Alveolar capillary dysplasia with misalignment of the pulmonary veins (ACDMPV) is
a rare lethal lung developmental disorder in which the majority of affected infants
present with neonatal respiratory failure and severe pulmonary hypertension that is
refractory to treatment (1, 2). Pathologically, the disease is characterized by a
paucity of distal capillaries and the presence of “misaligned” veins—pulmonary veins
located within the same bronchovascular sheath as the pulmonary artery and airway
(2). Recently, it has been shown that these misaligned veins are actually anastomotic
shunt vessels (3). Most affected infants also have abnormalities in other organ systems,
including the cardiac, gastrointestinal, and genitourinary systems (2, 4). Over the
past several years, children with milder forms of ACDMPV who present later and survive
longer with anti–pulmonary hypertensive therapies have been increasingly recognized,
although the prognosis is still poor, with lung transplantation being the only available
long-term therapy (5, 6).
A breakthrough in understanding the cause of ACDMPV came with the discovery that genic
deletions of and mutations in the FOXF1 (forkhead box F1) gene account for the majority
of ACDMPV cases (7). FOXF1 is a transcription factor essential for vascular development.
Homozygous foxf1-null mice are embryonic lethal because of abnormal vascular development
of the allantois and yolk sac (8). Haploinsufficient foxf1
+/− mice recapitulate some of the features of ACDMPV, with affected animals having
lung hypoplasia and reduced angiogenesis, abnormal gall bladder morphogenesis, and
increased (but not universal) perinatal mortality. Interestingly the pathology of
foxf1
+/− mice does not include findings of misaligned pulmonary veins, as seen in the human
disorder (8). Haploinsufficiency is the presumed mechanism for FOXF1 mutations causing
human lung disease, as disease results from monoallelic gene deletions and null (nonsense
and frameshift) mutations (4, 7). Regulation of FOXF1 is complex, as disease-associated
mutations are clustered within the DNA-binding domain of FOXF1, and deletions in the
5′ untranslated region involving two long noncoding RNAs also result in the phenotype
of ACDMPV (9).
In this issue of the Journal, Pradhan and colleagues (pp. 1045–1056) expand our knowledge
of the molecular mechanisms by which FOXF1 mutations cause disease and offer a glimmer
of hope for treatment for this universally fatal disorder (10). They selected for
study a mutation identified in an infant with ACDMPV that resulted in the substitution
of phenylalanine for S52F (serine in codon 52). The S52F mutation is located within
an evolutionary conserved, frequently mutated, computationally predicted SH2-binding
domain important for interactions with the protein STAT3 (signal transducer and activator
of transcription 3). The authors demonstrated that the S52F-FOXF1 protein did not
bind STAT3 in vitro, indicating the importance of the serine at position 52 in the
interaction of FOXF1 and STAT3, although several other FOXF1 mutations within another
computationally predicted SH2 binding domain (Y284A, I285Q, S291*) did not disrupt
FOXF1’s interaction with STAT3. They then used CrispR/Cas9 to generate a mouse model
with one allele expressing the S52F mutation. Perinatal mortality was increased in
the wild type (WT)/S52F mice, although, similar to foxf1
+/− mice, it was not uniformly lethal, and the reasons some WT/S52F pups survive remains
unclear. However, this murine model largely recapitulates the histopathology of the
human phenotype, including pulmonary hypoplasia, misaligned pulmonary veins, pulmonary
arterial hypertrophy, and alveolar simplification. Furthermore, decreased transcription
of both the FOXF1 and STAT3 genes, as well as decreased transcription of additional
downstream target genes important in endothelial cell proliferation and angiogenesis,
was observed in the lungs of WT/S52F mice. Finally, they used nanoparticles to deliver
STAT3 complementary DNA intravascularly into newborn WT/S52F mice and demonstrated
efficient targeting of lung endothelial cells with increased STAT3 protein and phosphorylation,
increased expression of endothelial cell markers indicating improved angiogenesis,
improved alveogenesis, and decreased inflammation. Whether there was increased survival
or improved lung function in treated mice was unaddressed.
Although ACDMPV is a rare disease, with the recognition of the causative role of FOXF1
mutations and deletions, clinical genetic testing is now routinely available, allowing
for noninvasive diagnosis. As a result, the number of identified cases has increased
dramatically in recent years, as exemplified by the additional 28 cases included in
the report (10). Could delivery of STAT3 complementary DNA using nanoparticles, which
are being used in clinical trials for human malignancies, be used to treat human infants
with ACDMPV? There are several important potential limitations and barriers to this
approach. First, it is not clear how many other FOXF1 mutations disrupt interactions
with STAT3 and are associated with decreased STAT3 signaling, as the authors’ data
with respect to several other mutations indicated that they did not interfere with
FOXF1–STAT3 interactions. Interestingly, decreased phosphorylated STAT3 was observed
in human lung tissue from an infant with an unrelated FOXF1 frameshift mutation downstream
of the first STAT3 consensus binding sequence. Augmenting STAT3 signaling might thus
be an effective approach for some FOXF1 mutations, as well as an approach that could
be applied to augment other downstream signaling critical for angiogenesis. A more
practical barrier is that ACDMPV usually arises as a sporadic disorder due to de novo
mutations (4, 9). Although familial cases are recognized and prenatal diagnosis has
been performed (11), these cases are the exceptions. Most infants present after birth
with respiratory failure and persistent pulmonary hypertension, which may result from
other disease mechanisms. Even if the diagnosis is suspected initially, confirmatory
genetic studies may take several weeks, and affected infants may die before a diagnosis
is confirmed. By the time the diagnosis is established for surviving infants, secondary
lung damage from oxygen toxicity and ventilator-induced injury will have complicated
the infant’s course. Lung biopsy may allow for more rapid diagnosis, but some infants
with histologic ACDMPV do not have FOXF1 mutations or deletions (9). Finally, gain-of-function
mutations in STAT3 cause an autoimmune disease that can affect the lungs (12), so
dosing considerations will be critical so as not to replace one disease with another.
Despite these practical limitations, Pradhan and colleagues have generated an important
animal model and an important advance in understanding the molecular pathogenesis
of ACDMPV, and they suggest a path forward for the treatment of this devastating disorder.