamoto and colleagues reported on 181 patients
with DMD (all of whom had exonic mutations causing depletion of the
Dp427 isoform).\textsuperscript{28} Those patients who also had a
deficiency of the Dp116 isoform experienced substantially longer cardiac
dysfunction-free survival compared to those who did not have a mutation
affecting Dp116, despite all patients having similar cardiac function at
initial evaluation. This is a potential cause of the divergent cardiac
phenotypes we described earlier in this review and suggests the
possibility that depletion of the Dp116 dystrophin isoform could improve
cardiac function and, thus, patient survival.

Overall, cardiopulmonary modifier genes have great potential for use as
therapeutic agents. Therapies that amplify beneficial modifiers and
block detrimental modifiers could positively alter the key clinical
features of DMD, including cardiopulmonary function. Our data
implicating cardiac function as the main determinant of
survival\textsuperscript{17,20} suggest that a therapeutic strategy
focused on beneficial cardiac modifiers is most likely to prolong
lifespan in patient with DMD.

7\textbar{} CONCLUSIONS

In this article we have discussed the complexities of cardiopulmonary
natural history in patients with DMD, including the key concepts of
cardiopulmonary phenotypic variability and discordance, which cause
patients to have unexpectedly good or poor cardiopulmonary function
(``phenotypic outliers''). A true understanding of cardiopulmonary
natural history is necessary in order to accurately determine patient
prognosis and to design studies that accurately assess the effectiveness
of new therapies. With this in mind, current studies of new DMD
therapies are flawed for several reasons: they focus on timed tests and
other measures of skeletal muscle function, rather than cardiopulmonary
outcome measures; their results are based on aggregate data without
accounting for the sizable prevalence of phenotypic ``outliers''; and
their study populations are composed primarily of younger patients with
good cardiopulmonary function, i.e., subjects who have not yet declared
their cardiopulmonary phenotypes. We have discussed how the dystrophin
mutation alone is a poor predictor of cardiopulmonary function in an
individual patient. However, once the genetic modifiers that
beneficially and detrimentally affect cardiopulmonary function are
better understood, it should be possible to create a personalized
genetic profile that accurately predicts the prognosis of each
individual DMD patient. This would allow investigators to design better
treatment studies by assessing the effect of new therapies in the
context of each patient's particular cardiopulmonary natural history.
Amplification of beneficial cardiopulmonary genetic modifiers and
blocking of detrimental modifiers is a promising strategy for creating
new DMD therapies. Moreover, our data suggest that, when patients with
chronic respiratory failure are treated with assisted ventilation,
cardiac function determines their survival. Therefore, prioritizing the
development of effective cardiac therapies is most likely to prolong
patient survival. By focusing on these topics we aim to move
neuromuscular respiratory medicine beyond assisted ventilation and
coughing and into the age of translational medicine.

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27. Spitali P, Zaharieva I, Bohringer S, et al. TCTEX1D1 is a genetic
modifier of disease progression in Duchenne muscular dystrophy.\emph{Eur
J Hum Genet.} 2020;28(6):815-825.

28. Yamamoto T, Awano H, Zhang Z, et al. Cardiac Dysfunction in Duchenne
Muscular Dystrophy Is Less Frequent in Patients With Mutations in the
Dystrophin Dp116 Coding Region Than in Other Regions. \emph{Circ Genom
Precis Med.} 2018;11(1):e001782.

29. Vo AH, McNally EM. Modifier genes and their effect on Duchenne
muscular dystrophy. \emph{Curr Opin Neurol.} 2015;28(5):528-534.

30. Van Dorn CS, Puchalski MD, Weng HY, Bleyl SB, Butterfield RJ,
Williams RV. DMD mutation and LTBP4 haplotype do not predict onset of
left ventricular dysfunction in Duchenne muscular
dystrophy.\emph{Cardiol Young.} 2018;28(7):910-915.

31. Bello L, D'Angelo G, Villa M, et al. Genetic modifiers of
respiratory function in Duchenne muscular dystrophy. \emph{Ann Clin
Transl Neurol.} 2020;7(5):786-798.

32. Vianello S, Pantic B, Fusto A, et al. SPP1 genotype and
glucocorticoid treatment modify osteopontin expression in Duchenne
muscular dystrophy cells. \emph{Hum Mol Genet.} 2017;26(17):3342-3351.

\textbf{Figure Legends}

Figure 1: Natural history of forced vital capacity (FVC) with age in
boys with DMD. (a) shows FVC as \% predicted (mean \emph{+} 95\%
confidence interval). (b) shows FVC in absolute values (mL; mean\emph{+}
95\% confidence interval). Group A included patients with loss of
ambulation at \textless{}8 years of age. Group B included patients with
loss of ambulation between 8-11 years of age. Group C included patients
with loss of ambulation between 11 and 16 years of age. Adapted from
Humbertclaude et al.,\textsuperscript{14} and reproduced with
permission.

Figure 2: Cumulative survival for DMD patients with onset of left
ventricular dysfunction (LVD) \textless{}18 years
versus\emph{\textgreater{}} 18 years. Patients with onset of LVD at a
later age lived significantly longer than those with earlier onset of
LVD. Adapted from Wang et al.,\textsuperscript{15} and reproduced with
permission.

Figure 3: Forced vital capacity (FVC, in liters) with age in two
brothers with DMD and variable pulmonary function. Each reached peak FVC
around the age of 18, but patient 1 maintained FVC near the peak level,
whereas his brother experienced a progressive decline after reaching
peak FVC. Adapted from Birnkrant et al.,\textsuperscript{16} and
reproduced with permission.

Figure 4: Changes in cardiac shortening fraction with age for patients
with deletion of exon 44 (solid lines) and deletion of exon 51 (dashed
lines). Despite identical dystrophin genotypes, patients experience
variable timing in rates of decline in shortening fraction, but once the
shortening fraction falls below 29\% (solid horizontal line), the
decline becomes steadier and more uniform. Adapted from Wang et
al.,\textsuperscript{15} and reproduced with permission.

Figure 5: Cardiopulmonary phenotypic differences in patients with DMD
who were prolonged survivors (PS) versus those who experienced early
death (ED). (a) shows percentage of patients with a vital capacity of
0mL who experienced PS versus ED. (b) shows the mean ejection fraction
in those with PS and ED. Adapted from Birnkrant et
al.,\textsuperscript{20} and reproduced with permission.

Figure 6: Phenotypic discordance in patients with identical dystrophin
mutations. Four patients with deletion of exon 44 show diametrically
different profiles in heart and lung function despite an identical
dystrophin genotype. (A1, patient 1 in group A; A2, patient 2 in group
A; B1, patient 1 in group B; B2, patient 2 in group B; EF, ejection
fraction; FVC, forced vital capacity). Adapted from Jin et
al.,\textsuperscript{17} and reproduced with permission.

Figure 7: Survival curve showing time without cardiomyopathy on
echocardiogram. Adapted from Ashwath et al.,\textsuperscript{18} and
reproduced with permission.\selectlanguage{english}
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\textbf{Hosted file}

\verb`Tables for Pediatric Pulmonology Article Submission Aug 5 2020.docx`  available at \url{https://authorea.com/users/349092/articles/474243-cardiopulmonary-phenotypic-variability-and-discordance-in-duchenne-muscular-dystrophy}

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