Discussion
This study identified that increasing sw[Cl-]
values and ever having a respiratory culture positive for PSA are
associated with risk of reclassification from CRMS/CFSPID to CF in a
population with high IRT and two CFTR variants. These findings
emphasize that continuous follow up, allowing for observation of changes
in phenotype over time, will help define the prognosis for children with
CRMS/CFSPID. Multiple factors that could influence CF reclassification
were studied, and we found that children who reclassified to CF had more
frequent reporting of crackles, respiratory symptoms, and growth
concerns compared to those who retained the CRMS/CFSPID designation.
CF-reclassified children also had higher rates of pneumonia,
hospitalizations for respiratory symptoms, use of antibiotics, and a
steeper slope of rising sw[Cl-] values. Measures
of ppFEV1, LCI2.5, IRT, and history of
asthma/reactive airways disease were not significantly associated
with the risk of CRMS/CFSPID-to-CF reclassification; this is notable
considering the clinical relevance of these variables within the CF
population.
A rising sw[Cl-] predicts CF reclassification,
confirming previous reports. The precise predictive threshold is unclear
and may vary by cohort. In prior studies, CRMS/CFSPID cohorts that have
reclassified to CF had sw[Cl-] increases from
5.3-7.3 mmol/L/year and an increase in mean
sw[Cl-] from 40 (±11) to 82 (±26)
mmol/L.21 Our group and others have also reported that
healthy controls and those who retained the CRMS/CFSPID designation
through childhood did not have significant increase in
sw[Cl-] over time.2021 Here, we present that a rate of
sw[Cl-] increase of 4.71 mmol/L/year (95%CI 2.45
– 6.97) is an independent risk factor for CF reclassification. These
combined findings may guide providers to be more attentive to children
with CRMS/CFSPID showing increase sw[Cl-] ≥4
mmol/L/year, and more reasured by others showing increase at
~1 mmol/L/year. This also suggests that there is more
value in making clinical decision based on multiple data points
collected over time, rather than based on one data point from an intial
assessment. Notably, 50% of those who reclassified had an intial
sw[Cl-] <30 mmol/L, which is considered
“CF unlikely” by current guidelines.16 Although an
initial intermediate sw[Cl-] (30-59 mmol/L)
naturally raises concern for likelihood of CFTR
dysfunction,23 recent longitudinal data from the
CF-reclassified group suggests that providers should also focus on the
rate at which sw[Cl-] rises as an indication for
change in diagnosis, even before it reaches the diagnostic threshold of
≥60 mmol/L.21
CFTR genotypes from children who reclassified to CF included at
least one F508del (58%) or a nonsense variant (G542X and R1162X;Table 2 ). Genotypes containing a CF-causing variant
and 5T;TG12 or 5T;TG13 were seen in 7 (58%) of the 12 who reclassified.
Notably, as the CF NBS algorithm in California includes CFTRsequencing, it is unlikely that other variants influenced the phenotypes
in these individuals. As we previously reported, 38% of children with a
CF-causing variant and 5T;TG13 reclassified to CF in their first 8 years
of life.20 This is aligned with previous reports that
determination of TG repeat number allows for more accurate prediction of
benign versus pathogenic 5T alleles.24 Other variants
observed in the second allele of the CF-reclassified group were missense
variants of varying clinical consequence per CFTR2, and which are
approved for CFTR modulators: c.1853T>C (I618T),
c.3454G>C (D1152H) , and c.3154T>G (F1052V).
(https://cftr2.org) The non CF-causing
variant c.3705T>G (S1235R) was observed as part of a
complex allele. Variants c.94C>A (L32M) and
c.1393-42G>A (1525-42G>A) are frequent in
California and are of unknown consequence.25
LCI2.5 was non-discriminatory between those with
CRMS/CFSPID who reclassified to CF and those who did not (Figure
1, Supplemental Figure 2, Table 3) and did not influence
reclassification when tested in the multivariate logistic regression
model. As children with CRMS/CFSPID may have abnormal
LCI2.5 despite being asymptomatic,11longer follow-up will reveal if those with abnormal measurements at a
young age will progress to have symptoms and abnormal
ppFEV1 later in life.26 Munck and
colleagues reported that asymptomatic children with CRMS/CFSPID may have
bronchiectasis;27 therefore, MBW should continue to be
explored as a sensitive and safe method to detect early lung disease in
this population.11,28
Assessment of CFTR protein function was performed in a subgroup of
participants. This method was chosen because it directly estimates
protein function, is not expected to change over time, and is anex-vivo test that can be tolerated by children. Our results
suggest greater overlap in HNE assay results between children who
reclassified to CF and those who retained the CRMS/CFSPID designation
compared to sw[Cl-] values in these two groups
(Figure 2 ). On the other hand, HNE assays show greater
distinction between the CRMS/CFSPID group and healthy controls than
sw[Cl-], as healthy controls and carriers may have
sw[Cl-] from <30 to >60
mmol/L.29 Additionally, HNE assay can be performed at
any time, whereas sw[Cl-] slope requires repeated
measurements over time. We recruited a selected genotype group to
evaluate using HNE assays based on higher risk of reclassification: a
CF-causing variant on one allele and R117H;7T, 5T;TG12, or 5T;TG13 on
the other.20,30 A larger sample is needed to draw
conclusions based on genotype groups. Additionally, a non-linear
association between HNE and sw[Cl-] has been
reported in a cohort of individuals with CF 18 that
was not found in this CRMS/CFSPID cohort. However, this association was
difficult to interpret when sw[Cl-] was
<60 mmol/L. Finally, the HNE assay reported here was completed
in 10 subjects but a total of 17 underwent sampling (including some
individuals who were sampled twice; data not shown). The
~50% test yield due to contamination and poor cell
growth may be a reflection of the discomfort caused by sampling, which
could be a barrier to utilizing this assay for young pediatric patients.
However, considering the potential benefit of having the accurate
estimation of CFTR function early in life, HNE assays should be explored
further in a larger sample of the CRMS/CFSPID population.
Microbiology characteristics included at least one positive respiratory
culture for PSA in 12% of the CRMS/CFSPID group, similar to other US
reports and lower than in European cohorts (~10% and
24% respectively)3,27 (Table 1 ). One child
with two positive cultures in the previous 12 months was placed in the
chronic category; in this case, the second positive culture was
immediately after 28 days of inhaled tobramycin (Tobi®), which triggered
a second course of treatment.
CFQ-R questionnaires were used as a patient reporting outcome for
patients ≥6 years of age.14 Among the different
domains, social showed significant higher results for children with
CRMS/CFSPID, likely because these children are asymptomatic and their
social lives are less impacted. Respiratory and emotional domains were
statistically similar, which may be a reflection of early interventions
related to early diagnosis of CF through NBS and to the young age group.
IRT results were statistically similar between the CF-reclassified
subjects and those who retained the CRMS/CFSPID designation
(Table 3 ). This contrasts our previous findings, where an
IRT>80 ng/mL increased the likelihood of CF
reclassification.20 Others have reported no difference
in baseline IRT when reclassified and non-reclassified children with
CRMS/CFSPID were compared.21 Thus, it remains unclear
whether the baseline IRT measured at birth can be used as a predictor of
CF reclassification.31
Other findings included no identification of CFTR-related disorder
(CFTR-RD) and no association of asthma or reactive airways disease with
reclassification to CF. CFTR-RD was defined in 2011 as a diagnosis
related to CFTR dysfunction that does not fulfill the diagnosis criteria
for CF and typically manifests in one affected organ system (congenital
bilateral absence of the vas deferens [CBAVD], pancreatitis, or
bronchiectasis).32 As CFTR variants associated
with CFTR-RD are also found in CRMS/CFSPID cohorts, it is possible that
some children with CRMS/CFSPID will eventually fulfill diagnostic
criteria for CFTR-RD but not CF. The CFTR-RD definition is under needed
revision, as it likely that a child with CRMS/CFSPID, two CFTRvariants, and evidence of CFTR dysfunction (i.e., intermediate
sw[Cl-]) who develops chronic respiratory symptoms
and bronchiectasis will be diagnosed as CF.4, 6Chronic cough and wheezing can be the initial manifestations of CF in
childhood;33 however, they may lead to the diagnosis
of asthma/reactive airway disease instead of CF, considering the
increasing prevalence of asthma in childhood.34Interestingly, CFTR dysfunction is associated with an increased
likelihood of asthma, as seen among CFTRcarriers.35 In this cohort, the asthma prevalence was
statistically similar between the CF-reclassified group and those who
kept the CRMS/CFSPID designation, and asthma did not increase the risk
of reclassification. It is unclear if the high prevalence seen in both
groups (42% in CF-reclassified and 36% in CRMS/CFSPID; Table
3 ) indicates misdiagnosis of CF, CFTR dysfunction, or a public health
trend. Nevertheless, primary care providers should avoid misdiagnosis
and refer a child with CRMS/CFSPID and who has chronic respiratory signs
and symptoms to a CF center for re-evaluation.36
This study is not without limitations. The recruitment was limited to
two sites, making the sample size relatively small and limited to a
specific geographic area. Both sites also had a high lost-to-follow-up
rate among CRMS/CFSPID children (51% at CHLA and 34% at KP),
potentially introducing bias to the population continuing to receive
care. We did attempt to address this by specifically recruiting from
this population – 13 of the 59 (22%) subjects in the CRMS/CFSPID group
were recruited from a lost-to-follow-up pool. Tobacco smoke exposure, a
known risk factor for CFTR dysfunction,37 could not be
studied because of the low prevalence of exposure to secondhand smoking.
A final thought is that although currently published data showed that
most children with CRMS/CFSPID designation have a favorable outcome,
data is limited to a small number entered in national registries and
short follow up time.27,31,38 The short follow up may
be attributed to young age of this population, early discharge per
physician’s decision, and high rates of lost-to-follow
up.39 The second challenge for defining prognosis in
this population is high variability in duration of follow up, care
practices, and CF reclassification criteria despite improved
guidelines.40
CF providers and investigators are still in the beginning of
understanding the intricacies of the CRMS/CFSPID population, and longer
longitudinal observational studies are needed to better define the risk
of reclassification to CF. Although recent literature suggests that most
persons with CRMS/CFSPID will not develop any symptoms, more current
findings indicate that some will develop CF, others CFTR-RD, and a
larger group may be at risk for a wide range of cystic fibrosis-related
conditions such as those seen in carriers. Key strengths of our work are
the size of the cohort, the diversity of fully-sequenced CFTRgenotypes, and a wide age range with longitudinal data up to 13 years.
We showed that the two independent risk factors for manifesting signs
and symptoms to fulfil CF diagnosis criteria are rapid rising
sw[Cl-] values and the detection of PSA from
respiratory cultures. Yearly visits at the CF center and greater
awareness from the primary care providers will promote monitoring of
chronic respiratory symptoms and other signs and symptoms seen in early
CF disease, so treatment can start early fulfilling the preventive
purpose of NBS. The CF scientific community should continue to invest in
alternative CFTR function assays that can complement
sw[Cl-] testing in defining risk of disease for
each unique combination of CFTR variants. With all this put in
place we will better define each child’s needs from infancy, which will
avoid unnecessary medical care for those at low risk and close follow up
for those at a higher risk of developing CF disease.